[clang] [Clang] Optimize some `getBeginLoc` implementations (PR #141058)

[Aaron Ballman via cfe-commits]

================
@@ -2901,34 +2907,44 @@ class CallExpr : public Expr {
   //
   // * An optional of type FPOptionsOverride.
   //
-  // Note that we store the offset in bytes from the this pointer to the start
-  // of the trailing objects. It would be perfectly possible to compute it
-  // based on the dynamic kind of the CallExpr. However 1.) we have plenty of
-  // space in the bit-fields of Stmt. 2.) It was benchmarked to be faster to
-  // compute this once and then load the offset from the bit-fields of Stmt,
-  // instead of re-computing the offset each time the trailing objects are
-  // accessed.
+  // CallExpr subclasses are asssumed to be 32 bytes or less, and CallExpr
+  // itself is 24 bytes. To avoid having to recompute or store the offset of the
+  // trailing objects, we put it at 32 bytes (such that it is suitable for all
+  // subclasses) We use the 8 bytes gap left for instances of CallExpr to store
+  // the begin and end source locations. Caching the begin source location in
+  // particular as a significant impact on perf as getBeginLoc is assumed to be
+  // cheap.
+  // The layourt is as follow:
+  // CallExpr | Begin | End | Trailing Objects
+  // CXXMemberCallExpr | Trailing Objects
+  // A bit in CallExprBitfields indicates if source locations are presents.
 
+protected:
+  static constexpr unsigned offsetToTrailingObjects = 32;
----------------
AaronBallman wrote:

```suggestion
  static constexpr unsigned OffsetToTrailingObjects = 32;
```

https://github.com/llvm/llvm-project/pull/141058