Why doesn't .NET/C# optimize for tail-call recursion? - c#

I found this question about which languages optimize tail recursion. Why C# doesn't optimize tail recursion, whenever possible?
For a concrete case, why isn't this method optimized into a loop (Visual Studio 2008 32-bit, if that matters)?:
private static void Foo(int i)
{
if (i == 1000000)
return;
if (i % 100 == 0)
Console.WriteLine(i);
Foo(i+1);
}


JIT compilation is a tricky balancing act between not spending too much time doing the compilation phase (thus slowing down short lived applications considerably) vs. not doing enough analysis to keep the application competitive in the long term with a standard ahead-of-time compilation.
Interestingly the NGen compilation steps are not targeted to being more aggressive in their optimizations. I suspect this is because they simply don't want to have bugs where the behaviour is dependent on whether the JIT or NGen was responsible for the machine code.
The CLR itself does support tail call optimization, but the language-specific compiler must know how to generate the relevant opcode and the JIT must be willing to respect it.
F#'s fsc will generate the relevant opcodes (though for a simple recursion it may just convert the whole thing into a while loop directly). C#'s csc does not.
See this blog post for some details (quite possibly now out of date given recent JIT changes). Note that the CLR changes for 4.0 the x86, x64 and ia64 will respect it.


This Microsoft Connect feedback submission should answer your question. It contains an official response from Microsoft, so I'd recommend going by that.
Thanks for the suggestion. We've
considered emiting tail call
instructions at a number of points in
the development of the C# compiler.
However, there are some subtle issues
which have pushed us to avoid this so
far: 1) There is actually a
non-trivial overhead cost to using the
.tail instruction in the CLR (it is
not just a jump instruction as tail
calls ultimately become in many less
strict environments such as functional
language runtime environments where
tail calls are heavily optimized). 2)
There are few real C# methods where it
would be legal to emit tail calls
(other languages encourage coding
patterns which have more tail
recursion, and many that rely heavily
on tail call optimization actually do
global re-writing (such as
Continuation Passing transformations)
to increase the amount of tail
recursion). 3) Partly because of 2),
cases where C# methods stack overflow
due to deep recursion that should have
succeeded are fairly rare.
All that said, we continue to look at
this, and we may in a future release
of the compiler find some patterns
where it makes sense to emit .tail
instructions.
By the way, as it has been pointed out, it is worth noting that tail recursion is optimised on x64.


C# does not optimize for tail-call recursion because that's what F# is for!
For some depth on the conditions that prevent the C# compiler from performing tail-call optimizations, see this article: JIT CLR tail-call conditions.
Interoperability between C# and F#
C# and F# interoperate very well, and because the .NET Common Language Runtime (CLR) is designed with this interoperability in mind, each language is designed with optimizations that are specific to its intent and purposes. For an example that shows how easy it is to call F# code from C# code, see Calling F# code from C# code; for an example of calling C# functions from F# code, see Calling C# functions from F#.
For delegate interoperability, see this article: Delegate interoperability between F#, C# and Visual Basic.
Theoretical and practical differences between C# and F#
Here is an article that covers some of the differences and explains the design differences of tail-call recursion between C# and F#: Generating Tail-Call Opcode in C# and F#.
Here is an article with some examples in C#, F#, and C++\CLI: Adventures in Tail Recursion in C#, F#, and C++\CLI
The main theoretical difference is that C# is designed with loops whereas F# is designed upon principles of Lambda calculus. For a very good book on the principles of Lambda calculus, see this free book: Structure and Interpretation of Computer Programs, by Abelson, Sussman, and Sussman.
For a very good introductory article on tail calls in F#, see this article: Detailed Introduction to Tail Calls in F#. Finally, here is an article that covers the difference between non-tail recursion and tail-call recursion (in F#): Tail-recursion vs. non-tail recursion in F sharp.


I was recently told that the C# compiler for 64 bit does optimize tail recursion.
C# also implements this. The reason why it is not always applied, is that the rules used to apply tail recursion are very strict.


You can use the trampoline technique for tail-recursive functions in C# (or Java). However, the better solution (if you just care about stack utilization) is to use this small helper method to wrap parts of the same recursive function and make it iterative while keeping the function readable.


I had a happy surprise today :-)
I am reviewing my teaching material for my upcoming course on recursion with C#.
And it seems that finally tail call optimization has made its way into C#.
I am using NET5 with LINQPad 6 (optimization activated).
Here is the Tail call optimizable Factorial function I used:
long Factorial(int n, long acc = 1)
{
if (n <= 1)
return acc;
return Factorial(n - 1, n * acc);
}
And here is the X64 assembly code generated for this function:
See, there is no call, only a jmp. The function is agressively optimized as well (no stack frame setup/teardown). Oh Yes!


As other answers mentioned, CLR does support tail call optimization and it seems it was under progressive improvements historically. But supporting it in C# has an open Proposal issue in the git repository for the design of the C# programming language Support tail recursion #2544.
You can find some useful details and info there. For example #jaykrell mentioned
Let me give what I know.
Sometimes tailcall is a performance win-win. It can save CPU. jmp is
cheaper than call/ret It can save stack. Touching less stack makes for
better locality.
Sometimes tailcall is a performance loss, stack win.
The CLR has a complex mechanism in which to pass more parameters to
the callee than the caller recieved. I mean specifically more stack
space for parameters. This is slow. But it conserves stack. It will
only do this with the tail. prefix.
If the caller parameters are
stack-larger than callee parameters, it usually a pretty easy win-win
transform. There might be factors like parameter-position changing
from managed to integer/float, and generating precise StackMaps and
such.
Now, there is another angle, that of algorithms that demand
tailcall elimination, for purposes of being able to process
arbitrarily large data with fixed/small stack. This is not about
performance, but about ability to run at all.
Also let me mention (as extra info), When we are generating a compiled lambda using expression classes in System.Linq.Expressions namespace, there is an argument named 'tailCall' that as explained in its comment it is
A bool that indicates if tail call optimization will be applied when compiling the created expression.
I was not tried it yet, and I am not sure how it can help related to your question, but Probably someone can try it and may be useful in some scenarios:
var myFuncExpression = System.Linq.Expressions.Expression.Lambda<Func< … >>(body: … , tailCall: true, parameters: … );
var myFunc = myFuncExpression.Compile();

Related

Why does tail call optimization need an op code?

So I've read many times before that technically .NET does support tail call optimization (TCO) because it has the opcode for it, and just C# doesn't generate it.
I'm not exactly sure why TCO needs an opcode or what it would do. As far as I know, the requirement for being able to do TCO is that the results of a recursive call are not combined with any variables in the current function scope. If you don't have that, then I don't see how an opcode prevents you from having to keep a stack frame open. If you do have that, then can't the compiler always easily compile it to something iterative?
So what is the point of an opcode? Obviously there's something I'm missing. In cases where TCO is possible at all, can't it always be handled at the compiler level than at the opcode level? What's an example of where it can't?

Following the links you already provided, this is the part which seems to me, answers your question pretty closely..
Source
The CLR and tail calls
When you're dealing with languages managed by the CLR, there are two kinds of compilers in play. There's the compiler that goes from your language's source code down to IL (C# developers know this as csc.exe), and then there's the compiler that goes from IL to native code (the JIT 32/64 bit compilers that are invoked at run time or NGEN time). Both the source->IL and IL->native compilers understand the tail call optimization. But the IL->native compiler--which I'll just refer to as JIT--has the final say on whether the tail call optimization will ultimately be used. The source->IL compiler can help to generate IL that is conducive to making tail calls, including the use of the "tail." IL prefix (more on that later). In this way, the source->IL compiler can structure the IL it generates to persuade the JIT into making a tail call. But the JIT always has the option to do whatever it wants.
When does the JIT make tail calls?
I asked Fei Chen and Grant Richins, neighbors down the hall from me who happen to work on the JIT, under what conditions the various JITs will employ the tail call optimization. The full answer is rather detailed. The quick summary is that the JITs try to use the tail call optimization whenever they can, but there are lots of reasons why the tail call optimization can't be used. Some reasons why tail calling is a non-option:
Caller doesn't return immediately after the call (duh :-))
Stack arguments between caller and callee are incompatible in a way that would require shifting things around in the caller's frame before the callee could execute
Caller and callee return different types
We inline the call instead (inlining is way better than tail calling, and opens the door to many more optimizations)
Security gets in the way
The debugger / profiler turned off JIT optimizations
The most interesting part in context of your question, which makes it super clear in my opinion, among many scenarios, is example of security mentioned above...
Security in .NET in many cases depends on the stack being accurate... at runtime.. Which is why, as highlighted above, the burden is shared by both the source to CIL compiler, and (runtime) CIL-to-native JIT compilers, with the final say being with the latter.

Guess: In a simple language like x86 assembler where you manage the stack "manually", you don't need an opcode - you can just set up the call stack appropriately.
But in something higher-level like .NET CIL, the stack is partially managed for you, and the whole act of invoking a function is a single opcode (e.g. call). So you need a different opcode to implement TCO - one that does "pass control flow to this function, but without creating a new stack frame".

Efficiency comparison of recursion and non recursive function in Java

As I understand, recursive functions are generally less efficient than equivalent non-recursive functions because of the overhead of function calls. However, I have recently encountered a text book saying this is not necessary true with Java (and C#).
It does not say why, but I assume this might be because the Java compiler optimizes recursive functions in some way.
Does anyone know the details of why this is so?

The text book is probably referring to tail-call optimization; see #Travis's answer for details.
However, the textbook is incorrect in the context of Java. Current Java compilers do not implement tail-call optimization, apparently because it would interfere with the Java security implementation, and would alter the behaviour of applications that introspect on the call stack for various purposes.
References:
Does the JVM prevent tail call optimizations?
This Sun bug requesting tail-call support ... still open.
This page (and the referenced paper) suggest that perhaps it wouldn't be that hard after all ...
There are hints that tail-call optimization might make it into Java 8.

This is usually only true for tail-recursion (http://en.wikipedia.org/wiki/Tail_call).
Tail-recursion is semantically equivalent to an incremented loop, and can therefore be optimized to a loop. Below is a quote from the article that I linked to (emphasis mine):
Tail calls are significant because
they can be implemented without adding
a new stack frame to the call stack.
Most of the frame of the current
procedure is not needed any more, and
it can be replaced by the frame of the
tail call, modified as appropriate.
The program can then jump to the
called subroutine. Producing such code
instead of a standard call sequence is
called tail call elimination, or tail
call optimization.
In functional programming languages,
tail call elimination is often
guaranteed by the language standard,
and this guarantee allows using
recursion, in particular tail
recursion, in place of loops

Some reasons why recursive implementations can be as efficient as iterative ones under certain circumstances:
Compilers can be clever enough to optimise out the function call for certain functions, e.g. by converting a tail-recursive function into a loop. I strongly suspect some of the modern JIT compilers for Java do this.
Modern processors do branch prediction and speculative execution, which can mean that the cost of a function call is minimal, or at least not much more than the cost of an iterative loop
In situations where you need a small amount local storage on each level of recursion, it is often more efficient to put this on the stack via a recursive function call than to allocate it in some other way (e.g. via a queue in heap memory).
My general advice however is don't bother worrying about this - the difference is so small that it is very unlikely to make a difference in your overall performance.

Guy Steele, one of the fathers of Java, wrote a paper in 1977
Debunking the "Expensive Procedure Call" Myth
or, Procedure Call Implementations Considered Harmful
or, LAMBDA: The Ultimate GOTO
Abstract:
Folklore states that GOTO statements are
"cheap', while procedure calls are 'expensive'. This
myth is largely a result of poorly designed language
Implementations.
That's funny, because even today, Java has no tail call optimization:)

To the best of my knowledge, Java does not do any sort of recursion optimization. Knowing this is important - not because of efficiency, but because recursion at an excessive depth (a few thousand should do it) will cause a stack overflow and crash your program. (Really, considering the name of this site, I'm surprised nobody brought this up before me).

I don't think so, in my experience in solving some programming problems in sites like UVA or SPOJ I had to remove the recursion in order to solve the problem within established time to solve the problem.
One way that you can think is: in recursive calls, any time that the recursion occurs, the jvm must allocate resources for the function that has being called, in non recursive functions most part of the memory is already allocated.

Examples of CLR compiler optimizations

I'm doing a presentation in few months about .Net performance and optimization, I wanted to provide some samples of unnecessary optimization, things that will be done by the compiler anyways.
where can I find some explanation on what optimizations the compiler is actually capable of maybe some before and after code?

check out these links
C# Compiler Optimizations
compiler optimization
msdn
Also checkout this book on MSIL
1. Microsoft Intermediate Language: Comparison Between C# and VB.NET / Niranjan Kumar

What I think would be even better than examples of "things that will be done by the compiler anyways" would be examples of scenarios where the compiler doesn't perform "optimizations" that the developer assumes will yield a performance improvement but which, in fact, won't.
For example sometimes a developer will assume that caching a value locally will improve performance, when actually the savings of having one less value on the stack outweighs the miniscule cost of a field access that can be inlined.
Or the developer might assume that "force-inlining" a method call (essentially by stripping out the call itself and replacing with copied/pasted code) will be worthwhile, when in reality keeping the method call as-is would result in its getting inlined by the compiler only when it makes sense (when the benefit of inlining outweighs the growth in code size).
This is only a general idea, of course. I don't have concrete code samples that I can point to; but maybe you can scrounge some up if you look for them.

Performance difference between C++ and C# for mathematics

I would like to preface this with I'm not trying to start a fight. I was wondering if anyone had any good resources that compared C++ and C# for mathematically intensive code? My gut impression is that C# should be significantly slower, but I really have no evidence for this feeling. I was wondering if anyone here has ever run across a study or tested this themselves? I plan on running some tests myself, but would like to know if anyone has done this in a rigorous manner (google shows very little). Thanks.
EDIT: For intensive, I mean a lot of sin/cos/exp happening in tight loops

I have to periodically compare the performance of core math under runtimes and languages as part of my job.
In my most recent test, the performance of C# vs my optimized C++ control-case under the key benchmark — transform of a long array of 4d vectors by a 4d matrix with a final normalize step — C++ was about 30x faster than C#. I can get a peak throughput of one vector every 1.8ns in my C++ code, whereas C# got the job done in about 65ns per vector.
This is of course a specialized case and the C++ isn't naive: it uses software pipelining, SIMD, cache prefetch, the whole nine yards of microoptimization.

C# will be slower in general, but not significantly so. In some cases, depending on the structure of the code, C# can actually be faster, as JIT analysis can frequently improve the performance of a long-running algorithm.
Edit: Here's a nice discussion of C# vs C++ performance
Edit 2:
"In general" is not really accurate. As you say, the JIT compiler can actually turn your MSIL into faster native code that the C++ compiler because it can optimize for the hardware it is running on.
You must admit, however, that the act of JIT compiling itself is resource intensive, and there are runtime checks that occur in managed code. Pre-compiled and pre-optimized code will always be faster than just JITted code. Every benchmark comparison shows it. But long-running processes that can have a fair amount of runtime analysis can be improved over pre-compiled, pre-optimized native code.
So what I said was 100% accurate. For the general case, managed code is slightly slower than pre-compiled, pre-optimized code. It's not always a significant performance hit, however, and for some cases JIT analysis can improve performance over pre-optimized native code.

For straight mathematical functions asking if C# is faster than C++ is not the best question. What you should be asking
Is the assembly produced by the CLR JITer more or less efficient than assembly generated by the C++ compiler
The C# compiler has much less influence on the speed of purely mathmatical operations than the CLR JIT does. It would have almost identical performance as other .Net languages (such as VB.Net if you turn off overflow checing).

There are extensive benchmarks here:
http://shootout.alioth.debian.org/u32q/benchmark.php?test=all&lang=csharp&lang2=gpp&box=1
Note this compares the Mono JIT to C++. AFIAK there are no extensive benchmarks of Microsoft's implementation out there, so almost everything you will hear is hearsay. :(

I think you're asking the wrong question. You should be asking if C++ on can beat out the .NET family of languages in mathematical computation. Have a gander at F# timing comparisons for Runge Kutta

You do not define "mathematically intensive" very well (understatement for: not at all).
An attempt to a breakdown:
For the basic Sin/Cos/Log functions I would not expect much difference.
For linear algebra (matrices) I would expect .NET to loose out, the (always enforced) bounds checking on arrays is only optimized away under some circumstances.
You will probably have to benchmark something close to your intended domain.

I would consider using Mono.Simd to accelerate some operations. The minus is that on MS runtime it's not accelerated.

I haven't checked recently, but the last time I did check, Microsoft's license agreement for the .NET runtime required you to agree NOT to publish any benchmarks of its performance. That tends to limit the amount of solid information that gets published.
A few others have implied it, but I'll state it directly: I think you're engaging in (extremely) premature optimization -- or trying to anyway.
Edit:
Doing a bit of looking, the license has changed (a long time ago, in fact). The current terms
say you're allowed to publish benchmarks -- but only if you meet their conditions. Some of those conditions look (to me) nearly impossible to meet. For example, you can only publish provided: "your benchmark testing was performed using all performance tuning and best practice guidance set forth in the product documentation and/or on Microsoft's support Web sites". Given the size and number of Microsoft's web sites, I don't see how anybody stands a chance of being certain they're following all the guidance they might provide.
Although that web page talks about .NET 1.1, the newer licenses seem to refer back to it as well.
So, what I remembered was technically wrong, but effectively correct anyway.

For basic math library functions there won't be much difference because C# will call out to the same compiled code that C++ would use. For more interesting math that you won't find in the math library there are several factors that make C# worse. The Current JIT doesn't support SSE instructions that you would have access to in C++.

C++ performance vs. Java/C#

Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
My understanding is that C/C++ produces native code to run on a particular machine architecture. Conversely, languages like Java and C# run on top of a virtual machine which abstracts away the native architecture. Logically it would seem impossible for Java or C# to match the speed of C++ because of this intermediate step, however I've been told that the latest compilers ("hot spot") can attain this speed or even exceed it.
Perhaps this is more of a compiler question than a language question, but can anyone explain in plain English how it is possible for one of these virtual machine languages to perform better than a native language?

JIT vs. Static Compiler
As already said in the previous posts, JIT can compile IL/bytecode into native code at runtime. The cost of that was mentionned, but not to its conclusion:
JIT has one massive problem is that it can't compile everything: JIT compiling takes time, so the JIT will compile only some parts of the code, whereas a static compiler will produce a full native binary: For some kind of programs, the static compiler will simply easily outperform the JIT.
Of course, C# (or Java, or VB) is usually faster to produce viable and robust solution than is C++ (if only because C++ has complex semantics, and C++ standard library, while interesting and powerful, is quite poor when compared with the full scope of the standard library from .NET or Java), so usually, the difference between C++ and .NET or Java JIT won't be visible to most users, and for those binaries that are critical, well, you can still call C++ processing from C# or Java (even if this kind of native calls can be quite costly in themselves)...
C++ metaprograming
Note that usually, you are comparing C++ runtime code with its equivalent in C# or Java. But C++ has one feature that can outperform Java/C# out of the box, that is template metaprograming: The code processing will be done at compilation time (thus, increasing vastly compilation time), resulting into zero (or almost zero) runtime.
I have yet so see a real life effect on this (I played only with concepts, but by then, the difference was seconds of execution for JIT, and zero for C++), but this is worth mentioning, alongside the fact template metaprograming is not trivial...
Edit 2011-06-10: In C++, playing with types is done at compile time, meaning producing generic code which calls non-generic code (e.g. a generic parser from string to type T, calling standard library API for types T it recognizes, and making the parser easily extensible by its user) is very easy and very efficient, whereas the equivalent in Java or C# is painful at best to write, and will always be slower and resolved at runtime even when the types are known at compile time, meaning your only hope is for the JIT to inline the whole thing.
...
Edit 2011-09-20: The team behind Blitz++ (Homepage, Wikipedia) went that way, and apparently, their goal is to reach FORTRAN's performance on scientific calculations by moving as much as possible from runtime execution to compilation time, via C++ template metaprogramming. So the "I have yet so see a real life effect on this" part I wrote above apparently does exist in real life.
Native C++ Memory Usage
C++ has a memory usage different from Java/C#, and thus, has different advantages/flaws.
No matter the JIT optimization, nothing will go has fast as direct pointer access to memory (let's ignore for a moment processor caches, etc.). So, if you have contiguous data in memory, accessing it through C++ pointers (i.e. C pointers... Let's give Caesar its due) will goes times faster than in Java/C#. And C++ has RAII, which makes a lot of processing a lot easier than in C# or even in Java. C++ does not need using to scope the existence of its objects. And C++ does not have a finally clause. This is not an error.
:-)
And despite C# primitive-like structs, C++ "on the stack" objects will cost nothing at allocation and destruction, and will need no GC to work in an independent thread to do the cleaning.
As for memory fragmentation, memory allocators in 2008 are not the old memory allocators from 1980 that are usually compared with a GC: C++ allocation can't be moved in memory, true, but then, like on a Linux filesystem: Who needs hard disk defragmenting when fragmentation does not happen? Using the right allocator for the right task should be part of the C++ developer toolkit. Now, writing allocators is not easy, and then, most of us have better things to do, and for the most of use, RAII or GC is more than good enough.
Edit 2011-10-04: For examples about efficient allocators: On Windows platforms, since Vista, the Low Fragmentation Heap is enabled by default. For previous versions, the LFH can be activated by calling the WinAPI function HeapSetInformation). On other OSes, alternative allocators are provided (see https://secure.wikimedia.org/wikipedia/en/wiki/Malloc for a list)
Now, the memory model is somewhat becoming more complicated with the rise of multicore and multithreading technology. In this field, I guess .NET has the advantage, and Java, I was told, held the upper ground. It's easy for some "on the bare metal" hacker to praise his "near the machine" code. But now, it is quite more difficult to produce better assembly by hand than letting the compiler to its job. For C++, the compiler became usually better than the hacker since a decade. For C# and Java, this is even easier.
Still, the new standard C++0x will impose a simple memory model to C++ compilers, which will standardize (and thus simplify) effective multiprocessing/parallel/threading code in C++, and make optimizations easier and safer for compilers. But then, we'll see in some couple of years if its promises are held true.
C++/CLI vs. C#/VB.NET
Note: In this section, I am talking about C++/CLI, that is, the C++ hosted by .NET, not the native C++.
Last week, I had a training on .NET optimization, and discovered that the static compiler is very important anyway. As important than JIT.
The very same code compiled in C++/CLI (or its ancestor, Managed C++) could be times faster than the same code produced in C# (or VB.NET, whose compiler produces the same IL than C#).
Because the C++ static compiler was a lot better to produce already optimized code than C#'s.
For example, function inlining in .NET is limited to functions whose bytecode is less or equal than 32 bytes in length. So, some code in C# will produce a 40 bytes accessor, which won't be ever inlined by the JIT. The same code in C++/CLI will produce a 20 bytes accessor, which will be inlined by the JIT.
Another example is temporary variables, that are simply compiled away by the C++ compiler while still being mentioned in the IL produced by the C# compiler. C++ static compilation optimization will result in less code, thus authorizes a more aggressive JIT optimization, again.
The reason for this was speculated to be the fact C++/CLI compiler profited from the vast optimization techniques from C++ native compiler.
Conclusion
I love C++.
But as far as I see it, C# or Java are all in all a better bet. Not because they are faster than C++, but because when you add up their qualities, they end up being more productive, needing less training, and having more complete standard libraries than C++. And as for most of programs, their speed differences (in one way or another) will be negligible...
Edit (2011-06-06)
My experience on C#/.NET
I have now 5 months of almost exclusive professional C# coding (which adds up to my CV already full of C++ and Java, and a touch of C++/CLI).
I played with WinForms (Ahem...) and WCF (cool!), and WPF (Cool!!!! Both through XAML and raw C#. WPF is so easy I believe Swing just cannot compare to it), and C# 4.0.
The conclusion is that while it's easier/faster to produce a code that works in C#/Java than in C++, it's a lot harder to produce a strong, safe and robust code in C# (and even harder in Java) than in C++. Reasons abound, but it can be summarized by:
Generics are not as powerful as templates (try to write an efficient generic Parse method (from string to T), or an efficient equivalent of boost::lexical_cast in C# to understand the problem)
RAII remains unmatched (GC still can leak (yes, I had to handle that problem) and will only handle memory. Even C#'s using is not as easy and powerful because writing a correct Dispose implementations is difficult)
C# readonly and Java final are nowhere as useful as C++'s const (There's no way you can expose readonly complex data (a Tree of Nodes, for example) in C# without tremendous work, while it's a built-in feature of C++. Immutable data is an interesting solution, but not everything can be made immutable, so it's not even enough, by far).
So, C# remains an pleasant language as long as you want something that works, but a frustrating language the moment you want something that always and safely works.
Java is even more frustrating, as it has the same problems than C#, and more: Lacking the equivalent of C#'s using keyword, a very skilled colleague of mine spent too much time making sure its resources where correctly freed, whereas the equivalent in C++ would have been easy (using destructors and smart pointers).
So I guess C#/Java's productivity gain is visible for most code... until the day you need the code to be as perfect as possible. That day, you'll know pain. (you won't believe what's asked from our server and GUI apps...).
About Server-side Java and C++
I kept contact with the server teams (I worked 2 years among them, before getting back to the GUI team), at the other side of the building, and I learned something interesting.
Last years, the trend was to have the Java server apps be destined to replace the old C++ server apps, as Java has a lot of frameworks/tools, and is easy to maintain, deploy, etc. etc..
...Until the problem of low-latency reared its ugly head the last months. Then, the Java server apps, no matter the optimization attempted by our skilled Java team, simply and cleanly lost the race against the old, not really optimized C++ server.
Currently, the decision is to keep the Java servers for common use where performance while still important, is not concerned by the low-latency target, and aggressively optimize the already faster C++ server applications for low-latency and ultra-low-latency needs.
Conclusion
Nothing is as simple as expected.
Java, and even more C#, are cool languages, with extensive standard libraries and frameworks, where you can code fast, and have result very soon.
But when you need raw power, powerful and systematic optimizations, strong compiler support, powerful language features and absolute safety, Java and C# make it difficult to win the last missing but critical percents of quality you need to remain above the competition.
It's as if you needed less time and less experienced developers in C#/Java than in C++ to produce average quality code, but in the other hand, the moment you needed excellent to perfect quality code, it was suddenly easier and faster to get the results right in C++.
Of course, this is my own perception, perhaps limited to our specific needs.
But still, it is what happens today, both in the GUI teams and the server-side teams.
Of course, I'll update this post if something new happens.
Edit (2011-06-22)
"We find that in regards to performance, C++ wins out by
a large margin. However, it also required the most extensive
tuning efforts, many of which were done at a level of sophistication
that would not be available to the average programmer.
[...] The Java version was probably the simplest to implement, but the hardest to analyze for performance. Specifically the effects around garbage collection were complicated and very hard to tune."
Sources:
https://days2011.scala-lang.org/sites/days2011/files/ws3-1-Hundt.pdf
http://www.computing.co.uk/ctg/news/2076322/-winner-google-language-tests
Edit (2011-09-20)
"The going word at Facebook is that 'reasonably written C++ code just runs fast,' which underscores the enormous effort spent at optimizing PHP and Java code. Paradoxically, C++ code is more difficult to write than in other languages, but efficient code is a lot easier [to write in C++ than in other languages]."
– Herb Sutter at //build/, quoting Andrei Alexandrescu
Sources:
http://channel9.msdn.com/Events/BUILD/BUILD2011/TOOL-835T
http://video.ch9.ms/build/2011/slides/TOOL-835T_Sutter.pptx

Generally, C# and Java can be just as fast or faster because the JIT compiler -- a compiler that compiles your IL the first time it's executed -- can make optimizations that a C++ compiled program cannot because it can query the machine. It can determine if the machine is Intel or AMD; Pentium 4, Core Solo, or Core Duo; or if supports SSE4, etc.
A C++ program has to be compiled beforehand usually with mixed optimizations so that it runs decently well on all machines, but is not optimized as much as it could be for a single configuration (i.e. processor, instruction set, other hardware).
Additionally certain language features allow the compiler in C# and Java to make assumptions about your code that allows it to optimize certain parts away that just aren't safe for the C/C++ compiler to do. When you have access to pointers there's a lot of optimizations that just aren't safe.
Also Java and C# can do heap allocations more efficiently than C++ because the layer of abstraction between the garbage collector and your code allows it to do all of its heap compression at once (a fairly expensive operation).
Now I can't speak for Java on this next point, but I know that C# for example will actually remove methods and method calls when it knows the body of the method is empty. And it will use this kind of logic throughout your code.
So as you can see, there are lots of reasons why certain C# or Java implementations will be faster.
Now this all said, specific optimizations can be made in C++ that will blow away anything that you could do with C#, especially in the graphics realm and anytime you're close to the hardware. Pointers do wonders here.
So depending on what you're writing I would go with one or the other. But if you're writing something that isn't hardware dependent (driver, video game, etc), I wouldn't worry about the performance of C# (again can't speak about Java). It'll do just fine.
One the Java side, #Swati points out a good article:
https://www.ibm.com/developerworks/library/j-jtp09275

Whenever I talk managed vs. unmanaged performance, I like to point to the series Rico (and Raymond) did comparing C++ and C# versions of a Chinese/English dictionary. This google search will let you read for yourself, but I like Rico's summary.
So am I ashamed by my crushing defeat?
Hardly. The managed code got a very
good result for hardly any effort. To
defeat the managed Raymond had to:
Write his own file I/O stuff
Write his own string class
Write his own allocator
Write his own international mapping
Of course he used available lower
level libraries to do this, but that's
still a lot of work. Can you call
what's left an STL program? I don't
think so, I think he kept the
std::vector class which ultimately was
never a problem and he kept the find
function. Pretty much everything else
is gone.
So, yup, you can definately beat the
CLR. Raymond can make his program go
even faster I think.
Interestingly, the time to parse the
file as reported by both programs
internal timers is about the same --
30ms for each. The difference is in
the overhead.
For me the bottom line is that it took 6 revisions for the unmanaged version to beat the managed version that was a simple port of the original unmanaged code. If you need every last bit of performance (and have the time and expertise to get it), you'll have to go unmanaged, but for me, I'll take the order of magnitude advantage I have on the first versions over the 33% I gain if I try 6 times.

The compile for specific CPU optimizations are usually overrated. Just take a program in C++ and compile with optimization for pentium PRO and run on a pentium 4. Then recompile with optimize for pentium 4. I passed long afternoons doing it with several programs. General results?? Usually less than 2-3% performance increase. So the theoretical JIT advantages are almost none. Most differences of performance can only be observed when using scalar data processing features, something that will eventually need manual fine tunning to achieve maximum performance anyway. Optimizations of that sort are slow and costly to perform making them sometimes unsuitable for JIT anyway.
On real world and real application C++ is still usually faster than java, mainly because of lighter memory footprint that result in better cache performance.
But to use all of C++ capability you, the developer must work hard. You can achieve superior results, but you must use your brain for that. C++ is a language that decided to present you with more tools, charging the price that you must learn them to be able to use the language well.

JIT (Just In Time Compiling) can be incredibly fast because it optimizes for the target platform.
This means that it can take advantage of any compiler trick your CPU can support, regardless of what CPU the developer wrote the code on.
The basic concept of the .NET JIT works like this (heavily simplified):
Calling a method for the first time:
Your program code calls a method Foo()
The CLR looks at the type that implements Foo() and gets the metadata associated with it
From the metadata, the CLR knows what memory address the IL (Intermediate byte code) is stored in.
The CLR allocates a block of memory, and calls the JIT.
The JIT compiles the IL into native code, places it into the allocated memory, and then changes the function pointer in Foo()'s type metadata to point to this native code.
The native code is ran.
Calling a method for the second time:
Your program code calls a method Foo()
The CLR looks at the type that implements Foo() and finds the function pointer in the metadata.
The native code at this memory location is ran.
As you can see, the 2nd time around, its virtually the same process as C++, except with the advantage of real time optimizations.
That said, there are still other overhead issues that slow down a managed language, but the JIT helps a lot.

I like Orion Adrian's answer, but there is another aspect to it.
The same question was posed decades ago about assembly language vs. "human" languages like FORTRAN. And part of the answer is similar.
Yes, a C++ program is capable of being faster than C# on any given (non-trivial?) algorithm, but the program in C# will often be as fast or faster than a "naive" implementation in C++, and an optimized version in C++ will take longer to develop, and might still beat the C# version by a very small margin. So, is it really worth it?
You'll have to answer that question on a one-by-one basis.
That said, I'm a long time fan of C++, and I think it's an incredibly expressive and powerful language -- sometimes underappreciated. But in many "real life" problems (to me personally, that means "the kind I get paid to solve"), C# will get the job done sooner and safer.
The biggest penalty you pay? Many .NET and Java programs are memory hogs. I have seen .NET and Java apps take "hundreds" of megabytes of memory, when C++ programs of similar complexity barely scratch the "tens" of MBs.

I'm not sure how often you'll find that Java code will run faster than C++, even with Hotspot, but I'll take a swing at explaining how it could happen.
Think of compiled Java code as interpreted machine language for the JVM. When the Hotspot processor notices that certain pieces of the compiled code are going to be used many times, it performs an optimization on the machine code. Since hand-tuning Assembly is almost always faster than C++ compiled code, it's ok to figure that programmatically-tuned machine code isn't going to be too bad.
So, for highly repetitious code, I could see where it'd be possible for Hotspot JVM to run the Java faster than C++... until garbage collection comes into play. :)

Generally, your program's algorithm will be much more important to the speed of your application than the language. You can implement a poor algorithm in any language, including C++. With that in mind, you'll generally be able to write code the runs faster in a language that helps you implement a more efficient algorithm.
Higher-level languages do very well at this by providing easier access to many efficient pre-built data structures and encouraging practices that will help you avoid inefficient code. Of course, they can at times also make it easy to write a bunch of really slow code, too, so you still have to know your platform.
Also, C++ is catching up with "new" (note the quotes) features like the STL containers, auto pointers, etc -- see the boost library, for example. And you might occasionally find that the fastest way to accomplish some task requires a technique like pointer arithmetic that's forbidden in a higher-level language -- though they typcially allow you to call out to a library written in a language that can implement it as desired.
The main thing is to know the language you're using, it's associated API, what it can do, and what it's limitations are.

I don't know either...my Java programs are always slow. :-) I've never really noticed C# programs being particularly slow, though.

Here is another intersting benchmark, which you can try yourself on your own computer.
It compares ASM, VC++, C#, Silverlight, Java applet, Javascript, Flash (AS3)
Roozz plugin speed demo
Please note that the speed of javascript varries a lot depending on what browser is executing it. The same is true for Flash and Silverlight because these plugins run in the same process as the hosting browser. But the Roozz plugin run standard .exe files, which run in their own process, thus the speed is not influenced by the hosting browser.

You should define "perform better than..". Well, I know, you asked about speed, but its not everything that counts.
Do virtual machines perform more runtime overhead? Yes!
Do they eat more working memory? Yes!
Do they have higher startup costs (runtime initialization and JIT compiler) ? Yes!
Do they require a huge library installed? Yes!
And so on, its biased, yes ;)
With C# and Java you pay a price for what you get (faster coding, automatic memory management, big library and so on). But you have not much room to haggle about the details: take the complete package or nothing.
Even if those languages can optimize some code to execute faster than compiled code, the whole approach is (IMHO) inefficient. Imagine driving every day 5 miles to your workplace, with a truck! Its comfortable, it feels good, you are safe (extreme crumple zone) and after you step on the gas for some time, it will even be as fast as a standard car! Why don't we all have a truck to drive to work? ;)
In C++ you get what you pay for, not more, not less.
Quoting Bjarne Stroustrup: "C++ is my favorite garbage collected language because it generates so little garbage"
link text

The executable code produced from a Java or C# compiler is not interpretted -- it is compiled to native code "just in time" (JIT). So, the first time code in a Java/C# program is encountered during execution, there is some overhead as the "runtime compiler" (aka JIT compiler) turns the byte code (Java) or IL code (C#) into native machine instructions. However, the next time that code is encountered while the application is still running, the native code is executed immediately. This explains how some Java/C# programs appear to be slow initially, but then perform better the longer they run. A good example is an ASP.Net web site. The very first time the web site is accessed, it may be a bit slower as the C# code is compiled to native code by the JIT compiler. Subsequent accesses result in a much faster web site -- server and client side caching aside.

Some good answers here about the specific question you asked. I'd like to step back and look at the bigger picture.
Keep in mind that your user's perception of the speed of the software you write is affected by many other factors than just how well the codegen optimizes. Here are some examples:
Manual memory management is hard to do correctly (no leaks), and even harder to do effeciently (free memory soon after you're done with it). Using a GC is, in general, more likely to produce a program that manages memory well. Are you willing to work very hard, and delay delivering your software, in an attempt to out-do the GC?
My C# is easier to read & understand than my C++. I also have more ways to convince myself that my C# code is working correctly. That means I can optimize my algorithms with less risk of introducing bugs (and users don't like software that crashes, even if it does it quickly!)
I can create my software faster in C# than in C++. That frees up time to work on performance, and still deliver my software on time.
It's easier to write good UI in C# than C++, so I'm more likely to be able to push work to the background while UI stays responsive, or to provide progress or hearbeat UI when the program has to block for a while. This doesn't make anything faster, but it makes users happier about waiting.
Everything I said about C# is probably true for Java, I just don't have the experience to say for sure.

If you're a Java/C# programmer learning C++, you'll be tempted to keep thinking in terms of Java/C# and translate verbatim to C++ syntax. In that case, you only get the earlier mentioned benefits of native code vs. interpreted/JIT. To get the biggest performance gain in C++ vs. Java/C#, you have to learn to think in C++ and design code specifically to exploit the strengths of C++.
To paraphrase Edsger Dijkstra: [your first language] mutilates the mind beyond recovery.
To paraphrase Jeff Atwood: you can write [your first language] in any new language.

One of the most significant JIT optimizations is method inlining. Java can even inline virtual methods if it can guarantee runtime correctness. This kind of optimization usually cannot be performed by standard static compilers because it needs whole-program analysis, which is hard because of separate compilation (in contrast, JIT has all the program available to it). Method inlining improves other optimizations, giving larger code blocks to optimize.
Standard memory allocation in Java/C# is also faster, and deallocation (GC) is not much slower, but only less deterministic.

The virtual machine languages are unlikely to outperform compiled languages but they can get close enough that it doesn't matter, for (at least) the following reasons (I'm speaking for Java here since I've never done C#).
1/ The Java Runtime Environment is usually able to detect pieces of code that are run frequently and perform just-in-time (JIT) compilation of those sections so that, in future, they run at the full compiled speed.
2/ Vast portions of the Java libraries are compiled so that, when you call a library function, you're executing compiled code, not interpreted. You can see the code (in C) by downloading the OpenJDK.
3/ Unless you're doing massive calculations, much of the time your program is running, it's waiting for input from a very slow (relatively speaking) human.
4/ Since a lot of the validation of Java bytecode is done at the time of loading the class, the normal overhead of runtime checks is greatly reduced.
5/ At the worst case, performance-intensive code can be extracted to a compiled module and called from Java (see JNI) so that it runs at full speed.
In summary, the Java bytecode will never outperform native machine language, but there are ways to mitigate this. The big advantage of Java (as I see it) is the HUGE standard library and the cross-platform nature.

Orion Adrian, let me invert your post to see how unfounded your remarks are, because a lot can be said about C++ as well. And telling that Java/C# compiler optimize away empty functions does really make you sound like you are not my expert in optimization, because a) why should a real program contain empty functions, except for really bad legacy code, b) that is really not black and bleeding edge optimization.
Apart from that phrase, you ranted blatantly about pointers, but don't objects in Java and C# basically work like C++ pointers? May they not overlap? May they not be null? C (and most C++ implementations) has the restrict keyword, both have value types, C++ has reference-to-value with non-null guarantee. What do Java and C# offer?
>>>>>>>>>>
Generally, C and C++ can be just as fast or faster because the AOT compiler -- a compiler that compiles your code before deployment, once and for all, on your high memory many core build server -- can make optimizations that a C# compiled program cannot because it has a ton of time to do so. The compiler can determine if the machine is Intel or AMD; Pentium 4, Core Solo, or Core Duo; or if supports SSE4, etc, and if your compiler does not support runtime dispatch, you can solve for that yourself by deploying a handful of specialized binaries.
A C# program is commonly compiled upon running it so that it runs decently well on all machines, but is not optimized as much as it could be for a single configuration (i.e. processor, instruction set, other hardware), and it must spend some time first. Features like loop fission, loop inversion, automatic vectorization, whole program optimization, template expansion, IPO, and many more, are very hard to be solved all and completely in a way that does not annoy the end user.
Additionally certain language features allow the compiler in C++ or C to make assumptions about your code that allows it to optimize certain parts away that just aren't safe for the Java/C# compiler to do. When you don't have access to the full type id of generics or a guaranteed program flow there's a lot of optimizations that just aren't safe.
Also C++ and C do many stack allocations at once with just one register incrementation, which surely is more efficient than Javas and C# allocations as for the layer of abstraction between the garbage collector and your code.
Now I can't speak for Java on this next point, but I know that C++ compilers for example will actually remove methods and method calls when it knows the body of the method is empty, it will eliminate common subexpressions, it may try and retry to find optimal register usage, it does not enforce bounds checking, it will autovectorize loops and inner loops and will invert inner to outer, it moves conditionals out of loops, it splits and unsplits loops. It will expand std::vector into native zero overhead arrays as you'd do the C way. It will do inter procedural optimmizations. It will construct return values directly at the caller site. It will fold and propagate expressions. It will reorder data into a cache friendly manner. It will do jump threading. It lets you write compile time ray tracers with zero runtime overhead. It will make very expensive graph based optimizations. It will do strength reduction, were it replaces certain codes with syntactically totally unequal but semantically equivalent code (the old "xor foo, foo" is just the simplest, though outdated optimization of such kind). If you kindly ask it, you may omit IEEE floating point standards and enable even more optimizations like floating point operand re-ordering. After it has massaged and massacred your code, it might repeat the whole process, because often, certain optimizations lay the foundation for even certainer optimizations. It might also just retry with shuffled parameters and see how the other variant scores in its internal ranking. And it will use this kind of logic throughout your code.
So as you can see, there are lots of reasons why certain C++ or C implementations will be faster.
Now this all said, many optimizations can be made in C++ that will blow away anything that you could do with C#, especially in the number crunching, realtime and close-to-metal realm, but not exclusively there. You don't even have to touch a single pointer to come a long way.
So depending on what you're writing I would go with one or the other. But if you're writing something that isn't hardware dependent (driver, video game, etc), I wouldn't worry about the performance of C# (again can't speak about Java). It'll do just fine.
<<<<<<<<<<
Generally, certain generalized arguments might sound cool in specific posts, but don't generally sound certainly credible.
Anyways, to make peace: AOT is great, as is JIT. The only correct answer can be: It depends. And the real smart people know that you can use the best of both worlds anyways.

It would only happen if the Java interpreter is producing machine code that is actually better optimized than the machine code your compiler is generating for the C++ code you are writing, to the point where the C++ code is slower than the Java and the interpretation cost.
However, the odds of that actually happening are pretty low - unless perhaps Java has a very well-written library, and you have your own poorly written C++ library.

Actually, C# does not really run in a virtual machine like Java does. IL is compiled into assembly language, which is entirely native code and runs at the same speed as native code. You can pre-JIT an .NET application which entirely removes the JIT cost and then you are running entirely native code.
The slowdown with .NET will come not because .NET code is slower, but because it does a lot more behind the scenes to do things like garbage collect, check references, store complete stack frames, etc. This can be quite powerful and helpful when building applications, but also comes at a cost. Note that you could do all these things in a C++ program as well (much of the core .NET functionality is actually .NET code which you can view in ROTOR). However, if you hand wrote the same functionality you would probably end up with a much slower program since the .NET runtime has been optimized and finely tuned.
That said, one of the strengths of managed code is that it can be fully verifiable, ie. you can verify that the code will never access another processes's memory or do unsage things before you execute it. Microsoft has a research prototype of a fully managed operating system that has suprisingly shown that a 100% managed environment can actually perform significantly faster than any modern operating system by taking advantage of this verification to turn off security features that are no longer needed by managed programs (we are talking like 10x in some cases). SE radio has a great episode talking about this project.

In some cases, managed code can actually be faster than native code. For instance, "mark-and-sweep" garbage collection algorithms allow environments like the JRE or CLR to free large numbers of short-lived (usually) objects in a single pass, where most C/C++ heap objects are freed one-at-a-time.
From wikipedia:
For many practical purposes, allocation/deallocation-intensive algorithms implemented in garbage collected languages can actually be faster than their equivalents using manual heap allocation. A major reason for this is that the garbage collector allows the runtime system to amortize allocation and deallocation operations in a potentially advantageous fashion.
That said, I've written a lot of C# and a lot of C++, and I've run a lot of benchmarks. In my experience, C++ is a lot faster than C#, in two ways: (1) if you take some code that you've written in C#, port it to C++ the native code tends to be faster. How much faster? Well, it varies a whole lot, but it's not uncommon to see a 100% speed improvement. (2) In some cases, garbage collection can massively slow down a managed application. The .NET CLR does a terrible job with large heaps (say, > 2GB), and can end up spending a lot of time in GC--even in applications that have few--or even no--objects of intermediate life spans.
Of course, in most cases that I've encounted, managed languages are fast enough, by a long shot, and the maintenance and coding tradeoff for the extra performance of C++ is simply not a good one.

Here's an interesting benchmark
http://zi.fi/shootout/

Actually Sun's HotSpot JVM uses "mixed-mode" execution. It interprets the method's bytecode until it determines (usually through a counter of some sort) that a particular block of code (method, loop, try-catch block, etc.) is going to be executed a lot, then it JIT compiles it. The time required to JIT compile a method often takes longer than if the method were to be interpreted if it is a seldom run method. Performance is usually higher for "mixed-mode" because the JVM does not waste time JITing code that is rarely, if ever, run.
C# and .NET do not do this. .NET JITs everything which, often times, wastes time.

Go read about HP Labs' Dynamo, an interpreter for PA-8000 that runs on PA-8000, and often runs programs faster than they do natively. Then it won't seem at all surprising!
Don't think of it as an "intermediate step" -- running a program involves lots of other steps already, in any language.
It often comes down to:
programs have hot-spots, so even if you're slower running 95% of the body of code you have to run, you can still be performance-competitive if you're faster at the hot 5%
a HLL knows more about your intent than a LLL like C/C++, and so can generate more optimized code (OCaml has even more, and in practice is often even faster)
a JIT compiler has a lot of information that a static compiler doesn't (like, the actual data you happen to have this time)
a JIT compiler can do optimizations at run-time that traditional linkers aren't really allowed to do (like reordering branches so the common case is flat, or inlining library calls)
All in all, C/C++ are pretty lousy languages for performance: there's relatively little information about your data types, no information about your data, and no dynamic runtime to allow much in the way of run-time optimization.

You might get short bursts when Java or CLR is faster than C++, but overall the performance is worse for the life of the application:
see www.codeproject.com/KB/dotnet/RuntimePerformance.aspx for some results for that.

Here is answer from Cliff Click: http://www.azulsystems.com/blog/cliff/2009-09-06-java-vs-c-performanceagain

My understanding is that C/C++ produces native code to run on a particular machine architecture. Conversely, languages like Java and C# run on top of a virtual machine which abstracts away the native architecture. Logically it would seem impossible for Java or C# to match the speed of C++ because of this intermediate step, however I've been told that the latest compilers ("hot spot") can attain this speed or even exceed it.
That is illogical. The use of an intermediate representation does not inherently degrade performance. For example, llvm-gcc compiles C and C++ via LLVM IR (which is a virtual infinite-register machine) to native code and it achieves excellent performance (often beating GCC).
Perhaps this is more of a compiler question than a language question, but can anyone explain in plain English how it is possible for one of these virtual machine languages to perform better than a native language?
Here are some examples:
Virtual machines with JIT compilation facilitate run-time code generation (e.g. System.Reflection.Emit on .NET) so you can compile generated code on-the-fly in languages like C# and F# but must resort to writing a comparatively-slow interpreter in C or C++. For example, to implement regular expressions.
Parts of the virtual machine (e.g. the write barrier and allocator) are often written in hand-coded assembler because C and C++ do not generate fast enough code. If a program stresses these parts of a system then it could conceivably outperform anything that can be written in C or C++.
Dynamic linking of native code requires conformance to an ABI that can impede performance and obviates whole-program optimization whereas linking is typically deferred on VMs and can benefit from whole-program optimizations (like .NET's reified generics).
I'd also like to address some issues with paercebal's highly-upvoted answer above (because someone keeps deleting my comments on his answer) that presents a counter-productively polarized view:
The code processing will be done at compilation time...
Hence template metaprogramming only works if the program is available at compile time which is often not the case, e.g. it is impossible to write a competitively performant regular expression library in vanilla C++ because it is incapable of run-time code generation (an important aspect of metaprogramming).
...playing with types is done at compile time...the equivalent in Java or C# is painful at best to write, and will always be slower and resolved at runtime even when the types are known at compile time.
In C#, that is only true of reference types and is not true for value types.
No matter the JIT optimization, nothing will go has fast as direct pointer access to memory...if you have contiguous data in memory, accessing it through C++ pointers (i.e. C pointers... Let's give Caesar its due) will goes times faster than in Java/C#.
People have observed Java beating C++ on the SOR test from the SciMark2 benchmark precisely because pointers impede aliasing-related optimizations.
Also worth noting that .NET does type specialization of generics across dynamically-linked libraries after linking whereas C++ cannot because templates must be resolved before linking. And obviously the big advantage generics have over templates is comprehensible error messages.

On top of what some others have said, from my understanding .NET and Java are better at memory allocation. E.g. they can compact memory as it gets fragmented while C++ cannot (natively, but it can if you're using a clever garbage collector).

For anything needing lots of speed, the JVM just calls a C++ implementation, so it's a question more of how good their libs are than how good the JVM is for most OS related things.
Garbage collection cuts your memory in half, but using some of the fancier STL and Boost features will have the same effect but with many times the bug potential.
If you are just using C++ libraries and lots of its high level features in a large project with many classes you will probably wind up slower than using a JVM. Except much more error prone.
However, the benefit of C++ is that it allows you to optimize yourself, otherwise you are stuck with what the compiler/jvm does. If you make your own containers, write your own memory management that's aligned, use SIMD, and drop to assembly here and there, you can speed up at least 2x-4x times over what most C++ compilers will do on their own. For some operations, 16x-32x. That's using the same algorithms, if you use better algorithms and parallelize, increases can be dramatic, sometimes thousands of times faster that commonly used methods.

I look at it from a few different points.
Given infinite time and resources, will managed or unmanaged code be faster? Clearly, the answer is that unmanaged code can always at least tie managed code in this aspect - as in the worst case, you'd just hard-code the managed code solution.
If you take a program in one language, and directly translate it to another, how much worse will it perform? Probably a lot, for any two languages. Most languages require different optimizations and have different gotchas. Micro-performance is often a lot about knowing these details.
Given finite time and resources, which of two languages will produce a better result? This is the most interesting question, as while a managed language may produce slightly slower code (given a program reasonably written for that language), that version will likely be done sooner, allowing for more time spent on optimization.

A very short answer: Given a fixed budget you will achieve better performing java application than a C++ application (ROI considerations) In addition Java platform has more decent profilers, that will help you pinpoint your hotspots more quickly

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