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

Sat Mar 26 08:38:22 PDT 2016

Thanks, guys. I checked out tags/RELEASE_380/final and compiled that in
Release mode with assertions off, and verified that the resulting binaries
were "optimized" (see clang-tidy output below). Still runs about 33%
slower. But, I'm going to chalk this up to a older and probably stripped
down version of clang that I have on my system; this may not be an
apples-to-apples comparison. I do want the customizability that I get from
compiling from source, so I'm going to stick with this.

Thanks again,
Jim

$ clang-tidy --version

LLVM (http://llvm.org/):

LLVM version 3.8.0

  Optimized build.

  Built Mar 25 2016 (18:53:39).

  Default target: x86_64-apple-darwin15.0.0

  Host CPU: westmere

On Wed, Mar 23, 2016 at 12:19 PM, David Blaikie <dblaikie at gmail.com> wrote:

> Yeah, not sure, sorry - I don't often do performance measurements of Clang.
>
> Don's advice is probably about right.
>
> On Wed, Mar 23, 2016 at 5:29 AM, jpsota at gmail.com <jpsota at gmail.com>
> wrote:
>
>> Hi Don and David, just wanted to see if you guys had any quick thoughts
>> on any of this before I just resign to live with a 35% slower clang than
>> I'm used to (it's possible that the previous version from Xcode is much
>> slimmed down or otherwise very carefully tuned in some way).
>>
>> Thank you,
>> Jim
>>
>> On Fri, Mar 18, 2016 at 9:42 AM, jpsota at gmail.com <jpsota at gmail.com>
>> wrote:
>>
>>> David and Don, thanks for your tips. We're making progress as the new
>>> clang is only about 35% slower instead of 50% slower, but not quite at
>>> parity yet. Here's what I did:
>>>
>>> 1. deleted everything and started over, as Don suggested (I just checked
>>> out the source code from scratch again)
>>>
>>> 2. configured with cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release
>>> -DLLVM_ENABLE_ASSERTIONS=OFF ../llvm
>>>
>>> 3. built
>>>
>>> Now I'm seeing that clang-tidy --verbose correctly says this new version
>>> doesn't have assertions enabled, and is optimized (for some reason
>>> clang-tidy shows more details than clang when passing --verbose):
>>>
>>> clang-tidy --version
>>> LLVM (http://llvm.org/):
>>>   LLVM version 3.9.0svn
>>>   Optimized build.
>>>   Built Mar 17 2016 (16:05:27).
>>>   Default target: x86_64-apple-darwin15.0.0
>>>   Host CPU: westmere
>>>
>>> However, when doing my benchmarking with the xcode version of clang
>>> (3.7), it's still about 35% faster than my newly compiled version:
>>>
>>> Benchmarking compiling about 20 C++ files:
>>> Original from xcode (3.7): 27s
>>>
>>> New from svn (3.9): 37s
>>>
>>>
>>> I also noticed that the new clang binary is significantly larger than
>>> the original one. I'm sharing this as it may provide some clues to what's
>>> going on. I'm happy to grep for symbols if that would also be helpful.
>>>
>>> ORIGINAL FROM XCODE (3.7):
>>> ls -ltah
>>> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
>>> -rwxr-xr-x  1 root  wheel    42M Sep 22 19:25
>>> /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
>>>
>>> NEW FROM SVN (3.9):
>>> ls -ltah /Users/jim/toolchains/llvm/bin/clang-3.9
>>> -rwxr-xr-x  1 jim  staff    54M Mar 17 17:00
>>> /Users/jim/toolchains/llvm/bin/clang-3.9
>>>
>>>
>>> Anyone have any ideas on anything else to try to get my newly compiled
>>> clang-3.9 as fast as my system-installed clang-3.7? Are there additional
>>> options I should be passing to configure the cmake files, other than
>>> Release and turning off LLVM assertions?
>>>
>>> Thanks very much.
>>>
>>> Jim
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Thu, Mar 17, 2016 at 12:19 PM, don hinton <hintonda at gmail.com> wrote:
>>>
>>>> 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
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> cfe-users mailing list
>>>>> cfe-users at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>>>>>
>>>>>
>>>
>>
>
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