fix the BuildVector -> unpcklps logic to not do pointless shuffles

when the top elements of a vector are undefined.  This happens all
the time for X86-64 ABI stuff because only the low 2 elements of
a 4 element vector are defined.  For example, on:

_Complex float f32(_Complex float A, _Complex float B) {
  return A+B;
}

We used to produce (with SSE2, SSE4.1+ uses insertps):

_f32:                                   ## @f32
	movdqa	%xmm0, %xmm2
	addss	%xmm1, %xmm2
	pshufd	$16, %xmm2, %xmm2
	pshufd	$1, %xmm1, %xmm1
	pshufd	$1, %xmm0, %xmm0
	addss	%xmm1, %xmm0
	pshufd	$16, %xmm0, %xmm1
	movdqa	%xmm2, %xmm0
	unpcklps	%xmm1, %xmm0
	ret

We now produce:

_f32:                                   ## @f32
	movdqa	%xmm0, %xmm2
	addss	%xmm1, %xmm2
	pshufd	$1, %xmm1, %xmm1
	pshufd	$1, %xmm0, %xmm3
	addss	%xmm1, %xmm3
	movaps	%xmm2, %xmm0
	unpcklps	%xmm3, %xmm0
	ret

This implements rdar://8368414


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112378 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 11 insertions, 1 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 91fc26cd1e..8141634165 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -4304,8 +4304,18 @@ X86TargetLowering::LowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG) const {
     //   Step 2: unpcklps X, Y ==>    <3, 2, 1, 0>
     unsigned EltStride = NumElems >> 1;
     while (EltStride != 0) {
-      for (unsigned i = 0; i < EltStride; ++i)
+      for (unsigned i = 0; i < EltStride; ++i) {
+        // If V[i+EltStride] is undef and this is the first round of mixing,
+        // then it is safe to just drop this shuffle: V[i] is already in the
+        // right place, the one element (since it's the first round) being
+        // inserted as undef can be dropped.  This isn't safe for successive
+        // rounds because they will permute elements within both vectors.
+        if (V[i+EltStride].getOpcode() == ISD::UNDEF &&
+            EltStride == NumElems/2)
+          continue;
+        
         V[i] = getUnpackl(DAG, dl, VT, V[i], V[i + EltStride]);
+      }
       EltStride >>= 1;
     }
     return V[0];