[cfe-dev] Clang on Windows targteing gcc compiles very slowly

Jonathan Roelofs jonathan at codesourcery.com
Sat May 30 17:02:46 PDT 2015

On 5/30/15 9:20 AM, Edward Diener wrote:
> On 5/30/2015 10:30 AM, Reid Kleckner wrote:
>> You said you configured in release mode, but assertions might still be
>> on. That accounts for a 2x slowdown.

Can you run it in a profiler and tell us what's slow about it for your 
benchmark? We don't have a lot of information to work with here. A flame 
graph would probably be the most useful.


>> Otherwise I'd say that we haven't
>> profiled and optimized clang on windows very much.
>> Sent from phone
>> On May 29, 2015 4:51 PM, "Edward Diener" <eldlistmailingz at tropicsoft.com
>> <mailto:eldlistmailingz at tropicsoft.com>> wrote:
>>     I test Boost libraries mostly on Windows using various version of
>>     gcc 4.3 and up, VC++ 8.0 and up, and the latest version of clang
>>     which I build from source using mingw/gcc-4.8.1 in release mode.
>>     While clang is a great compiler when it comes to testing code,
>>     getting intelligent error message, and implementing the latest C++
>>     standard, it is noticably slower ( 2x or 3x at minimum ) than the
>>     other compilers I use.
>>     The clang command line parameters usually being used are usually
>>     along the lines of:
>>     -c -x c++ -O0 -g -fno-inline -Wall -g -march=i686 -m32
>>     with the addition of 'std=c++11' occasionally depending on the test.
>>     Does anybody have any idea why clang is so much slower than any of
>>     the other compilers I use ? It does not seem to matter whether it is
>>     in C++03 mode or C++11 mode, it is noticeably slower than the other
>>     compilers.
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Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded

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