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

jpsota@gmail.com via cfe-users cfe-users at lists.llvm.org
Wed Mar 16 14:12:48 PDT 2016

David and all, a couple more questions.  I stumbled upon
http://llvm.org/docs/Packaging.html and see a few other options. For a
typical clang build where I am not hacking on clang, but I do want good
error messages for debugging my programs, which options are recommended
[1]? It seems like I should use --disable-assertions, but I'm not sure
about enable-debug-symbols and enable-optimized (which seem at odds with
each other?). I'm basically just looking for the same options that would
come with a packaged build, but I do want to build from source.

Finally, when I'm done, how do I verify that the settings are correct?
clang --version doesn't tell me much about if I built it correctly. [2]

Thank you,

[1] --disable-assertions--enable-debug-symbols--enable-optimized

$ clang --version

clang version 3.9.0 (trunk 263648)

Target: x86_64-apple-darwin15.0.0

Thread model: posix

InstalledDir: /Users/jim/toolchains/llvm/bin

On Wed, Mar 16, 2016 at 2:05 PM, jpsota at gmail.com <jpsota at gmail.com> wrote:

> Got it -- thanks!
> On Wednesday, March 16, 2016, David Blaikie <dblaikie at gmail.com> wrote:
>> 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
>>> 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
>>> _______________________________________________
>>> cfe-users mailing list
>>> cfe-users at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
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