Bug 1595222 - Don't define an unused |AtomicReturnReg64| on x86. r=lth

Differential Revision: https://phabricator.services.mozilla.com/D52424

--HG--
extra : moz-landing-system : lando

js/src/jit/shared/AtomicOperations-shared-jit.cpp

@@ -135,9 +135,7 @@ static constexpr Register AtomicTemp = edx;
 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.
 
 #else
 #  error "Unsupported platform"
@@ -432,9 +430,10 @@ static uint32_t GenCmpxchg(MacroAssembler& masm, Scalar::Type size,
 #if defined(JS_CODEGEN_X86)
       MOZ_ASSERT(AtomicValReg64 == Register64(edx, eax));
       MOZ_ASSERT(AtomicVal2Reg64 == Register64(ecx, ebx));
-      masm.lock_cmpxchg8b(edx, eax, ecx, ebx, Operand(addr));
 
-      MOZ_ASSERT(AtomicReturnReg64 == Register64(edx, eax));
+      // 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));
 #else
       masm.compareExchange64(sync, addr, AtomicValReg64, AtomicVal2Reg64,
                              AtomicReturnReg64);