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

[Yingwei Zheng via llvm-commits]

================
@@ -258,3 +259,73 @@ define double @different_types_powi(double %x, i32 %y, i64 %z) {
   %mul = fmul reassoc double %p2, %p1
   ret double %mul
 }
+
+define double @fdiv_pow_powi(double %x) {
+; CHECK-LABEL: @fdiv_pow_powi(
+; CHECK-NEXT:    [[DIV:%.*]] = fmul reassoc nnan double [[X:%.*]], [[X]]
+; CHECK-NEXT:    ret double [[DIV]]
+;
+  %p1 = call double @llvm.powi.f64.i32(double %x, i32 3)
+  %div = fdiv reassoc nnan double %p1, %x
+  ret double %div
+}
+
+define float @fdiv_powf_powi(float %x) {
+; CHECK-LABEL: @fdiv_powf_powi(
+; CHECK-NEXT:    [[DIV:%.*]] = call reassoc nnan float @llvm.powi.f32.i32(float [[X:%.*]], i32 99)
+; CHECK-NEXT:    ret float [[DIV]]
+;
+  %p1 = call float @llvm.powi.f32.i32(float %x, i32 100)
+  %div = fdiv reassoc nnan float %p1, %x
+  ret float %div
+}
+
+; TODO: Multi-use may be also better off creating Powi(x,y-1) then creating
+; (mul, Powi(x,y-1),x) to replace the Powi(x,y).
+define double @fdiv_pow_powi_multi_use(double %x) {
+; CHECK-LABEL: @fdiv_pow_powi_multi_use(
+; CHECK-NEXT:    [[P1:%.*]] = call double @llvm.powi.f64.i32(double [[X:%.*]], i32 3)
+; CHECK-NEXT:    [[DIV:%.*]] = fdiv reassoc nnan double [[P1]], [[X]]
+; CHECK-NEXT:    tail call void @use(double [[P1]])
+; CHECK-NEXT:    ret double [[DIV]]
+;
+  %p1 = call double @llvm.powi.f64.i32(double %x, i32 3)
+  %div = fdiv reassoc nnan double %p1, %x
+  tail call void @use(double %p1)
+  ret double %div
+}
+
+; Negative test: Miss part of the fmf flag for the fdiv instruction
+define float @fdiv_powf_powi_missing_reassoc(float %x) {
+; CHECK-LABEL: @fdiv_powf_powi_missing_reassoc(
+; CHECK-NEXT:    [[P1:%.*]] = call float @llvm.powi.f32.i32(float [[X:%.*]], i32 100)
+; CHECK-NEXT:    [[DIV:%.*]] = fdiv nnan float [[P1]], [[X]]
+; CHECK-NEXT:    ret float [[DIV]]
+;
+  %p1 = call float @llvm.powi.f32.i32(float %x, i32 100)
+  %div = fdiv nnan float %p1, %x
+  ret float %div
+}
+
+define float @fdiv_powf_powi_missing_nnan(float %x) {
+; CHECK-LABEL: @fdiv_powf_powi_missing_nnan(
+; CHECK-NEXT:    [[P1:%.*]] = call float @llvm.powi.f32.i32(float [[X:%.*]], i32 100)
+; CHECK-NEXT:    [[DIV:%.*]] = fdiv reassoc float [[P1]], [[X]]
+; CHECK-NEXT:    ret float [[DIV]]
+;
+  %p1 = call float @llvm.powi.f32.i32(float %x, i32 100)
+  %div = fdiv reassoc float %p1, %x
+  ret float %div
+}
+
+; Negative test: Inllegal because (Y - 1) wraparound
----------------
dtcxzyw wrote:

```suggestion
; Negative test: Illegal because (Y - 1) wraparound
```
typo

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