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amd64: finish the tail in memset with an overlapping store

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Instead of finding the exact size to fit in we can just shift the target
by -8 + tail. Doing a blind write to a previously rep stosq'ed area comes
with a penalty so do it conditionally.

Sample win on EPYC when zeroing a 257 sized buffer (tail = 1) aligned to
16 bytes:
before: 44782846 ops/s
after:  46118614 ops/s

Idea stolen from NetBSD.

Sponsored by:	The FreeBSD Foundation
This commit is contained in:
mjg 2018-10-22 06:44:20 +00:00
parent 9ebffec59f
commit ac1eb54956
1 changed files with 6 additions and 3 deletions
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9
sys/amd64/amd64/support.S
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@ -524,9 +524,12 @@ END(memcpy_erms)
rep
stosq
movq %r9,%rax
movq %rdx,%rcx
andb $7,%cl
jne 1004b
andl $7,%edx
jnz 1f
POP_FRAME_POINTER
ret
1:
movq %r10,-8(%rdi,%rdx)
.endif
POP_FRAME_POINTER
ret