[PATCH] D39421: [InstCombine] Extracting common and-mask for shift operands of Or instruction

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

Why do we canonicalize shift-left before 'and'?

Ie, shouldn't we prefer this:

  define i8 @andshl(i8 %x) {
    %and = and i8 %x, 1
    %shl = shl i8 %and, 3
    ret i8 %shl
  }

instead of this:

  define i8 @andshl(i8 %x) {
    %and = shl i8 %x, 3
    %shl = and i8 %and, 8
    ret i8 %shl
  }

...because doing the 'and' before the shift always uses a smaller constant?


https://reviews.llvm.org/D39421