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

Matthias Kretz m.kretz@gsi.de
Tue Jun 8 12:12:23 GMT 2021

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.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>


	* 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
	* include/experimental/bits/simd_detail.h: Add defines for
	* 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
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