[llvm] [SLP] De-duplicate costing of perfect-diamond gather spills (PR #220569)

[Alexey Bataev via llvm-commits]

================
@@ -19040,6 +19048,73 @@ InstructionCost BoUpSLP::getSpillCost() {
     }
     return nullptr;
   };
+  auto IsCoveredByMatchingVectorEntry = [&](const TreeEntry *Gather,
+                                            const Loop *SpillLoop) -> bool {
+    assert(Gather->isGather());
+
+    Value *LookupValue = nullptr;
+    if (Gather->hasState()) {
+      LookupValue = Gather->getMainOp();
+    } else {
+      auto *It = find_if(Gather->Scalars, [](Value *V) {
+        return !isa<PoisonValue, UndefValue>(V);
+      });
+      if (It == Gather->Scalars.end())
+        return false;
+      LookupValue = *It;
+    }
+
+    // Find the real vector entry reused by this perfect-diamond gather.
+    const TreeEntry *SameTE =
+        getSameValuesTreeEntry(LookupValue, Gather->Scalars, /*SameVF=*/true);
+    if (!SameTE || SameTE == Gather || SameTE->State != TreeEntry::Vectorize ||
+        ScalarOrPseudoEntries.contains(SameTE) || !SameTE->UserTreeIndex)
+      return false;
+
+    // Only permit an ordinary vectorized user.
+    const TreeEntry *UserTE = SameTE->UserTreeIndex.UserTE;
+    if (!UserTE || UserTE->State != TreeEntry::Vectorize ||
+        ScalarOrPseudoEntries.contains(UserTE) ||
+        UserTE->getOpcode() == Instruction::PHI)
+      return false;
+
----------------
alexey-bataev wrote:

```suggestion
    // Different demotion state would make the two edge costs unequal.
    if (MinBWs.contains(SameTE) != MinBWs.contains(Gather))
      return false;
```

https://github.com/llvm/llvm-project/pull/220569