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

[Matthias Kretz]

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
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