[PATCH] D19661: [X86] Also try to zero elts when lowering v32i8 shuffle with PSHUFB.

[Sanjay Patel via llvm-commits]

spatel added inline comments.

================
Comment at: lib/Target/X86/X86ISelLowering.cpp:11427-11445
@@ -11426,9 +11426,21 @@
 
-  if (isSingleInputShuffleMask(Mask)) {
-    // There are no generalized cross-lane shuffle operations available on i8
-    // element types.
-    if (is128BitLaneCrossingShuffleMask(MVT::v32i8, Mask))
-      return lowerVectorShuffleAsLanePermuteAndBlend(DL, MVT::v32i8, V1, V2,
-                                                     Mask, DAG);
+  SmallBitVector Zeroable = computeZeroableShuffleElements(Mask, V1, V2);
+  bool SingleInputMask = true;
+  bool SingleInputAndZeroesMask = true;
+  for (int i = 0, Size = Mask.size(); i < Size; ++i) {
+    if (Mask[i] >= Size) {
+      SingleInputMask = false;
+      if (!Zeroable[i]) {
+        SingleInputAndZeroesMask = false;
+        break;
+      }
+    }
+  }
+  // There are no generalized cross-lane shuffle operations available on i8
+  // element types.
+  if (SingleInputMask && is128BitLaneCrossingShuffleMask(MVT::v32i8, Mask))
+    return lowerVectorShuffleAsLanePermuteAndBlend(DL, MVT::v32i8, V1, V2, Mask,
+                                                   DAG);
 
+  if (SingleInputAndZeroesMask) {
     SDValue PSHUFBMask[32];
----------------
ab wrote:
> I wanted to do that at first, but you need to look at the operands, and at that point you basically duplicated computeZeroableShuffleElements.
> 
> An alternative I considered was to do some kind of computeZeroableShuffleMask(Mask, V1, V2, NewMask), which returns a mask with SM_SentinelZero and SM_SentinelUndef. Then, users can check that it isSingleInputShuffleMask and do their thing.  WDYT?
That would be good - slow down the code explosion for x86 vector lowering.
But I don't think we have to hold up this patch for that; a TODO comment should be ok.

LGTM, but I'll let Simon give the final word because he has a much better understanding of everything going on with shuffles.


http://reviews.llvm.org/D19661