[DAGCombiner] improve readability

This is not quite NFC because I changed the SDLoc to use the more
standard 'N' (the starting node for the fold).

This transform is a special-case of a more general fold that we
do in IR, but it seems like the general fold is needed here too
to avoid a potential regression seen in D58017.

https://rise4fun.com/Alive/3jZm

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2c2f8fe..c462a6b 100644 (file)
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7922,26 +7922,26 @@ SDValue DAGCombiner::visitSRL(SDNode *N) {
     }
   }
 
-  // fold (srl (trunc (srl x, c1)), c2) -> 0 or (trunc (srl x, (add c1, c2)))
-  // TODO - support non-uniform vector shift amounts.
   if (N1C && N0.getOpcode() == ISD::TRUNCATE &&
       N0.getOperand(0).getOpcode() == ISD::SRL) {
-    if (auto N001C = isConstOrConstSplat(N0.getOperand(0).getOperand(1))) {
+    SDValue InnerShift = N0.getOperand(0);
+    // TODO - support non-uniform vector shift amounts.
+    if (auto *N001C = isConstOrConstSplat(InnerShift.getOperand(1))) {
       uint64_t c1 = N001C->getZExtValue();
       uint64_t c2 = N1C->getZExtValue();
-      EVT InnerShiftVT = N0.getOperand(0).getValueType();
-      EVT ShiftCountVT = N0.getOperand(0).getOperand(1).getValueType();
+      EVT InnerShiftVT = InnerShift.getValueType();
+      EVT ShiftAmtVT = InnerShift.getOperand(1).getValueType();
       uint64_t InnerShiftSize = InnerShiftVT.getScalarSizeInBits();
+      // srl (trunc (srl x, c1)), c2 --> 0 or (trunc (srl x, (add c1, c2)))
       // This is only valid if the OpSizeInBits + c1 = size of inner shift.
       if (c1 + OpSizeInBits == InnerShiftSize) {
-        SDLoc DL(N0);
+        SDLoc DL(N);
         if (c1 + c2 >= InnerShiftSize)
           return DAG.getConstant(0, DL, VT);
-        return DAG.getNode(ISD::TRUNCATE, DL, VT,
-                           DAG.getNode(ISD::SRL, DL, InnerShiftVT,
-                                       N0.getOperand(0).getOperand(0),
-                                       DAG.getConstant(c1 + c2, DL,
-                                                       ShiftCountVT)));
+        SDValue NewShiftAmt = DAG.getConstant(c1 + c2, DL, ShiftAmtVT);
+        SDValue NewShift = DAG.getNode(ISD::SRL, DL, InnerShiftVT,
+                                       InnerShift.getOperand(0), NewShiftAmt);
+        return DAG.getNode(ISD::TRUNCATE, DL, VT, NewShift);
       }
     }
   }