On Tue, Jan 9, 2018 at 11:26 AM, Jan Hubicka <hubi...@ucw.cz> wrote: > Hi, > gather instructions are rather hard to implement in hardware and except for > skylake+ chips (i.e. haswell and Zen) they seems to be rather slow; to the > degree I did not find real world loop where gather would help on Zen. > This patch simply adds a knob to disable its autogeneration (builtin still > works). I have considered two alternatives > 1) tune this with x86-tune-costs because gather is still profitable than > scalar code if we do very expensive operations (such as sequence of > divides) > on the values gathered/scattered > 2) implement expansion of gathers into primitive instructions. This is > faster > as semantics of gather is bit weird and not fully needed for our > vectorizer > > I did not have luck to get any good results out of 1 alone as cost model is > not > very realistic. 1+2 probably makes sense but we can do this incrementally as > that would make most sense to be implemented generically in vectorizer on the > top of this change.
Note that the vectorizer gives up on loops with gathers with no target support for gathers. It could simply open-code the gather though (and properly cost that open-coded variant), that's probably the way to go here. Richard. > Given that gather is problematic even on skylake+ as shown in the PR (which > has > most optimized implementation of it) it is good to have a knob to control its > codegen at first place. > > I have also disabled gathers for generic. This is because its use causes > some two-digit regression on zen for spec2k17 while there are no measurable > benefits on Intel. Note that this affects only > -march=<somethig supporting gather> -mtune=generic > as by default we do not use AVX2. > > Bootstrapped/regtested x86_64-linux, plan to commit it later today if there > are no complains. > > Honza > > PR target/81616 > * i386.c (ix86_vectorize_builtin_gather): Check TARGET_USE_GATHER. > * i386.h (TARGET_USE_GATHER): Define. > * x86-tune.def (X86_TUNE_USE_GATHER): New. > Index: config/i386/i386.c > =================================================================== > --- config/i386/i386.c (revision 256369) > +++ config/i386/i386.c (working copy) > @@ -38233,7 +38233,7 @@ ix86_vectorize_builtin_gather (const_tre > bool si; > enum ix86_builtins code; > > - if (! TARGET_AVX2) > + if (! TARGET_AVX2 || !TARGET_USE_GATHER) > return NULL_TREE; > > if ((TREE_CODE (index_type) != INTEGER_TYPE > Index: config/i386/i386.h > =================================================================== > --- config/i386/i386.h (revision 256369) > +++ config/i386/i386.h (working copy) > @@ -498,6 +498,8 @@ extern unsigned char ix86_tune_features[ > ix86_tune_features[X86_TUNE_SLOW_PSHUFB] > #define TARGET_AVOID_4BYTE_PREFIXES \ > ix86_tune_features[X86_TUNE_AVOID_4BYTE_PREFIXES] > +#define TARGET_USE_GATHER \ > + ix86_tune_features[X86_TUNE_USE_GATHER] > #define TARGET_FUSE_CMP_AND_BRANCH_32 \ > ix86_tune_features[X86_TUNE_FUSE_CMP_AND_BRANCH_32] > #define TARGET_FUSE_CMP_AND_BRANCH_64 \ > Index: config/i386/x86-tune.def > =================================================================== > --- config/i386/x86-tune.def (revision 256369) > +++ config/i386/x86-tune.def (working copy) > @@ -399,6 +399,10 @@ DEF_TUNE (X86_TUNE_SLOW_PSHUFB, "slow_ps > DEF_TUNE (X86_TUNE_AVOID_4BYTE_PREFIXES, "avoid_4byte_prefixes", > m_SILVERMONT | m_INTEL) > > +/* X86_TUNE_USE_GATHER: Use gather instructions. */ > +DEF_TUNE (X86_TUNE_USE_GATHER, "use_gather", > + ~(m_ZNVER1 | m_GENERIC)) > + > > /*****************************************************************************/ > /* AVX instruction selection tuning (some of SSE flags affects AVX, too) > */ > > /*****************************************************************************/
