[PATCH] D118264: [DAG] SelectionDAG::getNode(N1,N2) - detect N2 constant vector splats as well as scalars

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added a comment.

What we might have done is uncovered an existing inconsistency with or with undef folding: https://gcc.godbolt.org/z/e6aPfe3Ye which effects later folds, such as the bitcasting (bitcast <32 x i16> to i512).

  define i64 @xx(<2 x i64> %a0) {
    %s = sub <2 x i64> %a0, %a0
    %r = or <2 x i64> <i64 0, i64 undef>, %s
    %x = extractelement <2 x i64> %r, i32 1
    ret i64 %x
  }
  
  instcombine:
  define i64 @xx(<2 x i64> %a0) {
    ret i64 undef
  }
  
  llc:
  xx:
          movq    $-1, %rax
          retq


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  https://reviews.llvm.org/D118264/new/

https://reviews.llvm.org/D118264