[DAG] getNode - fold (sext (trunc x)) -> x iff the upper bits are already signbits (#151945)

Similar to what we already do for ZERO_EXTEND/ANY_EXTEND patterns.

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 71a175d..649a310 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -6422,6 +6422,20 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     if (N1.isUndef())
       // sext(undef) = 0, because the top bits will all be the same.
       return getConstant(0, DL, VT);
+
+    // Skip unnecessary sext_inreg pattern:
+    // (sext (trunc x)) -> x iff the upper bits are all signbits.
+    if (OpOpcode == ISD::TRUNCATE) {
+      SDValue OpOp = N1.getOperand(0);
+      if (OpOp.getValueType() == VT) {
+        unsigned NumSignExtBits =
+            VT.getScalarSizeInBits() - N1.getScalarValueSizeInBits();
+        if (ComputeNumSignBits(OpOp) > NumSignExtBits) {
+          transferDbgValues(N1, OpOp);
+          return OpOp;
+        }
+      }
+    }
     break;
   case ISD::ZERO_EXTEND:
     assert(VT.isInteger() && N1.getValueType().isInteger() &&