Re: [PATCH 11/11] libstdc++: Fix ODR issues with different -m flags

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Jun 9, 2021 at 2:53 PM Matthias Kretz <m.kretz@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.kretz@gsi.de> wrote:
> > > From: Matthias Kretz <kretz@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.kretz@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
> ──────────────────────────────────────────────────────────────────────────
>
>
>