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

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon 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
----------------
goldstein.w.n wrote:
> 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.
@nikic Are you intending to look at this regression in a followup?


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

https://reviews.llvm.org/D151807