Author: mjg
Date: Mon Oct 22 06:44:20 2018
New Revision: 339579
URL: https://svnweb.freebsd.org/changeset/base/339579
Log:
amd64: finish the tail in memset with an overlapping store
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
Modified:
head/sys/amd64/amd64/support.S
Modified: head/sys/amd64/amd64/support.S
==============================================================================
--- head/sys/amd64/amd64/support.S Mon Oct 22 04:12:51 2018
(r339578)
+++ head/sys/amd64/amd64/support.S Mon Oct 22 06:44:20 2018
(r339579)
@@ -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
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