CVS commit: src/common/lib/libc/arch/aarch64/atomic

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

Module Name:    src
Committed By:   riastradh
Date:           Sat Aug  6 21:31:33 UTC 2022

Modified Files:
        src/common/lib/libc/arch/aarch64/atomic: Makefile.inc __aarch64_lse.S

Log Message:
aarch64: Implement __aarch64_casN_sync.

gcc generates calls to this symbol in programs that use
__sync_*_compare_and_swap, which require full sequential consistency
barriers, including store-before-load ordering on both sides of the
atomic; none of the release/acquire operations guarantee that, so we
have to insert explicit DMB instructions.

Note: gcc's own definition omits some of the DMB instructions, but I
can't prove that it's correct that way -- stores preceding the CAS
must complete before the load part of the CAS, and the store part of
the CAS must complete before loads following the CAS.  Maybe there's
some way to prove that one of these orderings is guaranteed some
other way than a DMB but I'm not seeing it, and store-before-load
ordering is hard to understand.

Patch by skrll@ based on a patch by mrg@, soliloquy in commit message
by me.


To generate a diff of this commit:
cvs rdiff -u -r1.4 -r1.5 src/common/lib/libc/arch/aarch64/atomic/Makefile.inc
cvs rdiff -u -r1.6 -r1.7 \
    src/common/lib/libc/arch/aarch64/atomic/__aarch64_lse.S

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