[PATCH] D151807: [InstCombine] Revisit user of newly one-use instructions

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/test/Transforms/InstCombine/or-shifted-masks.ll:80
 define i32 @multiuse2(i32 %x) {
 ; CHECK-LABEL: @multiuse2(
 ; CHECK-NEXT:    [[I:%.*]] = shl i32 [[X:%.*]], 1
----------------
This one seems to keep the worse codegen (full -O3 pipeline):
Before:
```
define i32 @multiuse2(i32 %x) {
  %i = shl i32 %x, 1
  %i6 = shl i32 %x, 8
  %i10 = and i32 %i6, 32256
  %i12 = and i32 %i, 252
  %i13 = or i32 %i10, %i12
  ret i32 %i13
}
```
After:
```
define i32 @multiuse2(i32 %x) {
  %i = shl i32 %x, 1
  %i6 = and i32 %x, 96
  %i7 = shl nuw nsw i32 %i6, 8
  %i8 = shl nuw nsw i32 %i6, 1
  %i14 = shl i32 %x, 8
  %i9 = and i32 %i14, 7680
  %i10 = or i32 %i7, %i9
  %1 = and i32 %i, 60
  %i12 = or i32 %i8, %1
  %i13 = or i32 %i10, %i12
  ret i32 %i13
}
```
Its not a dealbreaker imo, but maybe leave a TODO on the fold the implements it indicating the combine is missing a case.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D151807/new/

https://reviews.llvm.org/D151807