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

[Haojian Wu via cfe-commits]

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@@ -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
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hokein wrote:

If I understand correctly, with this patch we’ll now allocate 32 bytes for each `CallExpr` subclass instance. This slightly increases AST memory usage (24 =>32) for `CXXMemberCallExpr` and `CUDAKernelCallExpr`. That’s probably fine -- from my observation, `CXXMemberCallExpr` accounts for ~2% of all AST nodes.

The 32-byte seems like a strong assumption. It won’t be sufficient if we move to 64-bit source locations. `CXXOperatorCallExpr` grows from 32 to at least 40 bytes because it stores a SourceRange (coincidentally, I’ve been revisiting 64-bit source locations recently).

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