[llvm-dev] [cfe-dev] llvm.org pre-built clang significantly slower than apple/xcode clang

[Jack Howarth via llvm-dev]

Jean-Daniel,
     The latest available release posted on their open source web site
argues that that they are disabling assertions in clang.

https://opensource.apple.com/source/clang/clang-800.0.42.1/Makefile.auto.html

     Jack

On Tue, Nov 20, 2018 at 5:21 PM Jean-Daniel <mailing at xenonium.com> wrote:

> I don’t think Apple disable assertion on the release build. I remember
> having clang and llvm crash because of assertion failure regularly at some
> point in the past.
> Nowadays, it is far more unusual to get a clang crash, so I can’t tell,
> but I doubt they change the configuration.
>
> Le 20 nov. 2018 à 16:32, Jack Howarth via cfe-dev <cfe-dev at lists.llvm.org>
> a écrit :
>
> The obvious question is whether the llvm.org builds are using -DLLVM_ENABLE_ASSERTIONS:OFF
> -DCMAKE_BUILD_TYPE:STRING=Release -DLLVM_LINK_LLVM_DYLIB:BOOL=ON which
> would improve the load time of the compiler by combining all of the llvm
> libs into a single dylib and would eliminate the speed decrease from using
> the default use of assertions in the built compiler.
> Jack
>
> On Tue, Nov 20, 2018 at 6:56 AM Tobias Hieta via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hello LLVM/Clang developers,
>>
>> We recently switched to use the same clang version on all our platforms.
>> This included switching from apple-clang from xcode to a pre-built binary
>> we downloaded from llvm.org. We noticed that this actually came with a
>> pretty big performance regression in compile times.
>>
>> If we do the simplest test program like this:
>>
>> #include <string>
>> #include <iostream>
>> int main()
>> {
>>     std::cout << "Hello world" << std::endl;
>> }
>>
>> and compile that with Xcode Clang (Xcode 10.1 apple-clang
>> clang-1000.11.45.5):
>> clang++ test.cpp -o test  0.31s user 0.06s system 97% cpu 0.380 total
>>
>> with clang 7 binaries found on llvm.org 7.0.0:
>> ~/Downloads/clang+llvm-7.0.0-x86_64-apple-darwin/bin/clang++ -o test
>> test.cpp  0.53s user 0.11s system 62% cpu 1.032 total
>>
>> If we now run that on our whole project:
>> with xcode clang:
>> 368.17s user 32.00s system 663% cpu 1:00.30 total
>>
>> with clang 7:
>> 423.31s user 31.65s system 662% cpu 1:08.69 total
>>
>> That's a pretty hefty difference. Any ideas what can account for this
>> discrepancy? Does apple-clang contain any special patches or build flags
>> that differ a lot from the binaries on llvm.org?
>>
>> I know about PGO - and I guess the best we could do is to get profile
>> data out of compiling my whole tree and use that when building clang - but
>> this process seems not very well documented and unsure if this would even
>> help.
>>
>> Thankful for any ideas or feedback.
>> Tobias
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>> cfe-dev at lists.llvm.org
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>>
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