fixup! Add atomics support to asmjs

llvm/lib/CheerpWriter/CheerpWriter.cpp

@@ -4872,13 +4872,19 @@ void CheerpWriter::compileAtomicCmpXchg(const AtomicCmpXchgInst& ai, PARENT_PRIO
 	if (shift == 0)
 		shiftPrio = LOWEST;
 
-	// We need to use the compare operand twice, so we load it into the second register first.
-	StringRef reg2 = namegen.getName(&ai, 1);
-	stream << "(" << reg2 << "=";
-	// We end up using compileOperand on the cmpOp twice. But this is ok, since we make sure
-	// that a compare operand to an AtomicCmpXchg instruction is never inlineable.
-	compileOperand(cmpOp, BIT_OR);
-	stream << "|0),";
+	// Compare Exchange can be called even if there are no uses, because it has side effects. In
+	// this specific case, we do not (and cannot) assign the result to a value and do the comparison.
+	StringRef reg2;
+	if (!ai.use_empty())
+	{
+		// We need to use the compare operand twice, so we load it into the second register first.
+		reg2 = namegen.getName(&ai, 1);
+		stream << "(" << reg2 << "=";
+		// We end up using compileOperand on the cmpOp twice. But this is ok, since we make sure
+		// that a compare operand to an AtomicCmpXchg instruction is never inlineable.
+		compileOperand(cmpOp, BIT_OR);
+		stream << "|0),";
+	}
 	// Now we compile the main atomic compare exchange function.
 	stream << namegen.getBuiltinName(NameGenerator::Builtin::ATOMICCMPXCHG) << "(";
 	if (t->isIntegerTy(1) || t->isIntegerTy(8))
@@ -4894,13 +4900,24 @@ void CheerpWriter::compileAtomicCmpXchg(const AtomicCmpXchgInst& ai, PARENT_PRIO
 	compileRawPointer(ptrOp, shiftPrio);
 	if (shift != 0)
 		stream << ">>" << shift;
-	stream << "," << reg2 << "|0,";
+	stream << ",";
+	// If this instruction has uses, the compare operand has been stored in the second register
+	// for this instruction. Otherwise, use the compare operand directly.
+	if (!ai.use_empty())
+		stream << reg2;
+	else
+		compileOperand(cmpOp, BIT_OR);
+	stream << "|0,";
 	compileOperand(newValOp, BIT_OR);
-	stream << "|0)|0;" << NewLine;
+	stream << "|0)|0";
 
 	// Finally compile the second part of this instruction, the comparison between the loaded value
 	// and the compare operand (which is in the second register for this instruction).
-	stream << reg2 << "=(" << namegen.getName(&ai, 0) << "|0)==(" << reg2 << "|0)";
+	if (!ai.use_empty())
+	{
+		stream << ";" << NewLine;
+		stream << reg2 << "=(" << namegen.getName(&ai, 0) << "|0)==(" << reg2 << "|0)";
+	}
 }
 
 CheerpWriter::COMPILE_INSTRUCTION_FEEDBACK CheerpWriter::compileCallInstruction(const CallBase& ci, PARENT_PRIORITY parentPrio)