[PATCH] D54174: [GlobalISel] LegalizationArtifactCombiner: Combine aext([asz]ext x) -> [asz]ext x

[Volkan Keles via Phabricator via llvm-commits]

volkan added a comment.

In https://reviews.llvm.org/D54174#1291716, @dsanders wrote:

> What's the reason for not putting this in a post-legalize combiner? It seems to me that we'll have to implement this in the Legalizer and the Combiner


The main reason is generating a better code by doing this in the legalizer. If we don't handle this in the legalizer, it would be really hard to combine
in a post-legalize combine as LegalizationArtifactCombiner replaces the pattern with shifts and ands.
Here is an example:

  %0:_(s32) = COPY $w0
  %1:_(s1) = G_TRUNC %0(s32)
  %2:_(s8) = G_SEXT %1(s1)
  %3:_(s32) = G_ANYEXT %2(s8)
  $w0 = COPY %3(s32)

Currently, we generate:

  %0:_(s32) = COPY $w0
  %13:_(s32) = G_CONSTANT i32 7
  %4:_(s8) = G_TRUNC %13(s32)
  %10:_(s32) = COPY %0(s32)
  %16:_(s32) = G_CONSTANT i32 255
  %17:_(s32) = COPY %13(s32)
  %11:_(s32) = G_AND %17, %16
  %12:_(s32) = G_SHL %10, %11
  %18:_(s32) = G_CONSTANT i32 24
  %19:_(s32) = COPY %12(s32)
  %20:_(s32) = G_SHL %19, %18
  %7:_(s32) = G_ASHR %20, %18
  %14:_(s32) = G_CONSTANT i32 255
  %15:_(s32) = COPY %13(s32)
  %8:_(s32) = G_AND %15, %14
  %9:_(s32) = G_ASHR %7, %8
  %3:_(s32) = COPY %9(s32)
  $w0 = COPY %3(s32)

With https://reviews.llvm.org/D54174 applied, we generate:

  %0:_(s32) = COPY $w0
  %4:_(s32) = G_CONSTANT i32 31
  %5:_(s32) = COPY %0(s32)
  %6:_(s32) = G_SHL %5, %4
  %3:_(s32) = G_ASHR %6, %4
  $w0 = COPY %3(s32)


https://reviews.llvm.org/D54174