[PATCH] D20774: [InstCombine] look through bitcasts to find selects

[Sanjay Patel via llvm-commits]

spatel added a comment.

It's going to take multiple patches/steps to get this to match in the DAG. Is there any concern about making the following transform in IR:

Before:

  define <2 x i64> @gibson(<2 x i64> %a, <2 x i64> %b) {
    %t0 = bitcast <2 x i64> %a to <4 x i32>
    %t1 = bitcast <2 x i64> %b to <4 x i32>
    %cmp = icmp sgt <4 x i32> %t0, %t1
    %sext = sext <4 x i1> %cmp to <4 x i32>
    %t2 = bitcast <4 x i32> %sext to <2 x i64>
    %and = and <2 x i64> %t2, %a
    %neg = xor <4 x i32> %sext, <i32 -1, i32 -1, i32 -1, i32 -1>
    %neg2 = bitcast <4 x i32> %neg to <2 x i64>
    %and2 = and <2 x i64> %neg2, %b
    %or = or <2 x i64> %and, %and2
    ret <2 x i64> %or
  }

After:

  define <2 x i64> @max(<2 x i64> %a, <2 x i64> %b) {
    %t0 = bitcast <2 x i64> %a to <4 x i32>
    %t1 = bitcast <2 x i64> %b to <4 x i32>
    %cmp = icmp sgt <4 x i32> %t0, %t1
    %or = select <4 x i1> %cmp, <4 x i32> %t0, <4 x i32> %t1
    %r1 = bitcast <4 x i32> %or to <2 x i64>
    ret <2 x i64> %r1
  }

We're trading 4 logic ops, a bitcast, and a sext for a select. Is that better / more canonical IR for all targets?


http://reviews.llvm.org/D20774