[LICM] Invalidate cached SCEV results in hoistMulAddAssociation

While reassociating expressions, LICM is required to invalidate SCEV
results, as otherwise subsequent passes in the pipeline that leverage
LICM foldings (e.g. IndVars), may reason on invalid expressions; thus
miscompiling. This is achieved by rewriting the reassociable
instruction from scratch.

Fixes: #91957.

Author: antoniofrighetto

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -2751,16 +2751,28 @@ static bool hoistMulAddAssociation(Instruction &I, Loop &L,
   IRBuilder<> Builder(Preheader->getTerminator());
   for (auto *U : Changes) {
     assert(L.isLoopInvariant(U->get()));
-    Instruction *Ins = cast<Instruction>(U->getUser());
+    auto *Ins = cast<BinaryOperator>(U->getUser());
     Value *Mul;
     if (I.getType()->isIntOrIntVectorTy()) {
       Mul = Builder.CreateMul(U->get(), Factor, "factor.op.mul");
       // Drop the poison flags on the original multiply.
       Ins->dropPoisonGeneratingFlags();
     } else
       Mul = Builder.CreateFMulFMF(U->get(), Factor, Ins, "factor.op.fmul");
-    U->set(Mul);
+
+    // Rewrite the reassociable instruction.
+    unsigned OpIdx = U->getOperandNo();
+    auto *LHS = OpIdx == 0 ? Mul : Ins->getOperand(0);
+    auto *RHS = OpIdx == 1 ? Mul : Ins->getOperand(1);
+    auto *NewBO = BinaryOperator::Create(Ins->getOpcode(), LHS, RHS,
+                                         Ins->getName() + ".reass", Ins);
+    NewBO->copyIRFlags(Ins);
+    if (VariantOp == Ins)
+      VariantOp = NewBO;
+    Ins->replaceAllUsesWith(NewBO);
+    eraseInstruction(*Ins, SafetyInfo, MSSAU);
   }
+
   I.replaceAllUsesWith(VariantOp);
   eraseInstruction(I, SafetyInfo, MSSAU);
   return true;

llvm/test/Transforms/LICM/update-scev-after-hoist.ll

@@ -3,11 +3,11 @@
 define i16 @main() {
 ; SCEV-EXPR:      Classifying expressions for: @main
 ; SCEV-EXPR-NEXT:  %mul = phi i16 [ 1, %entry ], [ %mul.n.3, %loop ]
-; SCEV-EXPR-NEXT:  -->  %mul U: full-set S: [-32768,32753)		Exits: 4096		LoopDispositions: { %loop: Variant }
+; SCEV-EXPR-NEXT:  -->  %mul U: [0,-15) S: [-32768,32753)		Exits: 4096		LoopDispositions: { %loop: Variant }
 ; SCEV-EXPR-NEXT:  %div = phi i16 [ 32767, %entry ], [ %div.n.3, %loop ]
 ; SCEV-EXPR-NEXT:  -->  %div U: [-2048,-32768) S: [-2048,-32768)		Exits: 7		LoopDispositions: { %loop: Variant }
-; SCEV-EXPR-NEXT:  %mul.n = mul i16 %mul, 8
-; SCEV-EXPR-NEXT:  -->  (2 * %mul) U: [0,-1) S: [-32768,32767)		Exits: 8192		LoopDispositions: { %loop: Variant }
+; SCEV-EXPR-NEXT:  %mul.n.reass.reass = mul i16 %mul, 8
+; SCEV-EXPR-NEXT:  -->  (8 * %mul) U: [0,-7) S: [-32768,32761)		Exits: -32768		LoopDispositions: { %loop: Variant }
 entry:
   br label %loop