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

[Vidhush Singhal via llvm-commits]

================
@@ -12658,6 +12669,268 @@ 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;
+  }
+
+  bool isaMallocInst(Instruction *I) {
+    CallInst *Call = dyn_cast<CallInst>(I);
+    auto CallName = Call->getCalledFunction()->getName();
+    if (CallName.str() == "malloc")
+      return true;
+
+    return false;
+  };
+
+  ChangeStatus updateImpl(Attributor &A) override {
+
+    Instruction *I = getIRPosition().getCtxI();
+
+    const IRPosition &IRP = getIRPosition();
+
+    if (!(isa<AllocaInst>(I) || isaMallocInst(I)))
+      return llvm::IntegerStateBase<bool, true,
+                                    false>::indicatePessimisticFixpoint();
+
+    const AAPointerInfo *PI =
+        A.getOrCreateAAFor<AAPointerInfo>(IRP, *this, DepClassTy::REQUIRED);
+
+    if (!PI)
+      return indicatePessimisticFixpoint();
+
+    if (!PI->getState().isValidState())
+      return indicatePessimisticFixpoint();
+
+    int64_t BinSize = PI->numOffsetBins();
+
+    // TODO: Handle for more than one Bin
+    if (BinSize > 1)
+      return indicatePessimisticFixpoint();
+
+    const auto &It = PI->begin();
+
+    if (BinSize == 0)
+      return indicatePessimisticFixpoint();
----------------
vidsinghal wrote:

Here if the BinSize is 0, then it can mean two things. 
a.) There is no access since its an escaping pointer 
b.) The ptr is not escaping buts its not accessed in which case it should be removed by dead code elimination? 

It is then fair to return indicatePessimisticFixpoint in this case?


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