[PATCH] D108091: [AggressiveInstCombine] Add shift left instruction to `TruncInstCombine` DAG

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope added a comment.

Seen some failures and I'm suspecting this commit.

For input like this:

  define i32 @foo(i32 %call62, i16 %call63) {
    %conv64142 = zext i32 %call62 to i64
    %conv65 = sext i16 %call63 to i64
    %sh_prom66 = and i64 %conv65, 4294967295
    %shl67 = shl i64 %conv64142, %sh_prom66
    %conv68 = trunc i64 %shl67 to i32
    ret i32 %conv68
  }

we now get the following after aggressive instcombine

  define i32 @foo(i32 %call62, i16 %call63) {
    %conv65 = sext i16 %call63 to i32
    %sh_prom66 = and i32 %conv65, -1
    %shl67 = shl i32 %call62, %sh_prom66
    ret i32 %shl67
  }

which is more poisonous according to https://alive2.llvm.org/ce/z/88tNrs

In my original test case %call63 is 32, so we shift leftby 32, which is ok when shifting the i64 but not after having rewritten into a 32-bit shift.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D108091/new/

https://reviews.llvm.org/D108091