Fix PR12359

- In addition to undefined, if V2 is zero vector, skip 2nd PSHUFB and POR as
  well as PSHUFB will zero elements with negative indices.

  Patch by Sriram Murali <sriram.murali@intel.com>

llvm-svn: 163018

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -5881,8 +5881,6 @@ SDValue LowerVECTOR_SHUFFLEv16i8(ShuffleVectorSDNode *SVOp,
   DebugLoc dl = SVOp->getDebugLoc();
   ArrayRef<int> MaskVals = SVOp->getMask();
 
-  bool V2IsUndef = V2.getOpcode() == ISD::UNDEF;
-
   // If we have SSSE3, case 1 is generated when all result bytes come from
   // one of  the inputs.  Otherwise, case 2 is generated.  If no SSSE3 is
   // present, fall back to case 3.
@@ -5906,7 +5904,11 @@ SDValue LowerVECTOR_SHUFFLEv16i8(ShuffleVectorSDNode *SVOp,
     V1 = DAG.getNode(X86ISD::PSHUFB, dl, MVT::v16i8, V1,
                      DAG.getNode(ISD::BUILD_VECTOR, dl,
                                  MVT::v16i8, &pshufbMask[0], 16));
-    if (V2IsUndef)
+
+    // As PSHUFB will zero elements with negative indices, it's safe to ignore
+    // the 2nd operand if it's undefined or zero.
+    if (V2.getOpcode() == ISD::UNDEF ||
+        ISD::isBuildVectorAllZeros(V2.getNode()))
       return V1;
 
     // Calculate the shuffle mask for the second input, shuffle it, and

llvm/test/CodeGen/X86/pr12359.ll

@@ -0,0 +1,10 @@
+; RUN: llc -asm-verbose -mtriple=x86_64-unknown-unknown -mcpu=corei7 < %s | FileCheck %s
+define <16 x i8> @shuf(<16 x i8> %inval1) {
+entry:
+  %0 = shufflevector <16 x i8> %inval1, <16 x i8> zeroinitializer, <16 x i32> <i32 0, i32 4, i32 3, i32 2, i32 16, i32 16, i32 3, i32 4, i32 0, i32 4, i32 3, i32 2, i32 16, i32 16, i32 3, i32 4>
+  ret <16 x i8> %0
+; CHECK: shuf
+; CHECK: # BB#0: # %entry
+; CHECK-NEXT: pshufb
+; CHECK-NEXT: ret
+}