[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();
+
+    if (It->first.Offset != 0)
+      return indicatePessimisticFixpoint();
+
+    uint64_t OffsetEnd = It->first.Offset + It->first.Size;
----------------
vidsinghal wrote:

I was thinking of a case where, you have only one bin and it is offset from the start of the allocation. 
For instance. 
```
struct foo {
   int a; 
   int b; 
   int c; 
}
```

if only c is used and other fields are dead, then the offset would not be 0 even though the #bins are 1?

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