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malloc: Ensure mtag code path in checked_request2size is cold

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This is a workaround (hack) for a gcc optimization issue (PR 99551).
Without this the generated code may evaluate the expression in the
cold path which causes performance regression for small allocations
in the memory tagging disabled (common) case.

Reviewed-by: DJ Delorie <dj@redhat.com>
This commit is contained in:
Szabolcs Nagy 2021-03-15 11:44:32 +00:00
parent 05f878c58e
commit 850dbf24ee
1 changed files with 7 additions and 2 deletions
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9
malloc/malloc.c
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@ -1357,8 +1357,13 @@ checked_request2size (size_t req, size_t *sz) __nonnull (1)
must be a macro that produces a compile time constant if passed
a constant literal. */
if (__glibc_unlikely (mtag_enabled))
req = (req + (__MTAG_GRANULE_SIZE - 1)) &
~(size_t)(__MTAG_GRANULE_SIZE - 1);
{
/* Ensure this is not evaluated if !mtag_enabled, see gcc PR 99551. */
asm ("");
req = (req + (__MTAG_GRANULE_SIZE - 1)) &
~(size_t)(__MTAG_GRANULE_SIZE - 1);
}
*sz = request2size (req);
return true;