[llvm] Add the "initializes" attribute inference (PR #97373)

[Jan Voung via llvm-commits]

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@@ -866,9 +1067,132 @@ static bool addAccessAttr(Argument *A, Attribute::AttrKind R) {
   return true;
 }
 
+static bool inferInitializes(Argument &A, Function &F) {
+  DenseMap<const BasicBlock *, UsesPerBlockInfo> UsesPerBlock;
+  auto [HasAnyWrite, HasWriteOutsideEntryBB] =
+      CollectArgumentUsesPerBlock(A, F, UsesPerBlock);
+  // No write anywhere in the function, bail.
+  if (!HasAnyWrite)
+    return false;
+
+  BasicBlock &EntryBB = F.getEntryBlock();
+  DenseMap<const BasicBlock *, ConstantRangeList> Initialized;
+  auto VisitBlock = [&](const BasicBlock *BB) -> ConstantRangeList {
+    auto UPB = UsesPerBlock.find(BB);
+
+    // If this block has uses and none are writes, the argument is not
+    // initialized in this block.
+    if (UPB != UsesPerBlock.end() && !UPB->second.HasWrites)
+      return ConstantRangeList();
+
+    ConstantRangeList CRL;
+
+    // Start with intersection of successors.
+    // If this block has any clobbering use, we're going to clear out the
+    // ranges at some point in this block anyway, so don't bother looking at
+    // successors.
+    if (UPB == UsesPerBlock.end() || !UPB->second.HasClobber) {
+      bool HasAddedSuccessor = false;
+      for (auto *Succ : successors(BB)) {
+        if (auto SuccI = Initialized.find(Succ); SuccI != Initialized.end()) {
+          if (HasAddedSuccessor) {
+            CRL = CRL.intersectWith(SuccI->second);
+          } else {
+            CRL = SuccI->second;
+            HasAddedSuccessor = true;
+          }
+        } else {
+          CRL = ConstantRangeList();
+          break;
+        }
+      }
+    }
+
+    if (UPB != UsesPerBlock.end()) {
+      // Sort uses in this block by instruction order.
+      SmallVector<std::pair<Instruction *, ArgumentAccessInfo>, 2> Insts;
+      append_range(Insts, UPB->second.Insts);
+      sort(Insts, [](std::pair<Instruction *, ArgumentAccessInfo> &LHS,
+                     std::pair<Instruction *, ArgumentAccessInfo> &RHS) {
+        return LHS.first->comesBefore(RHS.first);
+      });
+
+      // From the end of the block to the beginning of the block, set
+      // initializes ranges.
+      for (auto [_, Info] : reverse(Insts)) {
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jvoung wrote:

Avoid copying `Info` (auto &[...] ?)

https://github.com/llvm/llvm-project/pull/97373