[llvm] [SCCP] Support constant structure in PhiNode (PR #163713)

[Yingwei Zheng via llvm-commits]

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@@ -1383,49 +1385,65 @@ bool SCCPInstVisitor::isEdgeFeasible(BasicBlock *From, BasicBlock *To) const {
 // 7. If a conditional branch has a value that is overdefined, make all
 //    successors executable.
 void SCCPInstVisitor::visitPHINode(PHINode &PN) {
-  // If this PN returns a struct, just mark the result overdefined.
-  // TODO: We could do a lot better than this if code actually uses this.
-  if (PN.getType()->isStructTy())
-    return (void)markOverdefined(&PN);
-
-  if (getValueState(&PN).isOverdefined())
-    return; // Quick exit
-
   // Super-extra-high-degree PHI nodes are unlikely to ever be marked constant,
   // and slow us down a lot.  Just mark them overdefined.
   if (PN.getNumIncomingValues() > 64)
     return (void)markOverdefined(&PN);
 
-  unsigned NumActiveIncoming = 0;
+  if (isInstUnderDefined(PN))
+    return;
+  std::vector<unsigned> FeasibleIncomingIndices;
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dtcxzyw wrote:

Use SmallVector

https://github.com/llvm/llvm-project/pull/163713