[DAGCombiner] Don't create illegal narrow stores

Narrowing stores when the target doesn't support the narrow version
forces the target to expand into a load-modify-store sequence, which
is highly suboptimal. The information narrowing throws away (legality
of the inverse transform) is hard to re-analyze. If the target doesn't
support a store of the narrow type, don't narrow even in pre-legalize
mode.

No test as this is DAGCombiner and depends on target bits.

llvm-svn: 370576

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -14737,10 +14737,15 @@ ShrinkLoadReplaceStoreWithStore(const std::pair<unsigned, unsigned> &MaskInfo,
 
   // Check that it is legal on the target to do this.  It is legal if the new
   // VT we're shrinking to (i8/i16/i32) is legal or we're still before type
-  // legalization.
-  MVT VT = MVT::getIntegerVT(NumBytes*8);
+  // legalization (and the target doesn't explicitly think this is a bad idea).
+  MVT VT = MVT::getIntegerVT(NumBytes * 8);
+  const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   if (!DC->isTypeLegal(VT))
     return SDValue();
+  if (St->getMemOperand() &&
+      !TLI.allowsMemoryAccess(*DAG.getContext(), DAG.getDataLayout(), VT,
+                              *St->getMemOperand()))
+    return SDValue();
 
   // Okay, we can do this!  Replace the 'St' store with a store of IVal that is
   // shifted by ByteShift and truncated down to NumBytes.