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

[jpsota@gmail.com via cfe-users]

On Wed, Mar 16, 2016 at 5:55 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> 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.
>

Just using clang (and clang tools) -- not using LLVM as a library. So,
running without assertions seems right for my needs.


>
>
>> 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.
>
>

It seems that just enabling -DCMAKE_BUILD_TYPE=Release is not sufficient to
turn off assertions as I tried that and when I run clang-tidy --version it
says " LLVM version 3.9.0svn; Optimized build with assertions." So, I'm
going to try --disable-assertions --enable-optimized --disable-debug-symbols
(or the environment variable equivalents).

That seem right to you? It's still confusing to me that there's no
CMAKE_BUILD_TYPE that enables these sorts of options as a bundle, but I
guess that's what I get from compiling from source.

Thanks
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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