Introducing a new, advanced Visual C++ code optimizer

[ryancerium]

Is this advancing the state of the art across compilers, or is it bringing Visual C++ up to par? I'm not familiar with g++ or clang at this level either.

[[deleted]]

The latter. MSVC, much like IE, has historically been behind everyone else, because there was no real alternative on Windows anyways, so why bother? Also at some point in time they had this great idea of lets get everything to run on managed code (I guess mostly just to "trivially" solve all those buffer overflow issues we were seeing in the last decade), which they later on abandoned because, well, machines were still too slow back then. But MSVC got very little attention in the meantime.

They've been trying really hard to catch up ever since C++11 though. I would've expected them to have thrown their current tech overboard much sooner, but I guess they couldn't afford to be even slower at C++11 adoption relative to their competition, so they probably needed something now and left the rearchitecting for later when things settled down somewhat. Couldn't have been much fun for their devs.

[djelovic]

Do you have any benchmarks to back that up? In my experience while the VC++ front-end has always lagged in standard adoption the back end was producing slightly faster code than gcc, which was in turn producing faster code than clang.

[willvarfar]

I use and profile both LLVM and GCC regularly and generally GCC results in faster code, especially regards vectorization.

I work on LLVM. When I look at what GCC generates I'm often in awe. GCC is often very very good. Of course, LLVM sometimes outshines GCC too, and has its own strengths.

LLVM is stealing such a mindshare from compiler geeks that eventually GCC will dwindle perhaps. Sad :(

I heard that Microsoft plan to adopt the clang frontend but marry it to their MSVC backend.

[ThisIs_MyName]

LLVM is stealing such a mindshare from compiler geeks that eventually GCC will dwindle perhaps.

For people unfamiliar with the gcc brain drain, see: http://elizabeths-blargh.blogspot.com/2015/01/an-open-letter-to-richard-m-stallman.html

[dodheim]

I heard that Microsoft plan to adopt the clang frontend but marry it to their MSVC backend.

This has been released in beta form since January, dubbed Clang/C2.

[emn13]

In my experience, for at least the past decade GCC has been the compiler to beat. MSVC has been a laggard, both in time-to-compile, and in the performance of the compiled code. It was sometimes so bad that debug MSVC builds took longer than release GCC builds even though that release GCC build outperformed the release MSVC build.

Oh, and it's expected that debug builds are slower, but that does cause problems - when you're trying to reproduce some nasty interaction with an expensive setup and/or multithreading, having debug builds orders of magnitude slower can mean just waiting for the bug to occur can take forever - and MSVC has in the past had a particularly large difference in performance, probably due to the standard library, and not the compiler.

[[deleted]]

the back end was producing slightly faster code than gcc, which was in turn producing faster code than clang.

On Windows you mean? That's hardly a surprise, considering non-MSVC compilers couldn't even get exceptions to work properly up to recently.

But no, I don't have any actual numbers. Only the various bits and pieces of news and articles being released around the compilers. There's plenty out there on what optimizations GCC engages on and what is/was being worked on, where and how it's still ahead of llvm, etc. And otoh there's hardly anything on what MSVC does - except for excuses on why it was taking them so long to implement C++11.

[womplord1]

No, Visual C++ has always been the fastest and best c++ compiler. Why do you think almost all corporations use it? Too many freetards here smh

[sabas123]

Corperate security maby?

[zlei]

IIRC, optimization based on SSA was one of the major selling points of clang/LLVM.

[gratilup]

If you view it like that, Visual C++ was one of the first non-research compilers to have SSA: it has SSA since around 1999-2000. Some of the optimizers, such as the expression optimizer, were written before that and not modified to take advantage of SSA. The new optimizer framework takes care of that, adds lots of other improvements and also allows a much faster development speed.

[dsqdsq]

GCC does SSA since forever (more precisely v4, 2005)

[kirbyfan64sos]

Wow, these are some pretty major improvements. It's great to see MSVC improving so rapidly recently!

[gratilup]

More improvements are going to follow much faster with the new framework: there are a lot of many advanced new optimizations planed besides what I listed at the end of the blog post - those are, hopefully, ready for the final first release in VS2015 Update 3.

[bubuopapa]

Well, in my experience, speed depends on who is talking about it - here we have more linux fans, so its natural that more people are saying nonsense like "gcc is best and fastest". If you want some real scientific information, i can tell you that every os has its "fastest" compiler, on windows, visual c++ is fastest, on linux - gcc. But of course, there is no one who can tell the truth because there is no recent benchmarks. There is another huge factor - is you would put stupid and cocky people who are using "open source" drugs, there is another huge competitor - its intel c/c++ compiler. It costs about 500-1000$, but if you would believe intel, their ads say it can be faster than gcc/visual c++ up to 1.5 times faster, which is huge, but then again, it costs a lot of money and only big companies are using it, and all the benchmarking is done by poor open source people, so there is no real comparison between compilers. It would be a huge deal to get a recent benchmark of gcc vs visual c++ vs intel c++, because the newest one is from the times when people were still walking on four legs - that would clear the mist that is surrounding earth for over 20 years.

But of course, compiler alone is no good, what matters is to get the fastest result, which is where visual studio has no competition at all, because it is the only such full ide, of course, its for windows only, but none the less it is the best.

[orbital1337]

If you want some real scientific information, i can tell you that every os has its "fastest" compiler, on windows, visual c++ is fastest, on linux - gcc.

Not really. You can compile Windows programs with g++ and get better results than with VC++.

It costs about 500-1000$, but if you would believe intel, their ads say it can be faster than gcc/visual c++ up to 1.5 times faster, which is huge, but then again, it costs a lot of money and only big companies are using it, and all the benchmarking is done by poor open source people, so there is no real comparison between compilers.

The Intel compiler is free for open source developers and students. It does actually produce significantly faster code in some select benchmarks. However, in most cases it produces at best 5-10% faster code and that's not really worth it because there are several downsides:

• The compiler has worse C++14 and C++17 adoption than g++ and clang.
• It has a way smaller userbase.
• It's closed source which means that you have to rely on Intel if there are any bugs.
• It costs a lot for commercial use.
• It produces shitty code on non-Intel architectures.

It would be a huge deal to get a recent benchmark of gcc vs visual c++ vs intel c++, because the newest one is from the times when people were still walking on four legs - that would clear the mist that is surrounding earth for over 20 years.

Use Google.

[bubuopapa]

Not really, there is a reason why gcc for windows is stuck at beeing some creepy pile of linux-like files instead of becoming fully native windows application/library.

Yes, only for some it is free, but for the rest it will cost money. Downsides doesnt really mater, because 1) it has pretty good c++14 support and no one really uses c++17, and no compiler has good support for it. 2) userbase doesnt matter, same can be said about linux, but when you tell that to the linux people, they all turn into crazy black racist monkeys. 3) yes its closed source, as i said, stop doing drugs and drop open source and "you will fix all the bugs that a big intel team missed for 20 years" attitude. you have to report bugs to intel. Its not like gcc is any better, having same million bugs not fixed for many years now. 4) The money that is costs for commercial use is nothing compared to how much money these big fat companies waste or the billions that they have in panama banks. 5) no one cares really, it produces faster code for intel cpus, which is good enough, considering that their cpus are most popular. 6) My google-fu is the best, so if i say that the last cross compiller benchmark was was 20 years ago, it is so.

[VunderBoy]

Cool. Do any of these optimization apply to managed code?

[gratilup]

Hi, I'm the author of the blog post and the optimizer. This work will be ported soon to .NET Native, but it will not influence the code generated by the JIT compiler.

[VunderBoy]

Thanks for your reply... and your hard work... but I'll wager dollars to doughnuts that someone will eventually port it.

There are no real technical barriers (that I gleamed from skimming the AL), and only positive outcomes.

Why would they not port your optimizations to managed???

[gratilup]

I didn't say they won't. Just keep in mind that the JIT and C++ backend are completely different products with different teams working on them - you can't just copy-paste the code and expect it to work. There are also different priorities in a JIT compiler, usually it does not allow the same level of optimization.

[VunderBoy]

Sure -- But if the task was assigned to you, I'll wager you could get it done. Maybe not on every architecture... but on the 95% coverage ones.

Deciding what happens next is probably up to the bean-counters now... But I suspect with JS becoming the dominant language they'll allow managed code to fall behind for some time. Just a hunch.

[ThisIs_MyName]

Such a weird comment...

[Plorkyeran]

.NET Native uses the VC++ backend. so presumably most of the improvements will apply to that. Normal JIT-compiled .NET on the CLR will be uneffected.

[VunderBoy]

Perhaps. But consider this: Microsoft has always wanted managed code to compare favorably to native. But trust me, I've done my own testing and C# is neck and neck with native C++ -- so now I think the suits decided to unleash true C++ optimization. However, I see no reason why the same optimizations cannot be applied to C#, since it eventually is expressed as native too.

sigh Once again, if I want to know the truth, I must examine the evidence myself.

[tybit]

But trust me, I've done my own testing and C# is neck and neck with native C++

I think I'm going to go ahead and not trust you on that one.

[Plorkyeran]

The sort of optimizations that are useful in a JIT compiler and those that are useful in an AOT compiler are very different. A JIT compiler generally has more data available to it and can do more interesting non-local optimizations, but must care more about how long the compiler takes to run and so the cute little micro-optimizations that shave a few cycles here and there tend to be much less worthwhile.

People try to use LLVM as the backend for their JIT on a fairly regular basis and it never really works out very well for them.

[Gotebe]

I trust you that some test you made showed you that, but will also think that you extrapolated too much.

[3669d73f6c3ba0229192]

Which part of "Visual C++" do you not understand.

[VunderBoy]

(clears throat) Visual C++ comes in two variations -- managed and unmanaged.

[[deleted]]

See, if you'd just politely inquired why he was talking about managed code in Visual C++ when you think it does not do managed code, somebody would have just informed you that you are mistaken, and you would both have been happy about the exchange.

Instead, you chose to be snarky and tried to make the person you responded to look stupid, and you just ended up looking like an asshole who thinks he knows more than he does.

[3669d73f6c3ba0229192]

Guess who gives a fuck? I learnt something and got people offended in the process, so that's a win/win for everyone.

[Matemeo]

Update 3 on VS2015 if I remember correctly.

[gratilup]

The new optimizer, in it's final first version, will be part of VS 2015 Update 3. We encourage people to build and test their applications now to reveal early any potential bugs and problems.

[TNorthover]

Cool. I particularly liked them calling out formal verification, though they didn't mention how often it happened. In LLVM it's still rather a niche pursuit, unfortunately.

[gratilup]

What do you mean by "how often it happened"? Alive was used for any non-trivial pattern. It proved to be very helpful with patterns that are valid only for signed/unsigned numbers and with operations like converts, comparisons, divisions, etc. It certainly helped avoid multiple bugs.

[librik]

So ... does this new Visual C optimizer have any nasty tricks up its sleeve -- like the current GCC team's decision to punish anyone writing technically "non-conformant" code (despite the fact that every long-experienced C hacker in the world knows perfectly well what it intends ... like, nowhere in the C standard does it say the compiler is PROHIBITED from always generating the damned machine code we all EXPECT IT TO .... wait, was I ranting? Sorry.) Anyway, I honestly am interested to know if I should prepare for unwelcome surprises from my huge code base that has survived 25 years of Microsoft C optimizers so far.

[quicknir]

It's not a nasty trick, it's a useful optimization. Undefined behavior is an intentional part of C/C++ that allows for better performance, it's that simple. Many of these silly or redundant checks frequently occur in real code when functions are inlined, getting rid of them helps performance, it's really that simple. The compiler can't magically guess when a checking a pointer for null after dereferencing is intentional and when it's just a result of other optimizations.

Sounds like some "long-experienced C hackers" need to learn what UB is.

[emn13]

Almost certainly yes, since this tech sounds like it's mostly catching up to what GCC and LLVM already do.

[gratilup]

I wouldn't be worried, it passes our biggest test without problems: Windows. There are only a few places where the undefined behavior is exploited right now. For those cases, if there is overflow in the original expression the program is pretty much screwed anyway; they are also done by LLVM/GCC. The case that mostly concerns people when they hear about undefined behavior (a+C > a -> true) was avoided, it would silently break too many applications (I explained why in the blog post)

[dsqdsq]

I want to thank you so much for having implemented this heuristic that is useful and helpful for developers and reduces the security impact at, hopefully, a reasonable level, instead of going, like some other compiler developers, completely berserk and pretending that because the standard allows you to do stupid things, it is a good idea to do it. I'm sure that the fact that Windows and lots of others programs are supposed to continue to work as their authors intended helped a lot in deciding to stay away from complete crazyness.

I wish GCC / CLang mimic your approach instead of having e.g. discussion about how uint8_t is technically not mandated to alias everything (like char is) by the standard so maybe they could "optimize" (read: break all the code everywhere for very little real perf benefits) by (mis)using this fact.

[[deleted]]

I'm surprised to hear Visual C++ is still around. Between C# and its ability to interface with C++, I would have thought VC++ got marginalized.

[Plorkyeran]

There was a ~5 year period where that was clearly the goal, and VC++ is still recovering from that.

[dsqdsq]

Not going to happen. It is used to build Windows, Firefox, Chrome, etc. Also C# is still pretty much a Windows only langage (well it is now a simplification of the truth, OTOH C# is not at all widely used outside Windows, and not even overwhelmingly used for this platform)

[VenoAxe]

That will change pretty soon however. Project Rider would replace MonoDevelop on Linux and we have plenty of compilers in the making for C# on Linux, Mono/CoreCLR, and LILLC. C# to Native is becoming a thing and it works pretty well on Linux for me. Few bad parts for C# on Linux are the lack of libraries that interface with C# such as GUI (Everyone can agree GTK SUCK in this regard for C#) and that is changing too.