[PATCH] D117680: [InstCombine] Fold bswap(shl(x, C)) -> and(x, 255)

[Sanjay Patel via Phabricator via llvm-commits]

spatel added reviewers: lebedev.ri, craig.topper.
spatel added a comment.

This is one of several related patterns where we mask/shift a single byte from an end of the input:
https://alive2.llvm.org/ce/z/dtHMVr

  define i32 @bs_shl32(i32 %0) {
    %2 = shl i32 %0, 24
    %3 = call i32 @llvm.bswap.i32(i32 %2)
    ret i32 %3
  }
  
  define i32 @bs_lshr32(i32 %0) {
    %2 = lshr i32 %0, 24
    %3 = call i32 @llvm.bswap.i32(i32 %2)
    ret i32 %3
  }
  
  define i32 @bs_and32_000000ff(i32 %x) {
    %m = and i32 %x, 255
    %b = call i32 @llvm.bswap.i32(i32 %m)
    ret i32 %b
  }
  
  define i32 @bs_and32_ff000000(i32 %x) {
    %m = and i32 %x, -16777216
    %b = call i32 @llvm.bswap.i32(i32 %m)
    ret i32 %b
  }

It might help to see a larger, motivating example in source or IR, so we know what a potentially more general solution would look like.
The backend also misses these patterns, so we may want to mimic whatever we do in IR for codegen. I recently added a similar fold with D117508 <https://reviews.llvm.org/D117508>.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D117680/new/

https://reviews.llvm.org/D117680