[webkit-changes] [WebKit/WebKit] 3abcc5: [JSC] Fix conflict when newValue is eax in AtomicS...

[Yijia Huang]

  Branch: refs/heads/main
  Home:   https://github.com/WebKit/WebKit
  Commit: 3abcc59cf54142dbdfaccaf3fe4d9fc91a1a3b0b
      https://github.com/WebKit/WebKit/commit/3abcc59cf54142dbdfaccaf3fe4d9fc91a1a3b0b
  Author: Yijia Huang <yijia_huang at apple.com>
  Date:   2024-07-30 (Tue, 30 Jul 2024)

  Changed paths:
    A JSTests/stress/atomics-sub.js
    M Source/JavaScriptCore/assembler/MacroAssemblerX86Common.h

  Log Message:
  -----------
  [JSC] Fix conflict when newValue is eax in AtomicStrongCAS for X86_64
https://bugs.webkit.org/show_bug.cgi?id=276777
rdar://132482715

Reviewed by Keith Miller.

The cmpxchg instruction requires the `al` register (lower 8 bits of `eax`)
for the comparison and also updates the `al` register based on the
comparison result.

To perform the cmpxchg operation correctly in `AtomicStrongCAS`, the
`expectedAndResult` value must be in the `al` register. After the operation,
the result (either the original memory value or the new swapped value) must
be stored back in `expectedAndResult`.

The current implementation of `AtomicStrongCAS` already handles the cases where
`expectedAndResult`, `address.base`, or `address.index` might be aliases of `eax`.
This patch ensures proper handling when `newValue` is `eax` to avoid register
conflicts during the `AtomicStrongCAS` operations.

[1] https://www.felixcloutier.com/x86/cmpxchg

* JSTests/stress/atomics-sub.js: Added.
* Source/JavaScriptCore/assembler/MacroAssemblerX86Common.h:
(JSC::MacroAssemblerX86Common::atomicStrongCAS8):
(JSC::MacroAssemblerX86Common::atomicStrongCAS16):
(JSC::MacroAssemblerX86Common::atomicStrongCAS32):
(JSC::MacroAssemblerX86Common::branchAtomicStrongCAS8):
(JSC::MacroAssemblerX86Common::branchAtomicStrongCAS16):
(JSC::MacroAssemblerX86Common::branchAtomicStrongCAS32):

Canonical link: https://commits.webkit.org/281565@main



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