[gambit-list] Gambit Clang C/C++ status and compiler benchmarks?

Marc Feeley feeley at iro.umontreal.ca
Sat May 5 22:16:37 EDT 2018


I haven't experimented with clang very much.  The performance gap with gcc is so wide that it seems like a very long shot to get it to perform at least as well as gcc on Gambit.  gcc gives adequate performance so why look for better?  As Voltaire said, “better is the enemy of good”.

I brought this performance issue up with the Apple LLVM team 3 years ago.  At the time it was logged as rdar://problem/10739380 on their internal bug tracking system.  I have poked them from time to time to see if there is any progress, and at this point I’ve lost interest in seeing a resolution.  It is not on any critical path of mine.

If someone has the interest and time to followup with the developpers, be my guest.

Marc



> On May 5, 2018, at 1:06 PM, Adam <adam.mlmb at gmail.com> wrote:
> 
> Hi Marc,
> 
> Wow, Clang is immensely slow indeed.
> 
> Do you have any idea if Clang's low performance could be circumvented using Clang optimizer configuration, or, do you have any idea about qualities in Clang's design that destine it to produce slow code for Gambit?
> 
> Maybe there could be a conversation with Clang to ask them to have a look at why their compiler is performing so bad, maybe they would be interested in speeding up things, I think overall they do have an ambition for high performance.
> 
> Clang has become the bundled system-default C/C++/Objective C compiler for a handful OS:es now, so there is a growing incentive to use it. All the OS-bundled and packaged libraries on those platforms are Clang-compiled, and mixing Clang- and GCC-compiled code (e.g. shared libraries) in one executable, is not a very smooth experience today at least in my very limited experience.
> 
> Would you be interested in crossposting an email to Clang's developer mailing list (llvm-dev and maybe cfe-dev, http://lists.llvm.org/mailman/listinfo) where you address Clang's performance problem and suggest any reasons you see as plausible for why they are so much after?
> 
> Adam
> 
> 
> 2018-05-05 20:17 GMT+08:00 Marc Feeley <feeley at iro.umontreal.ca>:
> GCC is still the best choice when compiling Gambit.
> 
> When configured with the usual
> 
>   ./configure --enable-single-host
> 
> the build time of a “make -j8” on my 4 core laptop is over 10x slower when using CLANG.  As for speed of the generated code, CLANG produces code that is about 3x slower than GCC.
> 
> Here are the raw results, in seconds:
> 
>                     make -j8     test4
>  GCC 8.1.0           39.566      1.167
>  CLANG 802.0.38     426.371      4.229
> 
> When configured with
> 
>   ./configure --enable-single-host --enable-c-opt
> 
> which uses -O2 rather than the default -O1, the results are also favourable for GCC.  However the build times are closer and, interestingly, both compilers generate slightly slower code with -O2:
> 
>                     make -j8     test4
>  GCC 8.1.0          193.560      1.218
>  CLANG 802.0.38     229.945      4.410
> 
> Marc
> 
> 
> 
> > On May 5, 2018, at 5:54 AM, Adam <adam.mlmb at gmail.com> wrote:
> > 
> > Hi list,
> > 
> > So I think I figured out the answer myself.
> > 
> > First, the previous benchmark that I recalled having read, is the "8) Performance of GCC and CLANG when compiling Gambit" section in Gambit's install.txt file (https://github.com/gambit/gambit/blob/edac3c75f0d93f4f56a39c2b90621511f096dd72/INSTALL.txt#L577).
> > 
> > The essence here is that Clang 2.6, 2.7 and 2.8 wouldn't even compile Gambit, and 2.9-3.1 would compile Gambit but be fairly slow.
> > 
> > A lot has happened since Clang 3.1, which is the last Clang version covered by install.txt today. The latest version today is 6.0. Ref. https://en.wikipedia.org/wiki/Clang#Status_history . Clang is now the default C/C++/Objective-C compiler choice in a handful operating systems, for AMD64 and some more architectures.
> > 
> > Clang's language feature set is good, ref. https://en.cppreference.com/w/cpp/compiler_support , https://clang.llvm.org/cxx_status.html .
> > 
> > And Clang's speed is decent, recent benchmarks tend to find that Clang and GCC have a speed difference that's in the ballpark +-30%, up or down depending on benchmark, e.g. ref. https://stackoverflow.com/questions/3187414/clang-vs-gcc-which-produces-better-binaries#15043814 .
> > 
> > The term "LLVM" only denotes that Clang has an internal intermediary language form, and the term LLVM has no other meaning in the direction of executable bytecode like Java JIT VM:s. I.e. Clang is only a native C/C++/Obj-C compiler. A list of the Clang/LLVM projects is on the main page at http://www.llvm.org/ .
> > 
> > I have not tested yet but I do expect Clang to run Gambit stably and at a totally-okay speed.
> > 
> > Adam
> > 
> > 2018-05-05 16:13 GMT+08:00 Adam <adam.mlmb at gmail.com>:
> > Hi!
> > 
> > Does Gambit work perfectly with the Clang C/C++ compiler (compiling to assembly which is compiled to native code, nothing LLVM), if so is it for all Gambit versions, or since when?
> > 
> > I remember a benchmark of the time taken to, was it to compile Gambit-generated C code, or was it execution time of Gambit-C code as compiled by different GCC and Clang versions. What is the URL to that post in the mailing list archive?
> > 
> > Thanks!
> > Adam
> > 
> > _______________________________________________
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> > Gambit-list at iro.umontreal.ca
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> 
> 




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