[PATCH] D15981: [InstCombine] insert a new shuffle in a safe place (PR25999)

[Sanjay Patel via llvm-commits]

spatel added a comment.

After thinking about it some more, this doesn't work either.

It will fail if the vector that we're extracting from is defined in the same block:

  define <4 x double> @t(i1 %c, <2 x double> %a, <4 x double> %b) {
  bb1:
    br i1 %c, label %bb2, label %bb3
  
  bb2:
    %r = call <2 x double> @dummy(<2 x double> %a)
    br label %bb3
  
  bb3:
    %tmp1 = phi <2 x double> [ %a, %bb1 ], [ %r, %bb2 ]
    %tmp2 = phi <4 x double> [ %b, %bb1 ], [ zeroinitializer, %bb2 ]
    %d = fadd <2 x double> %tmp1, %tmp1
    %tmp3 = extractelement <2 x double> %d, i32 0
    %tmp4 = insertelement <4 x double> %tmp2, double %tmp3, i32 2
    ret <4 x double> %tmp4
  }
  
  declare <2 x double> @dummy(<2 x double>)

We need a way to insert the shuffle at the later of a safe insertion point and the define of the vector that we're extracting from, but I'm not sure how to code that. This sort of thing must exist somewhere else in LLVM?


http://reviews.llvm.org/D15981