Bug 1595222 - Don't define an unused |AtomicReturnReg64| on x86. r=lth
authorJeff Walden <jwalden@mit.edu>
Tue, 12 Nov 2019 13:43:31 +0000
changeset 501596 283981b64bead149add4f1c730fe78b69968dd7d
parent 501595 71e2a5b608a5a1cb02597993cdc26d8166edc082
child 501597 e7b53d25d5d49ae470eaf3c8eeabeb2ca27808f7
push id114172
push userdluca@mozilla.com
push dateTue, 19 Nov 2019 11:31:10 +0000
treeherdermozilla-inbound@b5c5ba07d3db [default view] [failures only]
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Bug 1595222 - Don't define an unused |AtomicReturnReg64| on x86. r=lth Differential Revision: https://phabricator.services.mozilla.com/D52424
--- a/js/src/jit/shared/AtomicOperations-shared-jit.cpp
+++ b/js/src/jit/shared/AtomicOperations-shared-jit.cpp
@@ -130,19 +130,17 @@ static constexpr Register AtomicVal2Reg 
 static constexpr Register AtomicTemp = edx;
 // 64-bit registers for cmpxchg8b.  ValReg/Val2Reg/Temp are not used in this
 // case.
 static constexpr Register64 AtomicValReg64(edx, eax);
 static constexpr Register64 AtomicVal2Reg64(ecx, ebx);
-// At the time of writing, ReturnReg64 is not edx:eax, but it is what our C/C++
-// compilers use.
-static constexpr Register64 AtomicReturnReg64(edx, eax);
+// AtomicReturnReg64 is unused on x86.
 #  error "Unsupported platform"
 // These are useful shorthands and hide the meaningless uint/int distinction.
 static constexpr Scalar::Type SIZE8 = Scalar::Uint8;
@@ -427,19 +425,20 @@ static uint32_t GenCmpxchg(MacroAssemble
     case SIZE64:
       GenGpr64Arg(masm, &iter, AtomicValReg64);
       GenGpr64Arg(masm, &iter, AtomicVal2Reg64);
 #if defined(JS_CODEGEN_X86)
       MOZ_ASSERT(AtomicValReg64 == Register64(edx, eax));
       MOZ_ASSERT(AtomicVal2Reg64 == Register64(ecx, ebx));
+      // The return register edx:eax is a compiler/ABI assumption that is *not*
+      // the same as ReturnReg64, so it's correct not to use that here.
       masm.lock_cmpxchg8b(edx, eax, ecx, ebx, Operand(addr));
-      MOZ_ASSERT(AtomicReturnReg64 == Register64(edx, eax));
       masm.compareExchange64(sync, addr, AtomicValReg64, AtomicVal2Reg64,
       MOZ_CRASH("Unknown size");