[llvm] [SelectionDAG] Deal with POISON for INSERT_VECTOR_ELT/INSERT_SUBVECTOR (part 1) (PR #143102)

[Simon Pilgrim via llvm-commits]

================
@@ -7968,11 +7987,37 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     if (VT == N2VT)
       return N2;
 
-    // If this is an insert of an extracted vector into an undef vector, we
-    // can just use the input to the extract.
+    // If this is an insert of an extracted vector into an undef/poison vector,
+    // we can just use the input to the extract. But not if skipping the
+    // extract+insert could make the result more poisonous.
     if (N1.isUndef() && N2.getOpcode() == ISD::EXTRACT_SUBVECTOR &&
-        N2.getOperand(1) == N3 && N2.getOperand(0).getValueType() == VT)
-      return N2.getOperand(0);
+        N2.getOperand(1) == N3 && N2.getOperand(0).getValueType() == VT) {
+      if (N1.getOpcode() == ISD::POISON)
+        return N2.getOperand(0);
+      if (VT.isFixedLengthVector() && N2VT.isFixedLengthVector()) {
+        unsigned LoBit = N3->getAsZExtVal();
+        unsigned HiBit = LoBit + N2VT.getVectorNumElements();
+        APInt EltMask =
+            APInt::getBitsSet(VT.getVectorNumElements(), LoBit, HiBit);
+        if (isGuaranteedNotToBePoison(N2.getOperand(0), ~EltMask))
+          return N2.getOperand(0);
+      } else if (isGuaranteedNotToBePoison(N2.getOperand(0)))
+        return N2.getOperand(0);
+    }
+
+    // If the inserted subvector is UNDEF, just use the input vector.
+    // But not if skipping the insert could make the result more poisonous.
+    if (N2.isUndef()) {
+      if (VT.isFixedLengthVector()) {
+        unsigned LoBit = N3->getAsZExtVal();
+        unsigned HiBit = LoBit + N2VT.getVectorNumElements();
+        APInt EltMask =
+            APInt::getBitsSet(VT.getVectorNumElements(), LoBit, HiBit);
+        if (isGuaranteedNotToBePoison(N1, EltMask))
+          return N1;
+      } else if (isGuaranteedNotToBePoison(N1))
+        return N1;
+    }
----------------
RKSimon wrote:

again - are we doing too much in getNode at this stage?

https://github.com/llvm/llvm-project/pull/143102