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Add MSVC support for pg_leftmost_one_pos32() and friends

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To allow testing for general support for fast bitscan intrinsics,
add symbols HAVE_BITSCAN_REVERSE and HAVE_BITSCAN_FORWARD.

Also do related cleanup in AllocSetFreeIndex(): Previously, we
tested for HAVE__BUILTIN_CLZ and copied the relevant internals of
pg_leftmost_one_pos32(), with a special fallback that does less
work than the general fallback for that function. Now that we have
a more general test, we just call pg_leftmost_one_pos32() directly
for platforms with intrinsic support. On gcc at least, there is no
difference in the binary for non-assert builds.

Discussion: https://www.postgresql.org/message-id/CAFBsxsEPc%2BFnX_0vmmQ5DHv60sk4rL_RZJ%2BMD6ei%3D76L0kFMvA%40mail.gmail.com
This commit is contained in:
John Naylor
2023-02-08 12:05:58 +07:00
parent 204b0cbecb
commit 6773197464
2 changed files with 70 additions and 11 deletions
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6
src/backend/utils/mmgr/aset.c
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@@ -289,7 +289,7 @@ AllocSetFreeIndex(Size size)
* or equivalently
* pg_leftmost_one_pos32(size - 1) - ALLOC_MINBITS + 1
*
* However, rather than just calling that function, we duplicate the
* However, for platforms without intrinsic support, we duplicate the
* logic here, allowing an additional optimization. It's reasonable
* to assume that ALLOC_CHUNK_LIMIT fits in 16 bits, so we can unroll
* the byte-at-a-time loop in pg_leftmost_one_pos32 and just handle
@@ -299,8 +299,8 @@ AllocSetFreeIndex(Size size)
* much trouble.
*----------
*/
#ifdef HAVE__BUILTIN_CLZ
idx = 31 - __builtin_clz((uint32) size - 1) - ALLOC_MINBITS + 1;
#ifdef HAVE_BITSCAN_REVERSE
idx = pg_leftmost_one_pos32((uint32) size - 1) - ALLOC_MINBITS + 1;
#else
uint32 t,
tsize;