[X86][AVX] lowerShuffleAsLanePermuteAndPermute - fully populate the lane shuffle...

As detailed on PR40730, we are not correctly filling in the lane shuffle mask (D53148/rL344446) - we fill in for the correct src lane but don't add it to the correct mask element, so any reference to the correct element is likely to see an UNDEF mask index.

This allows constant folding to propagate UNDEFs prior to the lane mask being (correctly) lowered to vperm2f128.

This patch fixes the issue by fully populating the lane shuffle mask - this is more than is necessary (if we only filled in the required mask elements we might be able to match other shuffle instructions - broadcasts etc.), but its the most cautious approach as this needs to be cherrypicked into the 8.0.0 release branch.

Differential Revision: https://reviews.llvm.org/D58237

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354117 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 0ecead8b5836251f14c8794919e2c821ed9cb0c5..2fa50635a6adabf19122b497cf9b65ea6e2d6ffa 100644 (file)
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -14081,7 +14081,6 @@ static SDValue lowerShuffleAsLanePermuteAndPermute(
   int NumEltsPerLane = NumElts / NumLanes;
 
   SmallVector<int, 4> SrcLaneMask(NumLanes, SM_SentinelUndef);
-  SmallVector<int, 16> LaneMask(NumElts, SM_SentinelUndef);
   SmallVector<int, 16> PermMask(NumElts, SM_SentinelUndef);
 
   for (int i = 0; i != NumElts; ++i) {
@@ -14096,10 +14095,20 @@ static SDValue lowerShuffleAsLanePermuteAndPermute(
       return SDValue();
     SrcLaneMask[DstLane] = SrcLane;
 
-    LaneMask[i] = (SrcLane * NumEltsPerLane) + (i % NumEltsPerLane);
     PermMask[i] = (DstLane * NumEltsPerLane) + (M % NumEltsPerLane);
   }
 
+  // Make sure we set all elements of the lane mask, to avoid undef propagation.
+  SmallVector<int, 16> LaneMask(NumElts, SM_SentinelUndef);
+  for (int DstLane = 0; DstLane != NumLanes; ++DstLane) {
+    int SrcLane = SrcLaneMask[DstLane];
+    if (0 <= SrcLane)
+      for (int j = 0; j != NumEltsPerLane; ++j) {
+        LaneMask[(DstLane * NumEltsPerLane) + j] =
+            (SrcLane * NumEltsPerLane) + j;
+      }
+  }
+
   // If we're only shuffling a single lowest lane and the rest are identity
   // then don't bother.
   // TODO - isShuffleMaskInputInPlace could be extended to something like this.

diff --git a/test/CodeGen/X86/pr40730.ll b/test/CodeGen/X86/pr40730.ll
index 679812c16f869b66098c4da614506f1954e4167d..12b372dea33bbe1fa1bd3b3887af1feea6f66ff2 100644 (file)
--- a/test/CodeGen/X86/pr40730.ll
+++ b/test/CodeGen/X86/pr40730.ll
@@ -19,7 +19,7 @@ define <8 x i32> @shuffle_v8i32_0dcd3f14(<8 x i32> %a, <8 x i32> %b) {
 ; CHECK:      .LCPI1_0:
 ; CHECK-NEXT: .quad   60129542157
 ; CHECK-NEXT: .quad   60129542157
-; CHECK-NEXT: .zero   8
+; CHECK-NEXT: .quad   68719476736
 ; CHECK-NEXT: .quad   60129542157
 
 define <8 x i32> @shuffle_v8i32_0dcd3f14_constant(<8 x i32> %a0)  {