[llvm] Simplify hot-path size computations in BumpPtrAllocator. (PR #101312)

[Nikita Popov via llvm-commits]

================
@@ -149,21 +149,21 @@ class BumpPtrAllocatorImpl
     // Keep track of how many bytes we've allocated.
     BytesAllocated += Size;
 
-    size_t Adjustment = offsetToAlignedAddr(CurPtr, Alignment);
-    assert(Adjustment + Size >= Size && "Adjustment + Size must not overflow");
+    char *AlignedPtr = reinterpret_cast<char *>(alignAddr(CurPtr, Alignment));
----------------
nikic wrote:

We should keep this as an integer, otherwise the addition below may be UB.

The current code was introduced in https://github.com/llvm/llvm-project/commit/44e27464181168faa2b2c4a6e92797ab96215cac by @zmodem. I think it's fine to essentially undo that change, but you should add an explicit check that the AlignedPtr + Size addition does not overflow.

https://github.com/llvm/llvm-project/pull/101312