Re: Disable autogeneration of gather instructions on Ryzen and generic

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Jan 9, 2018 at 11:58 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> On Tue, Jan 9, 2018 at 11:26 AM, Jan Hubicka <hubicka@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.
>
> Agreed, that is whay i meant by 2.  Generic code has all info to implement gathers/scatters
> by hand when they are not supported and estimate cost of the implementation.
> This is what I meant by the paragraph above.  I tried to look into it about month
> ago but got bit lost in vectorizer internals :(

Yeah, I think the only bits that cannot be done open-coded in an
efficient way is
the masking.

Richard.

> Honza
>>
>> 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)     */
>> >  /*****************************************************************************/