[PATCH] [DAGCombine] Don't generate atrocious shuffles for constant vectors

[Sanjay Patel]

Hi Michael -

This is almost exactly what I had in mind (and probably Simon did too as he was looking at this sort of problem as well).

But shortly after filing the bug, I realized that we also need to handle non-const splats better too. Eg, this is a simplification of what I'm seeing in loops where the induction variable is splatted in order to vectorize the loop:

  define <4 x i64> @splat(i64 %x) {
    %scalar_to_vector = insertelement <4 x i64> undef, i64 %x, i32 0
    %splat = shufflevector <4 x i64> %scalar_to_vector, <4 x i64> undef, <4 x i32> zeroinitializer
    ret <4 x i64> %splat
  }

Can you loosen the splat check to handle more than just constants? The particular case above seems ok with AVX2 (vbroadcastsd is generated), but AVX1 still shows:

  vmovq	%rdi, %xmm0
  vunpcklpd	%xmm0, %xmm0, %xmm0 ## xmm0 = xmm0[0,0]
  vinsertf128	$1, %xmm0, %ymm0, %ymm0


http://reviews.llvm.org/D7093

EMAIL PREFERENCES
  http://reviews.llvm.org/settings/panel/emailpreferences/