[PATCH v6] libstdc++: Implement [simd] for C++26

Matthias Kretz MatthiasKretz@gmx.net
Fri Mar 20 23:03:27 GMT 2026 

Tested on x86_64-pc-linux-gnu (with expensive tests enabled).

Changes since v5:

- #ifdef math_errhandling

- Replace … with [...]

- Implement LWG4412 (inline instead of static for std::simd::zero_element and 
uninit_element)

- Use std::__conditional_t. I don't care for the mangling anywhere, but I care 
a lot for compile time performance.

- Replace <tt> with backticks in comments.

- Replace \ with @ in doxygen commands.

- Fix the "value-reserving" typo.

- Squash (adjusted) https://forge.sourceware.org/mkretz/gcc/pulls/1/commits/
41d37c3bfdd521a864b007ebba63c8c52ca4015a to make default tests cheaper.


Also visible on the Forge as
- https://forge.sourceware.org/gcc/gcc-TEST/commit/
0ab5a8e005ba267001c22da922786b768e5ba67b

- https://forge.sourceware.org/gcc/gcc-TEST/commit/
bef1fb214620ce9b1a0177934318f23b5fa34003

- https://forge.sourceware.org/gcc/gcc-TEST/compare/
bef1fb214620ce9b1a0177934318f23b5fa34003..bd634e7ad5451211ade86a25621c57432ea48122


Replies (no further changes):

> >+#define _GLIBCXX_SIMD_TOSTRING_IMPL(x) #x
> >+#define _GLIBCXX_SIMD_TOSTRING(x) _GLIBCXX_SIMD_TOSTRING_IMPL(x)
> 
> Will this be able to use reflection some day? 🙂

It's actually unused in this patch and the next (simd.complex). I use it for 
my "warning on precondition violation" macro and in simd_math.h. It's probably 
better if I drop it until then?
And, at least for the precondition macro it needs to use "string " "concat " 
"like so", because I need a string literal for the [[gnu::warning]] attribute. 
Reflection won't give me literals, right? :)

> >+      [[__gnu__::__always_inline__]]
> >+      friend constexpr bool
> >+      __is_const_known(const basic_vec& __x)
> >+      { return __builtin_constant_p(__x._M_data); }
> 
> Is there any compilation speed benefit to using if consteval here for
> the 1-arg case, or is that only an advantage when it prevents the
> recursion?

That's a good question. Possibly yes, but I would assume the 
__builtin_constant_p isn't very expensive either. The multi-arg 
__is_const_known overload uses the shortcut mostly to avoid recursing into 
more of these functions.

------------ 8< ------------

This implementation differs significantly from the
std::experimental::simd implementation. One goal was a reduction in
template instantiations wrt. what std::experimental::simd did.

Design notes:

- bits/vec_ops.h contains concepts, traits, and functions for working
  with GNU vector builtins that are mostly independent from std::simd.
  These could move from std::simd:: to std::__vec (or similar). However,
  we would then need to revisit naming. For now we kept everything in
  the std::simd namespace with __vec_ prefix in the names. The __vec_*
  functions can be called unqualified because they can never be called
  on user-defined types (no ADL). If we ever get simd<UDT> support this
  will be implemented via bit_cast to/from integral vector
  builtins/intrinsics.

- bits/simd_x86.h extends vec_ops.h with calls to __builtin_ia32_* that
  can only be used after uttering the right GCC target pragma.

- basic_vec and basic_mask are built on top of register-size GNU vector
  builtins (for now / x86). Any larger vec/mask is a tree of power-of-2
  #elements on the "first" branch. Anything non-power-of-2 that is
  smaller than register size uses padding elements that participate in
  element-wise operations. The library ensures that padding elements
  lead to no side effects. The implementation makes no assumption on the
  values of these padding elements since the user can bit_cast to
  basic_vec/basic_mask.

Implementation status:

- The implementation is prepared for more than x86 but is x86-only for
  now.

- Parts of [simd] *not* implemented in this patch:

  - std::complex<floating-point> as vectorizable types
  - [simd.permute.dynamic]
  - [simd.permute.mask]
  - [simd.permute.memory]
  - [simd.bit]
  - [simd.math]
  - mixed operations with vec-mask and bit-mask types
  - some conversion optimizations (open questions wrt. missed
    optimizations in the compiler)

- This patch implements P3844R3 "Restore simd::vec broadcast from int",
  which is not part of the C++26 WD draft yet. If the paper does not get
  accepted the feature will be reverted.

- This patch implements D4042R0 "incorrect cast between simd::vec and
simd::mask via conversion to and from impl-defined vector types" (to be
published once the reported LWG issue gets a number).

- The standard feature test macro __cpp_lib_simd is not defined yet.

Tests:

- Full coverage requires testing
  1. constexpr,
  2. constant-propagating inputs, and
  3. unknown (to the optimizer) inputs
  - for all vectorizable types
  * for every supported width (1–64 and higher)
  + for all possible ISA extensions (combinations)
  = with different fast-math flags
  ... leading to a test matrix that's far out of reach for regular
  testsuite builds.

- The tests in testsuite/std/simd/ try to cover all of the API. The
  tests can be build in every combination listed above. Per default only
  a small subset is built and tested.

- Use GCC_TEST_RUN_EXPENSIVE=something to compile the more expensive
  tests (constexpr and const-prop testing) and to enable more /
  different widths for the test type.

- Tests can still emit bogus -Wpsabi warnings (see PR98734) which are
  filtered out via dg-prune-output.

Benchmarks:

- The current implementation has been benchmarked in some aspects on
  x86_64 hardware. There is more optimization potential. However, it is
  not always clear whether optimizations should be part of the library
  if they can be implemented in the compiler.

- No benchmark code is included in this patch.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add simd headers.
	* include/Makefile.in: Regenerate.
	* include/bits/version.def (simd): New.
	* include/bits/version.h: Regenerate.
	* include/bits/simd_alg.h: New file.
	* include/bits/simd_details.h: New file.
	* include/bits/simd_flags.h: New file.
	* include/bits/simd_iterator.h: New file.
	* include/bits/simd_loadstore.h: New file.
	* include/bits/simd_mask.h: New file.
	* include/bits/simd_mask_reductions.h: New file.
	* include/bits/simd_reductions.h: New file.
	* include/bits/simd_vec.h: New file.
	* include/bits/simd_x86.h: New file.
	* include/bits/vec_ops.h: New file.
	* include/std/simd: New file.
	* testsuite/std/simd/arithmetic.cc: New test.
	* testsuite/std/simd/arithmetic_expensive.cc: New test.
	* testsuite/std/simd/create_tests.h: New file.
	* testsuite/std/simd/creation.cc: New test.
	* testsuite/std/simd/creation_expensive.cc: New test.
	* testsuite/std/simd/loads.cc: New test.
	* testsuite/std/simd/loads_expensive.cc: New test.
	* testsuite/std/simd/mask2.cc: New test.
	* testsuite/std/simd/mask2_expensive.cc: New test.
	* testsuite/std/simd/mask.cc: New test.
	* testsuite/std/simd/mask_expensive.cc: New test.
	* testsuite/std/simd/reductions.cc: New test.
	* testsuite/std/simd/reductions_expensive.cc: New test.
	* testsuite/std/simd/shift_left.cc: New test.
	* testsuite/std/simd/shift_left_expensive.cc: New test.
	* testsuite/std/simd/shift_right.cc: New test.
	* testsuite/std/simd/shift_right_expensive.cc: New test.
	* testsuite/std/simd/simd_alg.cc: New test.
	* testsuite/std/simd/simd_alg_expensive.cc: New test.
	* testsuite/std/simd/sse_intrin.cc: New test.
	* testsuite/std/simd/stores.cc: New test.
	* testsuite/std/simd/stores_expensive.cc: New test.
	* testsuite/std/simd/test_setup.h: New file.
	* testsuite/std/simd/traits_common.cc: New test.
	* testsuite/std/simd/traits_impl.cc: New test.
	* testsuite/std/simd/traits_math.cc: New test.

 libstdc++-v3/include/Makefile.am              |   12 +
 libstdc++-v3/include/Makefile.in              |   12 +
 libstdc++-v3/include/bits/simd_alg.h          |   98 +
 libstdc++-v3/include/bits/simd_details.h      | 1394 ++++++++++
 libstdc++-v3/include/bits/simd_flags.h        |  187 ++
 libstdc++-v3/include/bits/simd_iterator.h     |  177 ++
 libstdc++-v3/include/bits/simd_loadstore.h    |  408 +++
 libstdc++-v3/include/bits/simd_mask.h         | 1972 ++++++++++++++
 .../include/bits/simd_mask_reductions.h       |  118 +
 libstdc++-v3/include/bits/simd_reductions.h   |  109 +
 libstdc++-v3/include/bits/simd_vec.h          | 2297 +++++++++++++++++
 libstdc++-v3/include/bits/simd_x86.h          | 1413 ++++++++++
 libstdc++-v3/include/bits/vec_ops.h           |  606 +++++
 libstdc++-v3/include/bits/version.def         |   13 +
 libstdc++-v3/include/bits/version.h           |    9 +
 libstdc++-v3/include/std/simd                 |   48 +
 libstdc++-v3/testsuite/std/simd/arithmetic.cc |  329 +++
 .../std/simd/arithmetic_expensive.cc          |    7 +
 .../testsuite/std/simd/create_tests.h         |   15 +
 libstdc++-v3/testsuite/std/simd/creation.cc   |   69 +
 .../testsuite/std/simd/creation_expensive.cc  |    7 +
 libstdc++-v3/testsuite/std/simd/loads.cc      |  121 +
 .../testsuite/std/simd/loads_expensive.cc     |    7 +
 libstdc++-v3/testsuite/std/simd/mask.cc       |  112 +
 libstdc++-v3/testsuite/std/simd/mask2.cc      |  108 +
 .../testsuite/std/simd/mask2_expensive.cc     |    7 +
 .../testsuite/std/simd/mask_expensive.cc      |    7 +
 libstdc++-v3/testsuite/std/simd/reductions.cc |   90 +
 .../std/simd/reductions_expensive.cc          |    7 +
 libstdc++-v3/testsuite/std/simd/shift_left.cc |   67 +
 .../std/simd/shift_left_expensive.cc          |    7 +
 .../testsuite/std/simd/shift_right.cc         |   91 +
 .../std/simd/shift_right_expensive.cc         |    7 +
 libstdc++-v3/testsuite/std/simd/simd_alg.cc   |  137 +
 .../testsuite/std/simd/simd_alg_expensive.cc  |    7 +
 libstdc++-v3/testsuite/std/simd/sse_intrin.cc |   42 +
 libstdc++-v3/testsuite/std/simd/stores.cc     |   67 +
 .../testsuite/std/simd/stores_expensive.cc    |    7 +
 libstdc++-v3/testsuite/std/simd/test_setup.h  |  809 ++++++
 .../testsuite/std/simd/traits_common.cc       |  710 +++++
 .../testsuite/std/simd/traits_impl.cc         |  185 ++
 .../testsuite/std/simd/traits_math.cc         |   62 +
 42 files changed, 11957 insertions(+)
 create mode 100644 libstdc++-v3/include/bits/simd_alg.h
 create mode 100644 libstdc++-v3/include/bits/simd_details.h
 create mode 100644 libstdc++-v3/include/bits/simd_flags.h
 create mode 100644 libstdc++-v3/include/bits/simd_iterator.h
 create mode 100644 libstdc++-v3/include/bits/simd_loadstore.h
 create mode 100644 libstdc++-v3/include/bits/simd_mask.h
 create mode 100644 libstdc++-v3/include/bits/simd_mask_reductions.h
 create mode 100644 libstdc++-v3/include/bits/simd_reductions.h
 create mode 100644 libstdc++-v3/include/bits/simd_vec.h
 create mode 100644 libstdc++-v3/include/bits/simd_x86.h
 create mode 100644 libstdc++-v3/include/bits/vec_ops.h
 create mode 100644 libstdc++-v3/include/std/simd
 create mode 100644 libstdc++-v3/testsuite/std/simd/arithmetic.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/arithmetic_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/create_tests.h
 create mode 100644 libstdc++-v3/testsuite/std/simd/creation.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/creation_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/loads.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/loads_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/mask.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/mask2.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/mask2_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/mask_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/reductions.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/reductions_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/shift_left.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/shift_left_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/shift_right.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/shift_right_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/simd_alg.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/simd_alg_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/sse_intrin.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/stores.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/stores_expensive.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/test_setup.h
 create mode 100644 libstdc++-v3/testsuite/std/simd/traits_common.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/traits_impl.cc
 create mode 100644 libstdc++-v3/testsuite/std/simd/traits_math.cc

 std::simd
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