[PATCH] libstdc++: Fix find_last_set(simd_mask) to ignore padding bits

[Matthias Kretz]

Ping.

I fear that time to make GCC 11.5 is running out.

-Matthias

On Tuesday, 18 June 2024 08:22:13 GMT+2 Matthias Kretz wrote:
> Tested on x86_64-linux-gnu (also -m32 and -mx32), aarch64-linux-gnu, and
> arm- linux-gnueabi(hf).
> 
> OK for trunk and backports? OK, to go for GCC 11.5 as early as possible?
> 
> ----------------------- 8< -----------------------
> 
> With the change to the AVX512 find_last_set implementation, the change
> to AVX512 operator!= is unnecessary. However, the latter was not
> producing optimal code and unnecessarily set the padding bits. In
> theory, the compiler could determine that with the new !=
> implementation, the bit operation for clearing the padding bits is a
> no-op and can be elided.
> 
> Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
> 
> libstdc++-v3/ChangeLog:
> 
> 	PR libstdc++/115454
> 	* include/experimental/bits/simd_x86.h (_S_not_equal_to): Use
> 	neq comparison instead of bitwise negation after eq.
> 	(_S_find_last_set): Clear unused high bits before computing
> 	bit_width.
> 	* testsuite/experimental/simd/pr115454_find_last_set.cc: New
> 	test.
> ---
>  .../include/experimental/bits/simd_x86.h      | 26 +++++-----
>  .../simd/pr115454_find_last_set.cc            | 49 +++++++++++++++++++
>  2 files changed, 62 insertions(+), 13 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/experimental/simd/
> pr115454_find_last_set.cc
> 
> 
> --
> ──────────────────────────────────────────────────────────────────────────
>  Dr. Matthias Kretz                           https://mattkretz.github.io
>  GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
>  stdₓ::simd
> ──────────────────────────────────────────────────────────────────────────


-- 
──────────────────────────────────────────────────────────────────────────
 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Center for Heavy Ion Research               https://gsi.de
 std::simd
──────────────────────────────────────────────────────────────────────────
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