On Mon, Feb 9, 2015 at 11:11 AM, Ricardo Telichevesky <rica...@teli.org> wrote: > Hello Renato, thanks for your reply, sorry about the confusion with > compilers - since there is a "clang" in the mac, I thought gcc was gcc.. > After your suggestion, I modified the code, and installed a bona fide gcc > compiler in the Mac.... I got more data that is kind of interesting, wanted > to share with the gcc community. Someone might have an idea why this > happens. > > No matter what compiler I use, gcc or llvm on Linux I seem to have this > problem of 64-bit code being 3x slower than 32-bit when very large > procedures are called. In my Macbook the 64-bit code is consistently faster > regardless of the compiler. I got my hands on an i7 Mac and the 64-bit code > is consistently 10% to 15% faster than the 32-bit code and - I am afraid to > say something "political" here, have no intention of offending anyone, just > the facts - with all the optimizations turned on, modified code to force > inlinining here and there, both gcc and llvm yield pretty much the same > performance, in fact both are outstanding, reduced the time I reported > previously from the 16.2s to 1.1s!!! AVX gave me an extra 10% over SSE > (tricky to get AVX to compile in the mac with gcc, but not impossible). > > Of course is hard to compare apples to apples here with Xeon Westmere at > 2Ghz and i7 IvyBridge at 3.5GHz, but almost 15x is quite a difference, given > the clock ratios are less than 2. > > Now I am trying to investigate if the problem is with Linux itself - which > is not so good because I need to release code on Linux, or there some > strange cache issue, or the way libraries are setup, or with the Westmere vs > IvyBridge. I will keep you posted if I find out more. >
To compare 64-bit vs 32-bit performance on Linux against MacOS, you MUST compile your code with -fPIC on Linux since -fPIC is the default on MacOS. -- H.J.
