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Comment by aDevilInMe | Hacker News Reader

Does Andrei have a sticky D button on his keyboard? He seems to mention it in every other sentence, where as I thought this post was about C and C++.

I would ask if there are any improvements between using wrap or just using clang.

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Warp is written in D, by the guy who created the language. It doesn't seem weird that Andrei would want to talk about that (especially considering the fact that I assume he's very much interested in seeing D in use in more places).

I had the same question about how it compares to clang though. I suppose now that it's open source, someone can do some testing.

" I assume he's very much interested in seeing D in use in more places"

Yes, I think you are correct and this is what I was getting at. It seems like a propaganda exercise to promote D, you only have to look at the end of the article to see this "And join the D language community for the D Conference 2014 on May 21-23 in Menlo Park, CA."

For the record I am very much aware of who Andrei is and the influences he has on certain languages.

What's wrong with that?

For me is propaganda, too!

I would be shocked if there are improvements over clang (and if there were, clang would fix them) in the general case.

clang has a pretty optimized preprocessor (It even has things like using SSE2 to do some neat things with character processing).

GCC has a fairly modern one, but it's still beatable (and clang beats it handily).

Well, as I mentioned on a parallel thread, I know of a project taking around 3 hours to compile, that doesn't look very optimized to me.

You know of a project that takes 3 hours of time to preprocess with clang?

I have serious doubts. Overall compilation time is kind of irrelevant to this discussion, because Warp is just a preprocessor.

While you can get some X speedup to gcc by replacing the preprocessor, X, as a factor of overall compilation time, is usually 0.2-0.5 in most cases, depending on size of file.

I expect the gains warp gets over gcc overall from preprocessing to be similar to those clang gets over gcc overall from preprocessing.

(Though it depends on the size of files being compiled, etc).

Most companies that want actual fast overall compilation and have the resources, build caching distributed compilation infrastructure (Google, Facebook).

As mentioned, if warp is really that much faster than clang's preprocessor that it mattered, clang would be fixed :)

Faire enough, I meant full build times.

What's stated doesn't say anything about optimization. It could be that it takes a long time to run processes that take a long time.

What is this even supposed to mean? Is the majority of that time spent preprocessing? How do other compilers perform? Your anecdote alone is absolutely useless.

Feel free to try it and see for yourself! Post results here.

Don't take this the wrong way, but you guys are the ones making performance claims that it's faster!

I would expect you guys to post the numbers comparing it to the two most used things out there :)

I don't post numbers anymore because I'd always wind up in arguments with people who simply didn't believe them, or thought I'd unfairly manipulated them, or cherry-picked the test cases, whatever. Hence I encourage you to run your own numbers.

Fair enough.

I had some trouble using the ubuntu 13 packages for gdc, so i downloaded it from the gdc project binaries as of the latest available there, as recommended by the readme.

Using that to compile warp with gdc with the flags it suggests (-release is not recognized by gdc, -O3 is), i get a warp that works.

For including every file in /usr/include/boost/*.hpp in one .cc file (which produces roughly 16 megabytes of C++ code), we get:

[dannyb@mainserver 12:40:56] ~ :) $ time gcc -E e.cc >f

  In file included from e.cc:101:0:
  /usr/include/boost/spirit.hpp:18:4: warning: #warning "This header is deprecated. Please use: boost/spirit/include/classic.hpp" [-Wcpp]
   #  warning "This header is deprecated. Please use: boost/spirit/include/classic.hpp"
    ^

gcc -E e.cc > f 3.18s user 0.25s system 97% cpu 3.528 total

[dannyb@mainserver 12:40:51] ~ :) $ time clang -E e.cc >f

  In file included from e.cc:101:
  /usr/include/boost/spirit.hpp:18:4: warning: "This header is deprecated. Please use: boost/spirit/include/classic.hpp" [-W#warnings]
  #  warning "This header is deprecated. Please use: boost/spirit/include/classic.hpp"
     ^
  1 warning generated.

clang -E e.cc > f 1.42s user 0.14s system 93% cpu 1.657 total

[dannyb@mainserver 12:40:33] ~ :( $ time ./warp/fwarpdrive_gcc4_8_1 -I/usr/include -I/usr/include/c++/4.8 -I/usr/include/x86_64-linux-gnu/c++/4.8 -I/usr/include/x86_64-linux-gnu -I/usr/lib/gcc/x86_64-linux-gnu/4.8/include/ -I/usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed/ e.cc >f

  cla/usr/include/boost/spirit.hpp(18) : warning: "This header is deprecated. Please use: boost/spirit/include/classic.hpp"

./warp/fwarpdrive_gcc4_8_1 -I/usr/include -I/usr/include/c++/4.8 e.cc 2.88s user 0.06s system 95% cpu 3.080 total

I've repeated these timings 10 times, and they are within 0.5% of these numbers each time.

I've also tried this on a large C++ project i have, that generates about 200 meg of preprocessed source (that i can't share, sadly) and got similar relative timings. I also tried it on some smaller projects. Based on data i have so far, clang blows warp out of the water by a factor of 2 in most cases i've tried it.

The above tests include stdout IO, but the relative numbers are the same without it:

[dannyb@mainserver 12:48:24] ~ :( $ time gcc -E e.cc -o f

  In file included from e.cc:101:0:
  /usr/include/boost/spirit.hpp:18:4: warning: #warning "This header is deprecated. Please use: boost/spirit/include/classic.hpp" [-Wcpp]
  #  warning "This header is deprecated. Please use: boost/spirit/include/classic.hpp"
    ^

gcc -E e.cc -o f 3.14s user 0.27s system 99% cpu 3.418 total

[dannyb@mainserver 12:48:33] ~ :) $ time clang -E e.cc -o f

  In file included from e.cc:101:
  /usr/include/boost/spirit.hpp:18:4: warning: "This header is deprecated. Please use: boost/spirit/include/classic.hpp" [-W#warnings]
  #  warning "This header is deprecated. Please use: boost/spirit/include/classic.hpp"
     ^
  1 warning generated.

clang -E e.cc -o f 1.41s user 0.13s system 94% cpu 1.631 total

[dannyb@mainserver 12:48:40] ~ :) $

(I reordered this one to make the timings in the same order as they were before)

[dannyb@mainserver 12:47:38] ~ :( $ time ./warp/fwarpdrive_gcc4_8_1 -o f -I/usr/include -I/usr/include/c++/4.8 -I/usr/include/x86_64-linux-gnu/c++/4.8 -I/usr/include/x86_64-linux-gnu -I/usr/lib/gcc/x86_64-linux-gnu/4.8/include/ -I/usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed/ e.cc

  /usr/include/boost/spirit.hpp(18) : warning: "This header is deprecated. Please use: boost/spirit/include/classic.hpp"

./warp/fwarpdrive_gcc4_8_1 -o f -I/usr/include -I/usr/include/c++/4.8 e.c 2.93s user 0.02s system 99% cpu 2.953 total

Warp is definitely faster than GCC, though.

Because of different #define's, Warp may take a very different path through header files than other preprocessors do. In fact, Warp doesn't have any predefined macros other than the ones required by the Standard. Hence, to use it with cpp or clang's preprocessor, it needs to be driven with a command that -D defines each macro.

There's a command (I forget at the moment what it is) that will tell cpp to list all its predefined macros. It's quite a few. You'll need to do that for clang to get an equivalent list, then drive Warp with that.

You'll be able to tell if it is taking the same path or not by using a diff on the outputs that ignores whitespace differences.

The reason Warp doesn't predefine all that stuff is because every install of gcc has a different list, and it's completely impractical to try and keep up with all that.

I did in fact, use warpdrive, which uses those predefines, as you can see in the commands.

I'm also familiar with the innerworkings on llvm and gcc (having hacked a lot on both), and generated the list of include paths i used with warpdrive (emulating gcc 4.8.1) to be exactly the same as GCC on my system uses for 4.8.1.

I also verified the preprocessed output is "sane" in each case, as per diff.

>Using that to compile warp with gdc with the flags it suggests (-release is not recognized by gdc, -O3 is),

Andrei wrote on Reddit:

"We build warp at Facebook using gdc with -fno-bound-checks -frelease -O4."

http://www.reddit.com/r/programming/comments/21m0bz/warp_a_f...

This is not the set of flags you have in the makefile on github though. I would expect people to use those :)

In any case, building warp with this brings the timings down to 2.31 seconds, so clang is still 40% faster (1.41 vs 2.31)

In any case, at least on my side, i don't have time to further explore, i'd love to see cases where warp is faster, but i haven't found them.

(There is also a certain irony of saying you don't post numbers because you get accused, then saying i used the wrong flags, but ...)

Thanks for doing this. I have read that clang uses some SIMD instructions to speed this up, and I don't know how much that contributes. Warp doesn't use any inline assembler.

And, as your numbers show, suggesting the change in compiler flags was entirely justified.

the SIMD usage is mainly to do two things.

In the lexer, it is used for block comment skipping. It will find the end of block comments 16 characters at a time (on both PPC and x86).

During line number computation, it will also find newlines 16 characters at a time.

This could actually (nowadays) be done 32 characters at a time on newer processors, but isn't.

I'm curious how much of an effect this has.

64 characters with AVX-512? =)

I didn't check to see if the instructions exist, but possibly :)

You do start to hit two issues though as oyu increase the size of the skipping:

1. Alignment 2. If the average block comment/line is < 64 characters, you may lose more time performing the instruction and then counting the trailing zeros in the result to find the place it ended.

I have no numbers to back up whether this matters, of course :)

AVX-512 does not seem to have PMOVMSKB, which is how I assume it is being done with SSE2. There are other ways to skin that cat, but it's unclear whether they have any advantage over using AVX2 with VPMOVMSKB.

I posted a patch here: https://gist.github.com/dberlin/9867614

It adds AVX2 and SSE4.2 instruction support. It makes no discernible difference performance wise that i can find :)

Heh, awesome! I'll try it out today.

This is flat-out fascinating. Thanks.

I forgot to ask - are you compiling Warp for a 64 bit executable? Use -m64 if you're not sure.

Just to make your results readable:

  gcc:   3.14s user 0.27s system 99% cpu 3.418 total
  clang: 1.41s user 0.13s system 94% cpu 1.631 total
  warp:  2.93s user 0.02s system 99% cpu 2.953 total
         2.31s (with recommended build settings)

A comparison against VC++ would also be interesting. I believe VC++ is commonly used on Windows, which I hear is a somewhat popular platform. (I suppose warp might not run on Windows though.)

In fact I developed Warp on Windows. It compiles and works fine. Warp source code is completely portable between Linux and Windows, with the following exceptions:

1. wchar_t is ushort on Windows, uint on Linux. 2. Warp uses a slightly modified file reader from the one in the D standard library, customized to the platform.

Grep the source for "version(Windows)".

Thanks! Another item for my to do list then :)

It's write in D, and there is a build of dmd for window, so should work in Windows.

I'd be curious to see how clang does, too. What matters to us is that warp is easy to get into so we can easily adapt it to our build system (in particular multithreaded preprocessing that saves on opening same included files multiple times).

He understands C++ well enough to have written "Modern C++ Design" ( http://amazon.com/dp/0201704315 ) back in 2001. If he's interested in D, so am I.

You can see this guy on http://forum.dlang.org/ almost everyday talking about D's designer with the community.

This makes no sense. Alex Stepanov continues to programming in C++. What language are you going to use now? (¿?)

>Does Andrei have a sticky D button on his keyboard?

Well, it is written in D. Walter (not Andrei) is talking about how he leveraged D's features to his advantage when writing Warp.

Preprocessing with clang would be very tricky in a system that uses gcc for the compilation proper (which we do). Clang adds its own predefined #defines, which may change the resulting code.

Sure, if you are stuck with GCC, and have no plans to move, it makes sense to improve GCC's preprocessor for a number of reasons (warp produces cacheable artifacts, etc).

But it's actually not that tricky, since you can just change the defines it makes (after all, if you are maintaining your own toolchain, you are maintaining your own toolchain).

In fact, you are already doing it with warp to emulate GCC's defines.

Suffice to say, we've done it before to provide clang diagnostics but build with GCC.

"stuck with GCC" is rather unfair. GCC still produces faster binaries on x86_64 Linux, which is very important for Facebook.

Yes, and no offense, but while other large companies have folks working on fixing this in public, i don't see facebook trying.

This is really not meant as a dig (really!), but more of "why i figured facebook was not trying to make a transition". The companies trying to do so, are contributing heavily to LLVM to make that transition :)

Develop on clang, do automated tests and release builds on GCC?

You can undefine predefined macros in clang using -U. I took a little time to write some commands that should produce something close to the right defines/undefines to let one compiler's preprocessor masquerade as another compiler's preprocessor:

http://coliru.stacked-crooked.com/a/17122f90ec070d55

I appreciate this piece of feedback. I've been consciously trying to keep the article interesting technically and avoid it being construed as an advertisement for D, to the extent a couple of coworkers and at least one other colleague (http://goo.gl/QZ5ELn) were unclear about warp being written in D at all. I'll do my best to tone things down further in the future. There's plenty of exciting stuff going on to be worth avoiding alienating people.

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