BumpPtrAllocator: Add a couple of fancy wrapper methods around identifyObject().

[NoQ]

Summary

So far all 5 uses of the API introduced in D51393 looked exactly like the proposed idenfityKnownAlignedObject().

[NoQ]

D54488 is the patch that makes use of the new API.

[george.karpenkov]

LGTM, but I would drop the word "convenient", and add an explicit \return section in doxygen.

[NoQ]

Update comments.

llvm/trunk/include/llvm/Support/Allocator.h

for (size_t Idx = 0, E = CustomSizedSlabs.size(); Idx < E; Idx++) {
       size_t Size = CustomSizedSlabs[Idx].second;
       if (P >= S && P < S + Size)
         return InCustomSizedSlabIdx - static_cast<int64_t>(P - S);
       InCustomSizedSlabIdx -= static_cast<int64_t>(Size);
     }
     return None;
   }
 
+  /// A wrapper around identifyObject that additionally asserts that
+  /// the object is indeed within the allocator.
+  /// \return An index uniquely and reproducibly identifying
+  /// an input pointer \p Ptr in the given allocator.
+  int64_t identifyKnownObject(const void *Ptr) {
+    Optional<int64_t> Out = identifyObject(Ptr);
+    assert(Out && "Wrong allocator used");
+    return *Out;
+  }
+
+  /// A wrapper around identifyKnownObject. Accepts type information
+  /// about the object and produces a smaller identifier by relying on
+  /// the alignment information. Note that sub-classes may have different
+  /// alignment, so the most base class should be passed as template parameter
+  /// in order to obtain correct results. For that reason automatic template
+  /// parameter deduction is disabled.
+  /// \return An index uniquely and reproducibly identifying
+  /// an input pointer \p Ptr in the given allocator. This identifier is
+  /// different from the ones produced by identifyObject and
+  /// identifyAlignedObject.
+  template <typename T>
+  int64_t identifyKnownAlignedObject(const void *Ptr) {
+    int64_t Out = identifyKnownObject(Ptr);
+    assert(Out % alignof(T) == 0 && "Wrong alignment information");
+    return Out / alignof(T);
+  }
+
   size_t getTotalMemory() const {
     size_t TotalMemory = 0;
     for (auto I = Slabs.begin(), E = Slabs.end(); I != E; ++I)
       TotalMemory += computeSlabSize(std::distance(Slabs.begin(), I));
     for (auto &PtrAndSize : CustomSizedSlabs)
       TotalMemory += PtrAndSize.second;
     return TotalMemory;
   }