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

[David Blaikie via cfe-users]

On Wed, Mar 16, 2016 at 2:12 PM, jpsota at gmail.com <jpsota at gmail.com> wrote:

> 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,
>

Define "my programs" - are you using LLVM as a library? If so, you probably
want a with-asserts build for development so you get better failures when
you use LLVM incorrectly. If you're just using Clang as a normal compiler -
a build without assertions should be just fine. If you hit a crash in the
compiler you can still file it, and we'll run in with an assertions-enabled
build to investigate further, generally.


> 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?).
>

If you're using LLVM as a library, but not expecting to be able to fix any
bugs in it yourself, probably optimized with assertiotns and without debug
info should be fine.

If you're just using clang as a compiler, release (optimized), no
assertions, no debug info.


> 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.
>

Not sure


> [2]
>
> Thank you,
> Jim
>
> [1] --disable-assertions--enable-debug-symbols--enable-optimized
> [2]
>
> $ 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
>>>> -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
>>>>
>>>> _______________________________________________
>>>> cfe-users mailing list
>>>> cfe-users at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>>>>
>>>>
>>>
>
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