[llvm] [DAGCombiner] Preserve nneg flag from inner zext when we combine (z/s/aext (zext X)) (PR #82199)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Craig Topper (topperc)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/82199.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+13-6) 
- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (+17-6) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2a09e44e192979..91ed3d9bd69317 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13710,8 +13710,12 @@ SDValue DAGCombiner::visitZERO_EXTEND(SDNode *N) {
 
   // fold (zext (zext x)) -> (zext x)
   // fold (zext (aext x)) -> (zext x)
-  if (N0.getOpcode() == ISD::ZERO_EXTEND || N0.getOpcode() == ISD::ANY_EXTEND)
-    return DAG.getNode(ISD::ZERO_EXTEND, DL, VT, N0.getOperand(0));
+  if (N0.getOpcode() == ISD::ZERO_EXTEND || N0.getOpcode() == ISD::ANY_EXTEND) {
+    SDNodeFlags Flags;
+    if (N0.getOpcode() == ISD::ZERO_EXTEND)
+      Flags.setNonNeg(N0->getFlags().hasNonNeg());
+    return DAG.getNode(ISD::ZERO_EXTEND, DL, VT, N0.getOperand(0), Flags);
+  }
 
   // fold (zext (aext_extend_vector_inreg x)) -> (zext_extend_vector_inreg x)
   // fold (zext (zext_extend_vector_inreg x)) -> (zext_extend_vector_inreg x)
@@ -13985,10 +13989,13 @@ SDValue DAGCombiner::visitANY_EXTEND(SDNode *N) {
   // fold (aext (aext x)) -> (aext x)
   // fold (aext (zext x)) -> (zext x)
   // fold (aext (sext x)) -> (sext x)
-  if (N0.getOpcode() == ISD::ANY_EXTEND  ||
-      N0.getOpcode() == ISD::ZERO_EXTEND ||
-      N0.getOpcode() == ISD::SIGN_EXTEND)
-    return DAG.getNode(N0.getOpcode(), DL, VT, N0.getOperand(0));
+  if (N0.getOpcode() == ISD::ANY_EXTEND || N0.getOpcode() == ISD::ZERO_EXTEND ||
+      N0.getOpcode() == ISD::SIGN_EXTEND) {
+    SDNodeFlags Flags;
+    if (N0.getOpcode() == ISD::ZERO_EXTEND)
+      Flags.setNonNeg(N0->getFlags().hasNonNeg());
+    return DAG.getNode(N0.getOpcode(), DL, VT, N0.getOperand(0), Flags);
+  }
 
   // fold (aext (aext_extend_vector_inreg x)) -> (aext_extend_vector_inreg x)
   // fold (aext (zext_extend_vector_inreg x)) -> (zext_extend_vector_inreg x)
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 421bb516ad242f..add92cf8b31e44 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5715,8 +5715,12 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
                                   N1.getValueType().getVectorElementCount()) &&
            "Vector element count mismatch!");
     assert(N1.getValueType().bitsLT(VT) && "Invalid sext node, dst < src!");
-    if (OpOpcode == ISD::SIGN_EXTEND || OpOpcode == ISD::ZERO_EXTEND)
-      return getNode(OpOpcode, DL, VT, N1.getOperand(0));
+    if (OpOpcode == ISD::SIGN_EXTEND || OpOpcode == ISD::ZERO_EXTEND) {
+      SDNodeFlags Flags;
+      if (OpOpcode == ISD::ZERO_EXTEND)
+        Flags.setNonNeg(N1->getFlags().hasNonNeg());
+      return getNode(OpOpcode, DL, VT, N1.getOperand(0), Flags);
+    }
     if (OpOpcode == ISD::UNDEF)
       // sext(undef) = 0, because the top bits will all be the same.
       return getConstant(0, DL, VT);
@@ -5732,8 +5736,11 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
                                   N1.getValueType().getVectorElementCount()) &&
            "Vector element count mismatch!");
     assert(N1.getValueType().bitsLT(VT) && "Invalid zext node, dst < src!");
-    if (OpOpcode == ISD::ZERO_EXTEND) // (zext (zext x)) -> (zext x)
-      return getNode(ISD::ZERO_EXTEND, DL, VT, N1.getOperand(0));
+    if (OpOpcode == ISD::ZERO_EXTEND) { // (zext (zext x)) -> (zext x)
+      SDNodeFlags Flags;
+      Flags.setNonNeg(N1->getFlags().hasNonNeg());
+      return getNode(ISD::ZERO_EXTEND, DL, VT, N1.getOperand(0), Flags);
+    }
     if (OpOpcode == ISD::UNDEF)
       // zext(undef) = 0, because the top bits will be zero.
       return getConstant(0, DL, VT);
@@ -5769,9 +5776,13 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     assert(N1.getValueType().bitsLT(VT) && "Invalid anyext node, dst < src!");
 
     if (OpOpcode == ISD::ZERO_EXTEND || OpOpcode == ISD::SIGN_EXTEND ||
-        OpOpcode == ISD::ANY_EXTEND)
+        OpOpcode == ISD::ANY_EXTEND) {
+      SDNodeFlags Flags;
+      if (OpOpcode == ISD::ZERO_EXTEND)
+        Flags.setNonNeg(N1->getFlags().hasNonNeg());
       // (ext (zext x)) -> (zext x)  and  (ext (sext x)) -> (sext x)
-      return getNode(OpOpcode, DL, VT, N1.getOperand(0));
+      return getNode(OpOpcode, DL, VT, N1.getOperand(0), Flags);
+    }
     if (OpOpcode == ISD::UNDEF)
       return getUNDEF(VT);
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/82199