[llvm] [SLP] Bail out on store-to-load forwarding hazards (PR #199606)

[Alexey Bataev via llvm-commits]

================
@@ -27384,6 +27418,142 @@ bool SLPVectorizerPass::runImpl(Function &F, ScalarEvolution *SE_,
   return Changed;
 }
 
+bool BoUpSLP::findStoreLoadForwardingConflict(StoreInst *BaseStore,
+                                              unsigned VF) {
+  if (!BaseStore || !LAIs)
+    return false;
+
+  StoreInst *FirstStore = BaseStore;
+
+  // Cache lookup: avoid re-walking the LAA dependence list when the same
+  // chain is retried at multiple vector factors.
+  auto Key = std::make_pair(FirstStore, VF);
+  auto CacheIt = StlfConflictCache.find(Key);
+  if (CacheIt != StlfConflictCache.end())
+    return CacheIt->second;
+
+  auto CacheAndReturn = [&](bool Result) -> bool {
+    StlfConflictCache[Key] = Result;
+    return Result;
+  };
+
+  Loop *L = LI->getLoopFor(FirstStore->getParent());
+  if (!L)
+    return CacheAndReturn(false);
+
+  Type *ValueTy = FirstStore->getValueOperand()->getType();
+  TypeSize StoreSize = DL->getTypeStoreSize(ValueTy);
+  if (StoreSize.isScalable())
+    return CacheAndReturn(false);
+  uint64_t ElementSize = StoreSize.getFixedValue();
+  if (ElementSize == 0)
+    return CacheAndReturn(false);
+  uint64_t VectorStoreBytes = uint64_t(VF) * ElementSize;
+  LLVM_DEBUG(dbgs() << "SLP: STLF check: VF=" << VF
+                    << " ElementSize=" << ElementSize
+                    << " VectorStoreBytes=" << VectorStoreBytes << "\n");
+
+  // Cheap early-gate: if the loop has no loads from the same underlying
+  // object as the store chain, STLF conflicts are impossible. This avoids
+  // paying for LAA on loops that obviously cannot conflict.
+  Value *StoreBase = getUnderlyingObject(FirstStore->getPointerOperand());
+  auto HasSameBaseLoad = [&]() {
+    for (BasicBlock *BB : L->blocks())
+      for (Instruction &I : *BB)
+        if (auto *LdI = dyn_cast<LoadInst>(&I))
+          if (LdI->isSimple() &&
+              getUnderlyingObject(LdI->getPointerOperand()) == StoreBase)
+            return true;
----------------
alexey-bataev wrote:

```suggestion
        if (auto *LdI = dyn_cast<LoadInst>(&I); LdI && LdI->isSimple() &&
              getUnderlyingObject(LdI->getPointerOperand()) == StoreBase)
            return true;
```

https://github.com/llvm/llvm-project/pull/199606