On Tue, Sep 3, 2024 at 3:07 PM Jan Hubicka <hubi...@ucw.cz> wrote: > > Hi, > We disable gathers for zen4. It seems that gather has improved a bit compared > to zen4 and Zen5 optimization manual suggests "Avoid GATHER instructions when > the indices are known ahead of time. Vector loads followed by shuffles result > in a higher load bandwidth." however the situation seems to be more > complicated. > > gather is 5-10% loss on parest benchmark as well as 30% loss on sparse dot > products in TSVC. Curiously enough breaking these out into microbenchmark > reversed the situation and it turns out that the performance depends on > how indices are distributed. gather is loss if indices are sequential, > neutral if they are random and win for some strides (4, 8). > > This seems to be similar to earlier zens, so I think (especially for > backporting znver5 support) that it makes sense to be conistent and disable > gather unless we work out a good heuristics on when to use it. Since we > typically do not know the indices in advance, I don't see how that can be > done. > > I opened PR116582 with some examples of wins and loses
Note there's no way to emulate masked gathers (well - emit control flow), so they remain the choice when AVX512 is enabled and you have conditional loads. Similar for stores and scatter though there performance may be well absymal - something for the cost model to resolve. Note I think x86 doesn't yet expose AVX512 masked gather/scatter - the builtin target hook doesn't support it and the backend doesn't have any mask_gather_load or mask_scatter_store optabs to go the now prefered internal-fn way. Open-coding 8-way gather is also heavy in code size and thus might effect ucode re-use for large loops (OTOH gathers may take up much space in the ucode cache or be not there at all). Richard. > Bootstrapped/regtested x86_64-linux, committed. > > > gcc/ChangeLog: > > * config/i386/x86-tune.def (X86_TUNE_USE_GATHER_2PARTS): Disable for > ZNVER5. > (X86_TUNE_USE_SCATTER_2PARTS): Disable for ZNVER5. > (X86_TUNE_USE_GATHER_4PARTS): Disable for ZNVER5. > (X86_TUNE_USE_SCATTER_4PARTS): Disable for ZNVER5. > (X86_TUNE_USE_GATHER_8PARTS): Disable for ZNVER5. > (X86_TUNE_USE_SCATTER_8PARTS): Disable for ZNVER5. > > diff --git a/gcc/config/i386/x86-tune.def b/gcc/config/i386/x86-tune.def > index da1a3d6a3c6..ed26136faee 100644 > --- a/gcc/config/i386/x86-tune.def > +++ b/gcc/config/i386/x86-tune.def > @@ -476,35 +476,35 @@ DEF_TUNE (X86_TUNE_AVOID_4BYTE_PREFIXES, > "avoid_4byte_prefixes", > /* X86_TUNE_USE_GATHER_2PARTS: Use gather instructions for vectors with 2 > elements. */ > DEF_TUNE (X86_TUNE_USE_GATHER_2PARTS, "use_gather_2parts", > - ~(m_ZNVER1 | m_ZNVER2 | m_ZNVER3 | m_ZNVER4 | m_CORE_HYBRID > + ~(m_ZNVER | m_CORE_HYBRID > | m_YONGFENG | m_SHIJIDADAO | m_CORE_ATOM | m_GENERIC | m_GDS)) > > /* X86_TUNE_USE_SCATTER_2PARTS: Use scater instructions for vectors with 2 > elements. */ > DEF_TUNE (X86_TUNE_USE_SCATTER_2PARTS, "use_scatter_2parts", > - ~(m_ZNVER4)) > + ~(m_ZNVER4 | m_ZNVER5)) > > /* X86_TUNE_USE_GATHER_4PARTS: Use gather instructions for vectors with 4 > elements. */ > DEF_TUNE (X86_TUNE_USE_GATHER_4PARTS, "use_gather_4parts", > - ~(m_ZNVER1 | m_ZNVER2 | m_ZNVER3 | m_ZNVER4 | m_CORE_HYBRID > + ~(m_ZNVER | m_CORE_HYBRID > | m_YONGFENG | m_SHIJIDADAO | m_CORE_ATOM | m_GENERIC | m_GDS)) > > /* X86_TUNE_USE_SCATTER_4PARTS: Use scater instructions for vectors with 4 > elements. */ > DEF_TUNE (X86_TUNE_USE_SCATTER_4PARTS, "use_scatter_4parts", > - ~(m_ZNVER4)) > + ~(m_ZNVER4 | m_ZNVER5)) > > /* X86_TUNE_USE_GATHER: Use gather instructions for vectors with 8 or more > elements. */ > DEF_TUNE (X86_TUNE_USE_GATHER_8PARTS, "use_gather_8parts", > - ~(m_ZNVER1 | m_ZNVER2 | m_ZNVER4 | m_CORE_HYBRID | m_CORE_ATOM > + ~(m_ZNVER | m_CORE_HYBRID | m_CORE_ATOM > | m_YONGFENG | m_SHIJIDADAO | m_GENERIC | m_GDS)) > > /* X86_TUNE_USE_SCATTER: Use scater instructions for vectors with 8 or more > elements. */ > DEF_TUNE (X86_TUNE_USE_SCATTER_8PARTS, "use_scatter_8parts", > - ~(m_ZNVER4)) > + ~(m_ZNVER4 | m_ZNVER5)) > > /* X86_TUNE_AVOID_128FMA_CHAINS: Avoid creating loops with tight 128bit or > smaller FMA chain. */
