[llvm] [Attributor] New attribute to identify what byte ranges are alive for an allocation (PR #66148)

[Johannes Doerfert via llvm-commits]

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@@ -12672,6 +12678,238 @@ struct AAAddressSpaceCallSiteArgument final : AAAddressSpaceImpl {
 };
 } // namespace
 
+/// ----------- Allocation Info ----------
+namespace {
+struct AAAllocationInfoImpl : public AAAllocationInfo {
+  AAAllocationInfoImpl(const IRPosition &IRP, Attributor &A)
+      : AAAllocationInfo(IRP, A) {}
+
+  std::optional<TypeSize> getAllocatedSize() const override {
+    assert(isValidState() && "the AA is invalid");
+    return AssumedAllocatedSize;
+  }
+
+  std::optional<TypeSize> findInitialAllocationSize(Instruction *I,
+                                                    const DataLayout &DL) {
+
+    // TODO: implement case for malloc like instructions
+    switch (I->getOpcode()) {
+    case Instruction::Alloca: {
+      AllocaInst *AI = cast<AllocaInst>(I);
+      return AI->getAllocationSize(DL);
+    }
+    default:
+      return std::nullopt;
+    }
+  }
+
+  ChangeStatus updateImpl(Attributor &A) override {
+
+    const IRPosition &IRP = getIRPosition();
+    Instruction *I = IRP.getCtxI();
+
+    // TODO: update check for malloc like calls
+    if (!isa<AllocaInst>(I))
+      return indicatePessimisticFixpoint();
+
+    bool IsKnownNoCapture;
+    if (!AA::hasAssumedIRAttr<Attribute::NoCapture>(
+            A, this, IRP, DepClassTy::OPTIONAL, IsKnownNoCapture))
+      return indicatePessimisticFixpoint();
+
+    // If the pointer escapes, we don't want to change its allocation size.
+    if (!isNonEscapingLocalObject(I))
+      return indicatePessimisticFixpoint();
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jdoerfert wrote:

You already check NoCapture above, why this additional check, I don't see what this adds.

https://github.com/llvm/llvm-project/pull/66148