[llvm] [InstCombine] optimize powi(X,Y) * X with Ofast (PR #69998)

[Matt Arsenault via llvm-commits]

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@@ -571,6 +571,55 @@ Instruction *InstCombinerImpl::foldFPSignBitOps(BinaryOperator &I) {
   return nullptr;
 }
 
+Instruction *InstCombinerImpl::foldPowiReassoc(BinaryOperator &I) {
+  Value *X, *Y, *Z;
+
+  // Make sure all operands have reassoc flag if they are powi.
+  if (!all_of(I.operands(), [](Value *V) {
+        if (match(V, m_Intrinsic<Intrinsic::powi>(m_Value(), m_Value()))) {
+          Instruction *Powi = cast<Instruction>(V);
+          return Powi->hasAllowReassoc();
+        }
+        return true;
+      }))
+    return nullptr;
+
+  auto createPowiExpr = [](BinaryOperator &I, InstCombinerImpl &IC, Value *X,
+                           Value *Y, Value *Z) {
+    Value *YZ;
+    InstCombiner::BuilderTy &Builder = IC.Builder;
+
+    if (auto *C = dyn_cast<ConstantInt>(Z)) {
+      if (C->isOne())
+        YZ = Builder.CreateAdd(Y, ConstantInt::get(Y->getType(), 1));
+    } else
+      YZ = Builder.CreateAdd(Y, Z);
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arsenm wrote:

I don't understand this YZ logic; if you can just do CreateAdd for an arbitrary add you can do it for the constant case. As-is this will by use of undefined value for constants that aren't 1 

https://github.com/llvm/llvm-project/pull/69998