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

[Simon Pilgrim via llvm-commits]

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@@ -7923,23 +7923,42 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     // INSERT_VECTOR_ELT into out-of-bounds element is an UNDEF, except
     // for scalable vectors where we will generate appropriate code to
     // deal with out-of-bounds cases correctly.
-    if (N3C && N1.getValueType().isFixedLengthVector() &&
-        N3C->getZExtValue() >= N1.getValueType().getVectorNumElements())
+    if (N3C && VT.isFixedLengthVector() &&
+        N3C->getZExtValue() >= VT.getVectorNumElements())
       return getUNDEF(VT);
 
     // Undefined index can be assumed out-of-bounds, so that's UNDEF too.
     if (N3.isUndef())
       return getUNDEF(VT);
 
-    // If the inserted element is an UNDEF, just use the input vector.
-    if (N2.isUndef())
+    // If inserting poison, just use the input vector.
+    if (N2.getOpcode() == ISD::POISON)
       return N1;
 
+    // Inserting undef into undef/poison is still undef.
+    if (N2.getOpcode() == ISD::UNDEF && N1.isUndef())
+      return getUNDEF(VT);
+
+    // If the inserted element is an UNDEF, just use the input vector.
+    // But not if skipping the insert could make the result more poisonous.
+    if (N2.isUndef()) {
+      if (N3C && VT.isFixedLengthVector()) {
+        APInt EltMask =
+            APInt::getOneBitSet(VT.getVectorNumElements(), N3C->getZExtValue());
+        if (isGuaranteedNotToBePoison(N1, EltMask))
+          return N1;
+      } else if (isGuaranteedNotToBePoison(N1))
+        return N1;
+    }
----------------
RKSimon wrote:

How many regressions do we get it we drop all this from getNode() and just leave the combine to do it?

We've often ended up doing more then basic canonicalisation in getNode() just to avoid one specific test regression.....

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