[PATCH] D33338: [InstCombineCasts] Take in account final size when transforming sext->lshr->trunc patterns

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

Here are some tests for the smaller patterns that we might want to match:

  ; FIXME: The bool bit got smeared across a wide val, but then we zero'd out those bits. This is just a zext.
  
  define i16 @bool_zext(i1 %x) {
    %sext = sext i1 %x to i16
    %hibit = lshr i16 %sext, 15
    ret i16 %hibit
  }
  
  define <2 x i8> @bool_zext_splat(<2 x i1> %x) {
    %sext = sext <2 x i1> %x to <2 x i8>
    %hibit = lshr <2 x i8> %sext, <i8 7, i8 7>
    ret <2 x i8> %hibit
  }
  
  ; FIXME: We could use a narrow arithmetic shift first and then zext.
  
  define i16 @smear_sign_and_widen(i4 %x) {
    %sext = sext i4 %x to i16
    %hibit = lshr i16 %sext, 12
    ret i16 %hibit
  }
  
  define <2 x i8> @smear_sign_and_widen_splat(<2 x i6> %x) {
    %sext = sext <2 x i6> %x to <2 x i8>
    %hibit = lshr <2 x i8> %sext, <i8 2, i8 2>
    ret <2 x i8> %hibit
  }
  
  ; FIXME: All of the replicated sign bits are wiped out by the lshr. This could be lshr+zext.
  
  define i16 @fake_sext(i3 %x) {
    %sext = sext i3 %x to i16
    %sh = lshr i16 %sext, 15
    ret i16 %sh
  }
  
  define <2 x i8> @fake_sext_splat(<2 x i3> %x) {
    %sext = sext <2 x i3> %x to <2 x i8>
    %sh = lshr <2 x i8> %sext, <i8 7, i8 7>
    ret <2 x i8> %sh
  }


https://reviews.llvm.org/D33338