Fix comment describing compare/exchange behavior

Change-Id: Iacc8bd65e9f8ba268dbe7dae6aa244a25c6f770a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29548
Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/Reactor/LLVMReactor.cpp

diff --git a/src/Reactor/LLVMReactor.cpp b/src/Reactor/LLVMReactor.cpp
index c196880..e2a82a0 100644
--- a/src/Reactor/LLVMReactor.cpp
+++ b/src/Reactor/LLVMReactor.cpp
@@ -1496,7 +1496,7 @@
 	Value *Nucleus::createAtomicCompareExchange(Value *ptr, Value *value, Value *compare, std::memory_order memoryOrderEqual, std::memory_order memoryOrderUnequal)
 	{
 		RR_DEBUG_INFO_UPDATE_LOC();
-		// Note: AtomicCmpXchgInstruction returns a 1-member struct containing the result, not the result directly.
+		// Note: AtomicCmpXchgInstruction returns a 2-member struct containing {result, success-flag}, not the result directly.
 		return V(::builder->CreateExtractValue(
 				::builder->CreateAtomicCmpXchg(V(ptr), V(compare), V(value), atomicOrdering(true, memoryOrderEqual), atomicOrdering(true, memoryOrderUnequal)),
 				llvm::ArrayRef<unsigned>(0u)));