icf: compare alloca position for testing equivalence

This fixes an incorrect detection of identical code involving alloca
ordering.

Previously for allocas, we only checked if the allocated type, operands
and alignment were the same. However this doesn't take into account that
two identical alloca instructions at different relative points in the
function can not be taken for the same.

Fixed by adding a map that keeps track of allocas that take place at
relatively the same position, and using that map to check alloca
equivalence.

llvm/include/llvm/Cheerp/IdenticalCodeFolding.h

@@ -68,6 +68,7 @@ class IdenticalCodeFolding
 	std::unordered_map<std::pair<const llvm::Function*, const llvm::Function*>, bool, pair_hash> functionEquivalence;
 	std::unordered_map<const llvm::Function*, bool> externalFunctionMapping;
 	std::vector<llvm::Function*> deleteList;
+	std::unordered_map<const llvm::AllocaInst*, const llvm::AllocaInst*> allocaPairs;
 };
 
 class HashAccumulator64 {

llvm/lib/CheerpUtils/IdenticalCodeFolding.cpp

@@ -92,6 +92,8 @@ bool IdenticalCodeFolding::equivalentFunction(const llvm::Function* A, const llv
 	llvm::errs() << "function A: " << A->getName() << '\n';
 	llvm::errs() << "function B: " << B->getName() << '\n';
 #endif
+	// Clear up equivalent allocas of previous function
+	allocaPairs.clear();
 
 	// Do not fold wasm/asmjs with generic JS functions.
 	if (A->getSection() != StringRef("asmjs") || B->getSection() != StringRef("asmjs"))
@@ -232,9 +234,20 @@ bool IdenticalCodeFolding::equivalentInstruction(const llvm::Instruction* A, con
 		{
 			const AllocaInst* a = cast<AllocaInst>(A);
 			const AllocaInst* b = cast<AllocaInst>(B);
-			return CacheAndReturn(equivalentType(a->getAllocatedType(), b->getAllocatedType()) &&
+
+			auto it = allocaPairs.find(a);
+			if (it != allocaPairs.end() && it->second != b)
+				return CacheAndReturn(false);
+
+			const bool equivalent =  equivalentType(a->getAllocatedType(), b->getAllocatedType()) &&
 				equivalentOperand(a->getArraySize(), b->getArraySize()) &&
-				a->getAlign() == b->getAlign());
+				a->getAlign() == b->getAlign();
+
+			if (equivalent) {
+				allocaPairs[a] = b;
+				allocaPairs[b] = a;
+			}
+			return CacheAndReturn(equivalent);
 		}
 		case Instruction::Unreachable:
 		{