[PATCH] D38037: [InstCombine] Compacting or instructions whose operands are shift instructions

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a reviewer: mcrosier.
spatel added a comment.

I haven't looked much at the reassociation pass, but this seems like something that would be simpler to canonicalize there. Did you look at adding a rule there?

Eg, in the "or_or_shift" test, you have a tree of 'or' ops. If you reassociate those so that similar shifts are paired together, instcombine would manage to fold that using existing patterns (although maybe not optimally yet):

  define i32 @or_or_shift(i32 %x) local_unnamed_addr #0  {
    %1 = and i32 %x, 2
    %2 = and i32 %x, 4
    %3 = shl nuw nsw i32 %1, 6
    %4 = lshr exact i32 %1, 1
    %5 = shl nuw nsw i32 %2, 6
    %6 = lshr exact i32 %2, 1
    %7 = or i32 %3, %5    <--- shl with shl
    %8 = or i32 %4, %6    <--- lshr with lshr
    %9 = or i32 %7, %8
    ret i32 %9
  }

$ ./opt -instcombine reass.ll -S

  define i32 @or_or_shift(i32 %x) local_unnamed_addr {
    %1 = shl i32 %x, 6
    %2 = and i32 %1, 384
    %3 = lshr i32 %x, 1
    %4 = and i32 %3, 3
    %5 = or i32 %2, %4
    ret i32 %5
  }


Repository:
  rL LLVM

https://reviews.llvm.org/D38037