[llvm] [InstCombine] optimize powi(X,Y)/X with Ofast (PR #67236)

[via llvm-commits]

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@@ -1764,6 +1764,16 @@ Instruction *InstCombinerImpl::visitFDiv(BinaryOperator &I) {
     return replaceInstUsesWith(I, Pow);
   }
 
+  if (I.hasAllowReassoc() && I.hasNoInfs() &&
+      match(Op0, m_OneUse(m_Intrinsic<Intrinsic::powi>(m_Specific(Op1),
+                                                       m_Value(Y))))) {
+    Constant *NegOne = ConstantInt::getAllOnesValue(Y->getType());
+    Value *Y1 = Builder.CreateAdd(Y, NegOne);
+    Type *Types[] = {Op1->getType(), Y1->getType()};
+    Value *Pow = Builder.CreateIntrinsic(Intrinsic::powi, Types, {Op1, Y1}, &I);
+    return replaceInstUsesWith(I, Pow);
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goldsteinn wrote:

> It will crash with **ArrayRef.h:257: Assertion `Index < Length && "Invalid index!"' failed** when I change it to
> 
> ```
> Value *Pow = Builder.CreateBinaryIntrinsic(Intrinsic::powi, Op1, Y1, &I);
> ```

Its properly invalid, I forgot different types :(

https://github.com/llvm/llvm-project/pull/67236