Sanjay Patel 750f4c591d [InstCombine] allow peeking through zext of shift amount to match rotate idioms (PR45701)
We might want to also allow trunc of the shift amount, but that seems less likely?

  define i32 @src(i32 %x, i1 %y) {
  %0:
    %rem = and i1 %y, 1
    %cmp = icmp eq i1 %rem, 0
    %sh_prom = zext i1 %rem to i32
    %sub = sub nsw nuw i1 0, %rem
    %sh_prom1 = zext i1 %sub to i32
    %shr = lshr i32 %x, %sh_prom1
    %shl = shl i32 %x, %sh_prom
    %or = or i32 %shl, %shr
    %r = select i1 %cmp, i32 %x, i32 %or
    ret i32 %r
  }
  =>
  define i32 @tgt(i32 %x, i1 %y) {
  %0:
    %t = zext i1 %y to i32
    %r = fshl i32 %x, i32 %x, i32 %t
    ret i32 %r
  }

  Transformation seems to be correct!

https://alive2.llvm.org/ce/z/xgMvE3

http://bugs.llvm.org/PR45701
2020-07-20 16:18:11 -04:00
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This directory contains test cases for the instcombine transformation.  The
dated tests are actual bug tests, whereas the named tests are used to test
for features that the this pass should be capable of performing.