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

[Jean-Daniel via cfe-dev]

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 <http://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 <mailto: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 <http://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 <http://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 <http://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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