CVS commit: src/common/lib/libc/arch/x86_64/atomic

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Tue Jul 16 22:45:10 UTC 2024

Modified Files:
        src/common/lib/libc/arch/x86_64/atomic: atomic.S

Log Message:
amd64: Fix performance regression in uniprocessor atomics/membars.

Back in 2022, I eliminated the MFENCE hotpatch in membar_sync because
it's essentially always more expensive than LOCK ADD with no benefit
for CPU/CPU store-before-load ordering.  (It is relevant only for
non-temporal stores or write-combining memory.)

https://mail-index.netbsd.org/source-changes/2022/07/30/msg140047.html

But in that change, I made a mistake and _also_ eliminated the LOCK
hotpatch on uniprocessor amd64.  And our assembler gas helpfully
interprets uppercase LOCK just like lowercase lock and assembles them
the same way, so I didn't notice.

This change restores the LOCK hotpatch, so that when booting on a
uniprocessor system (or a uniprocessor guest on a multicore host),
the LOCK prefix is replaced by NOP for a cheaper instruction.

Found by puzzling over how my explanation for PR kern/57199 could
possibly be correct when (on an amd64 guest) ddb x/i membar_sync kept
showing the lock prefix even in uniprocessor boots.


To generate a diff of this commit:
cvs rdiff -u -r1.30 -r1.31 src/common/lib/libc/arch/x86_64/atomic/atomic.S

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.