This patch addresses an incorrect transformation in the DAG combiner.

The included test case is derived from one of the GCC compatibility tests.
The problem arises after the selection DAG has been converted to type-legalized
form.  The combiner first sees a 64-bit load that can be converted into a
pre-increment form.  The original load feeds into a SRL that isolates the
upper 32 bits of the loaded doubleword.  This looks like an opportunity for
DAGCombiner::ReduceLoadWidth() to replace the 64-bit load with a 32-bit load.

However, this transformation is not valid, as the replacement load is not
a pre-increment load.  The pre-increment load produces an extra result,
which feeds a subsequent add instruction.  The replacement load only has
one result value, and this value is propagated to all uses of the pre-
increment load, including the add.  Because the add is looking for the
second result value as its operand, it ends up attempting to add a constant
to a token chain, resulting in a crash.

So the patch simply disables this transformation for any load with more than
two result values.

llvm-svn: 172480

1 files changed, 15 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 359c4cf8e4c..a82410ae6a0 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5100,16 +5100,26 @@ SDValue DAGCombiner::ReduceLoadWidth(SDNode *N) {
 
   // If we haven't found a load, we can't narrow it.  Don't transform one with
   // multiple uses, this would require adding a new load.
-  if (!isa<LoadSDNode>(N0) || !N0.hasOneUse() ||
-      // Don't change the width of a volatile load.
-      cast<LoadSDNode>(N0)->isVolatile())
+  if (!isa<LoadSDNode>(N0) || !N0.hasOneUse())
+    return SDValue();
+
+  // Don't change the width of a volatile load.
+  LoadSDNode *LN0 = cast<LoadSDNode>(N0);
+  if (LN0->isVolatile())
     return SDValue();
 
   // Verify that we are actually reducing a load width here.
-  if (cast<LoadSDNode>(N0)->getMemoryVT().getSizeInBits() < EVTBits)
+  if (LN0->getMemoryVT().getSizeInBits() < EVTBits)
+    return SDValue();
+
+  // For the transform to be legal, the load must produce only two values
+  // (the value loaded and the chain).  Don't transform a pre-increment
+  // load, for example, which produces an extra value.  Otherwise the 
+  // transformation is not equivalent, and the downstream logic to replace
+  // uses gets things wrong.
+  if (LN0->getNumValues() > 2)
     return SDValue();
 
-  LoadSDNode *LN0 = cast<LoadSDNode>(N0);
   EVT PtrType = N0.getOperand(1).getValueType();
 
   if (PtrType == MVT::Untyped || PtrType.isExtended())