[cfe-users] Clang 3.9 running 50% slower than 3.7

[David Blaikie via cfe-users]

On Wed, Mar 16, 2016 at 7:25 AM, jpsota at gmail.com via cfe-users <
cfe-users at lists.llvm.org> wrote:

> Hi, I recently installed "Release" clang (svn r263305) from source on my
> OSX machine, and it's compiling a 20 file C++ program about 50% slower than
> the natively installed clang 3.7 (that came with xcode, I believe, although
> I don't use xcode). I currently have both sets of tools installed and am
> able to switch back and forth and verify using time that clang 3.7 takes
> about 30 seconds and clang 3.9 takes about 45 seconds, on average (all
> flags, settings, etc. are the same for both). I did build with "Release" as
> the build type, although I also did set DLLVM_ENABLE_ASSERTIONS=ON (could
> this be the problem?).
>

Yes, the build system should print a warning telling you that an assertions
enabled build can be up to ten times slower. Performance
comparisons/measurements of an assertions enabled compiler aren't something
we really do/tune for.


> More details below. Please let me know if you have any ideas about why
> this newer clang would be noticable slower.
>
> Generally, I'm just trying to use clang as a user, not a clang developer,
> so if you have general recommendations for how to configure this, please
> let me know.
>
> Thank you,
> Jim
>
> How I configured and installed:
>
> cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release
> -DLLVM_ENABLE_ASSERTIONS=ON ../llvm
> make
> cmake -DCMAKE_INSTALL_PREFIX=$CLANG_PREFIX -P cmake_install.cmake
>
>
> Here are some specifics:
>
> $ uname -a
> Darwin localhost 15.0.0 Darwin Kernel Version 15.0.0: Wed Aug 26 16:57:32
> PDT 2015; root:xnu-3247.1.106~1/RELEASE_X86_64 x86_64
>
> $ clang --version
> clang version 3.9.0 (trunk 263305)
> Target: x86_64-apple-darwin15.0.0
> Thread model: posix
> InstalledDir: /Users/jim/toolchains/llvm/bin
>
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>
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