Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -27262,9 +27262,24 @@
 
     if (V) {
       assert(EltBits == 8 || EltBits == 16 || EltBits == 32);
-      return DAG.getBitcast(
-          VT, DAG.getNode(ISD::SUB, DL, MaskVT,
-                          DAG.getNode(ISD::XOR, DL, MaskVT, V, Mask), Mask));
+      SDValue SubOp1 = DAG.getNode(ISD::XOR, DL, MaskVT, V, Mask);
+      SDValue SubOp2 = Mask;
+
+      // If the negate was on the false side of the select, then
+      // the operands of the SUB need to be swapped. PR 27251.
+      // This is because the pattern being matched above is
+      // (vselect M, (sub (0, X), X)  -> (sub (xor X, M), M)
+      // but if the pattern matched was
+      // (vselect M, X, (sub (0, X))), that is really negation of the pattern
+      // above, -(vselect M, (sub 0, X), X), and therefore the replacement
+      // pattern also needs to be a negation of the replacement pattern above.
+      // And -(sub X, Y) is just sub (Y, X), so swapping the operands of the
+      // sub accomplishes the negation of the replacement pattern.
+      if (V == Y)
+         std::swap(SubOp1, SubOp2);
+
+      return DAG.getBitcast(VT,
+                            DAG.getNode(ISD::SUB, DL, MaskVT, SubOp1, SubOp2));
     }
   }
 
Index: test/CodeGen/X86/vector-blend.ll
===================================================================
--- test/CodeGen/X86/vector-blend.ll
+++ test/CodeGen/X86/vector-blend.ll
@@ -1010,7 +1010,8 @@
 ; SSE2-NEXT:    pslld $31, %xmm1
 ; SSE2-NEXT:    psrad $31, %xmm1
 ; SSE2-NEXT:    pxor %xmm1, %xmm0
-; SSE2-NEXT:    psubd %xmm1, %xmm0
+; SSE2-NEXT:    psubd %xmm0, %xmm1
+; SSE2-NEXT:    movdqa %xmm1, %xmm0
 ; SSE2-NEXT:    retq
 ;
 ; SSSE3-LABEL: blend_neg_logic_v4i32_2:
@@ -1019,7 +1020,8 @@
 ; SSSE3-NEXT:    pslld $31, %xmm1
 ; SSSE3-NEXT:    psrad $31, %xmm1
 ; SSSE3-NEXT:    pxor %xmm1, %xmm0
-; SSSE3-NEXT:    psubd %xmm1, %xmm0
+; SSSE3-NEXT:    psubd %xmm0, %xmm1
+; SSSE3-NEXT:    movdqa %xmm1, %xmm0
 ; SSSE3-NEXT:    retq
 ;
 ; SSE41-LABEL: blend_neg_logic_v4i32_2: