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

Thu Mar 17 09:19:44 PDT 2016

It looks like you are rerunning cmake without first removing the cache,
CMakeCache.txt.  Since the option () command that sets
LLVM_ENABLE_ASSERTIONS didn't include FORCE, the previous cached value is
preserved.

Therefore, I'd recommend always removing the cache -- I actually blow away
the entire directory tree -- and starting from scratch.

To ensure (re)builds don't take too long, I also use ccache.  Although it
works great for llvm, clang, etc..., it does break the libcxx and libcxxabi
tests under check-all, but I've submitted a couple trivial patches that fix
that problem.

hth...
don
On Mar 17, 2016 11:26 AM, "David Blaikie via cfe-users" <
cfe-users at lists.llvm.org> wrote:

>
>
> On Wed, Mar 16, 2016 at 3:36 PM, jpsota at gmail.com <jpsota at gmail.com>
> wrote:
>
>>
>>
>> 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?
>>
>
> Yeah, sounds about right.
>
>
>> 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.
>>
>
> Yeah - not sure what the right recipe is for building things the same as
> the official releases. No doubt it's written down somewhere...
>
>
>>
>> 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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