On Fri, Jun 26, 2015 at 08:38:56PM +0300, Roman Lebedev wrote: > On Fri, Jun 26, 2015 at 6:33 PM, Bruce Guenter <br...@untroubled.org> wrote: > > Here's what I've found out: GCC does support function multiversioning > > (https://gcc.gnu.org/wiki/FunctionMultiVersioning) to provide the same > > function name for different architectures and dispatching them at run > > time. The same function marked with __attribute__((target("X"))) is > > indeed compiled differently within the same source for X is SSE2 vs AVX, > > taking advantage of the different instruction sets. > > Just facts: that is a GCC-specific thing, supported only by gcc-4.8+, > and completely unsupported by clang, not even by clang+llvm 3.7 nightly > snapshot.
Right. That was just an illustration that GCC had support for multiple versions of the same function name with automatic internal switching. It's still not particularly useful nor portable. > > However, GCC does *NOT* do any of this automatically, or at least I have > > not found any way to make it. A function (with or without a vectorized > > loop) is emitted exactly once each time it appears, and there are no > > internal branches to different (SSE vs AVX) code paths. > > > > So to take full advantage of it we would still need to repeat the > > function for each target hardware. > > Yes, here i definitely agree with you. > Regardless of why it will be done (process() + process_cl() + process_avx() > + ... OR > process() + process_cl() + process_simd()), that is the ONLY way that would > allow to > eventually implement some kind of automatic implementation verification > tests without > changing much of the code. That is what I was leaning towards, yes. I am actually in favor of going the compiler vectorization model via OpenMP SIMD. I like the idea of future proofing the code by making it more portable. In fact, it would work fine for me, as I can build the code separately on my different systems to get separate AVX vs SSE implementations. It just doesn't seem to be a viable solution for distributors without some kind of runtime switch. -- Bruce Guenter <br...@untroubled.org> http://untroubled.org/
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