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

[Matt Arsenault via llvm-commits]

================
@@ -22915,14 +22916,29 @@ SDValue DAGCombiner::visitINSERT_VECTOR_ELT(SDNode *N) {
       InVec == InVal.getOperand(0) && EltNo == InVal.getOperand(1))
     return InVec;
 
-  if (!IndexC) {
-    // If this is variable insert to undef vector, it might be better to splat:
-    // inselt undef, InVal, EltNo --> build_vector < InVal, InVal, ... >
-    if (InVec.isUndef() && TLI.shouldSplatInsEltVarIndex(VT))
-      return DAG.getSplat(VT, DL, InVal);
-    return SDValue();
+  // If this is variable insert to undef vector, it might be better to splat:
+  // inselt undef, InVal, EltNo --> build_vector < InVal, InVal, ... >
+  if (!IndexC && InVec.isUndef() && TLI.shouldSplatInsEltVarIndex(VT))
+    return DAG.getSplat(VT, DL, InVal);
+
+  // Try to drop insert of UNDEF/POISON elements. This is also done in getNode,
+  // but we also do it as a DAG combine since for example simplifications into
+  // SPLAT_VECTOR/BUILD_VECTOR may turn poison elements into undef/zero etc, and
+  // then suddenly the InVec is guaranteed to not be poison.
+  if (InVal.isUndef()) {
+    if (IndexC && VT.isFixedLengthVector()) {
+      APInt EltMask = APInt::getOneBitSet(VT.getVectorNumElements(),
+                                          IndexC->getZExtValue());
+      if (DAG.isGuaranteedNotToBePoison(InVec, EltMask))
+        return InVec;
+    } else if (DAG.isGuaranteedNotToBePoison(InVec)) {
+      return InVec;
+    }
   }
----------------
arsenm wrote:

> (Juggling with stacked pull requests in github is still something that I'm still not very confortable with. It really messes up my local workflow as I can't just "git rebase -i" to move between the patch stack.)

You need to use one of the tools (graphite or spr), these manually stacked with pre-commits are as much of a pain to review as to submit 

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