R600: Bug 20982 - Avoid undefined left shift of negative value

I'm not sure what the hardware actually does, so don't
bother trying to fold it for now.

llvm-svn: 218057

1 files changed, 10 insertions, 3 deletions

diff --git a/llvm/lib/Target/R600/AMDGPUISelLowering.cpp b/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
index 8440c39c18d..89dab098710 100644
--- a/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
@@ -2048,16 +2048,23 @@ SDValue AMDGPUTargetLowering::PerformDAGCombine(SDNode *N,
       return DAG.getZeroExtendInReg(BitsFrom, DL, SmallVT);
     }
 
-    if (ConstantSDNode *Val = dyn_cast<ConstantSDNode>(N->getOperand(0))) {
+    if (ConstantSDNode *CVal = dyn_cast<ConstantSDNode>(N->getOperand(0))) {
       if (Signed) {
+        // Avoid undefined left shift of a negative in the constant fold.
+        // TODO: I'm not sure what the behavior of the hardware is, this should
+        // probably follow that instead.
+        int32_t Val = CVal->getSExtValue();
+        if (Val < 0)
+          return SDValue();
+
         return constantFoldBFE<int32_t>(DAG,
-                                        Val->getSExtValue(),
+                                        Val,
                                         OffsetVal,
                                         WidthVal);
       }
 
       return constantFoldBFE<uint32_t>(DAG,
-                                       Val->getZExtValue(),
+                                       CVal->getZExtValue(),
                                        OffsetVal,
                                        WidthVal);
     }