LoadStoreVectorizer: Split even sized illegal chains properly

Implement isLegalToVectorizeLoadChain for AMDGPU to avoid
producing private address spaces accesses that will need to be
split up later. This was doing the wrong thing in the case
where the queried chain was an even number of elements.

A possible <4 x i32> store was being split into
store <2 x i32>
store i32
store i32

rather than
store <2 x i32>
store <2 x i32>

when legal.

llvm-svn: 295933

1 files changed, 6 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp b/llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp
index 20fbcf4533d..4409d7a404f 100644
--- a/llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp
@@ -432,9 +432,12 @@ Vectorizer::splitOddVectorElts(ArrayRef<Instruction *> Chain,
   unsigned ElementSizeBytes = ElementSizeBits / 8;
   unsigned SizeBytes = ElementSizeBytes * Chain.size();
   unsigned NumLeft = (SizeBytes - (SizeBytes % 4)) / ElementSizeBytes;
-  if (NumLeft == Chain.size())
-    --NumLeft;
-  else if (NumLeft == 0)
+  if (NumLeft == Chain.size()) {
+    if ((NumLeft & 1) == 0)
+      NumLeft /= 2; // Split even in half
+    else
+      --NumLeft;    // Split off last element
+  } else if (NumLeft == 0)
     NumLeft = 1;
   return std::make_pair(Chain.slice(0, NumLeft), Chain.slice(NumLeft));
 }