On Wed, Jun 9, 2021 at 2:53 PM Matthias Kretz <m.kr...@gsi.de> wrote: > > On Wednesday, 9 June 2021 14:22:00 CEST Richard Biener wrote: > > On Tue, Jun 8, 2021 at 2:23 PM Matthias Kretz <m.kr...@gsi.de> wrote: > > > From: Matthias Kretz <kr...@kde.org> > > > > > > Explicitly support use of the stdx::simd implementation in situations > > > where the user links TUs that were compiled with different -m flags. In > > > general, this is always a (quasi) ODR violation for inline functions > > > because at least codegen may differ in important ways. However, in the > > > resulting executable only one (unspecified which one) of them might be > > > used. For simd we want to support users to compile code multiple times, > > > with different -m flags and have a runtime dispatch to the TU matching > > > the target CPU. But if internal functions are not inlined this may lead > > > to unexpected performance loss or execution of illegal instructions. > > > Therefore, inline functions that are not marked as always_inline must > > > use an additional template parameter somewhere in their name, to > > > disambiguate between the different -m translations. > > > > Note that excessive use of always_inline can cause compile-time issues > > (see for example PR99785). > > Ah, I should verify whether that's also the reason my stdx::simd > implementation is slow to compile. > > However, I really must have the always_inline semantics in most of the places > stdx::simd uses it. Because most of these functions compile to either a single > function call or a single instruction (often f0 -> f1 -> f2 -> single > instruction). If the inliner even makes one single wrong inlining decision, > the whole program might slow down by integral factors, not only small > percentages. And without inlining these functions, -fno-inline builds (i.e. > many debug builds) become unbearably slow (aka useless).
Understood. Note I think that the slow compile is a bug and there must be a way to address it, there's just too large testcases at the moment to get a hand on what kind of callgraphs cause which problem and why and how we might want to address this. > > I wonder whether the inlines can be > > placed in an anonymous namespace instead of the difficult to maintain > > explict list of SIMD features? > > It's possible, and part of the patch: > > + namespace > + { > + struct _OdrEnforcer {}; > + } > [...] > + using __odr_helper > + = conditional_t<__machine_flags() == 0, _OdrEnforcer, > + _MachineFlagsTemplate<__machine_flags(), __floating_point_flags()>>; > > It can potentially blow up the code size and the instruction cache usage, > though. The trade-off isn't obvious to make. I guess I can't promise that > mixing different compiler flags is ODR violation free > > > It also doesn't solve the issue when > > instantiating the functions from a TU which contains #pragma GCC target > > sections to switch options, of course. > > Yes. Can I get PR83875? ;-) heh ;) Richard. > - Matthias > > > > Signed-off-by: Matthias Kretz <m.kr...@gsi.de> > > > > > > libstdc++-v3/ChangeLog: > > > * include/experimental/bits/simd.h: Move feature detection bools > > > and add __have_avx512bitalg, __have_avx512vbmi2, > > > __have_avx512vbmi, __have_avx512ifma, __have_avx512cd, > > > __have_avx512vnni, __have_avx512vpopcntdq. > > > (__detail::__machine_flags): New function which returns a unique > > > uint64 depending on relevant -m and -f flags. > > > (__detail::__odr_helper): New type alias for either an anonymous > > > type or a type specialized with the __machine_flags number. > > > (_SimdIntOperators): Change template parameters from _Impl to > > > _Tp, _Abi because _Impl now has an __odr_helper parameter which > > > may be _OdrEnforcer from the anonymous namespace, which makes > > > for a bad base class. > > > (many): Either add __odr_helper template parameter or mark as > > > always_inline. > > > * include/experimental/bits/simd_detail.h: Add defines for > > > AVX512BITALG, AVX512VBMI2, AVX512VBMI, AVX512IFMA, AVX512CD, > > > AVX512VNNI, AVX512VPOPCNTDQ, and AVX512VP2INTERSECT. > > > * include/experimental/bits/simd_builtin.h: Add __odr_helper > > > template parameter or mark as always_inline. > > > * include/experimental/bits/simd_fixed_size.h: Ditto. > > > * include/experimental/bits/simd_math.h: Ditto. > > > * include/experimental/bits/simd_scalar.h: Ditto. > > > * include/experimental/bits/simd_neon.h: Add __odr_helper > > > template parameter. > > > * include/experimental/bits/simd_ppc.h: Ditto. > > > * include/experimental/bits/simd_x86.h: Ditto. > > > > > > --- > > > > > > libstdc++-v3/include/experimental/bits/simd.h | 380 ++++++++++++------ > > > .../include/experimental/bits/simd_builtin.h | 41 +- > > > .../include/experimental/bits/simd_detail.h | 40 ++ > > > .../experimental/bits/simd_fixed_size.h | 39 +- > > > .../include/experimental/bits/simd_math.h | 45 ++- > > > .../include/experimental/bits/simd_neon.h | 4 +- > > > .../include/experimental/bits/simd_ppc.h | 4 +- > > > .../include/experimental/bits/simd_scalar.h | 71 +++- > > > .../include/experimental/bits/simd_x86.h | 4 +- > > > 9 files changed, 440 insertions(+), 188 deletions(-) > > > > > > -- > > > ────────────────────────────────────────────────────────────────────────── > > > > > > Dr. Matthias Kretz https://mattkretz.github.io > > > GSI Helmholtz Centre for Heavy Ion Research https://gsi.de > > > std::experimental::simd https://github.com/VcDevel/std-simd > > > > > > ────────────────────────────────────────────────────────────────────────── > > > -- > ────────────────────────────────────────────────────────────────────────── > Dr. Matthias Kretz https://mattkretz.github.io > GSI Helmholtz Centre for Heavy Ion Research https://gsi.de > std::experimental::simd https://github.com/VcDevel/std-simd > ────────────────────────────────────────────────────────────────────────── > > >