[PATCH] D129890: [SDAG] reduce cast ops around sign_extend_inreg

[Sanjay Patel via Phabricator via llvm-commits]

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There are a pair of large regressions in D127115 <https://reviews.llvm.org/D127115> for Thumb2 caused by not folding this pattern:
trunc (sign_ext_inreg (anyext iN X), iM) to iN --> sign_ext_inreg X to iM

This fold should be enough to resolve those test diffs.

Unfortunately, I haven't found a way to expose this pattern with current SDAG, so there are no tests included. Suggestions welcome if anyone has ideas.


https://reviews.llvm.org/D129890

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp


Index: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13133,6 +13133,16 @@
     return N0.getOperand(0);
   }
 
+  // trunc (sign_ext_inreg (anyext iN X), iM) to iN --> sign_ext_inreg X to iM
+  if (N0.getOpcode() == ISD::SIGN_EXTEND_INREG) {
+    SDValue N00 = N0.getOperand(0);
+    SDValue N01 = N0.getOperand(1);
+    if (N00.getOpcode() == ISD::ANY_EXTEND &&
+        N00.getOperand(0).getValueType() == VT)
+      return DAG.getNode(ISD::SIGN_EXTEND_INREG, SDLoc(N), VT,
+                         N00.getOperand(0), N01);
+  }
+
   // If this is anyext(trunc), don't fold it, allow ourselves to be folded.
   if (N->hasOneUse() && (N->use_begin()->getOpcode() == ISD::ANY_EXTEND))
     return SDValue();


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