[PATCH] D87080: [AST] Reduce the size of TemplateArgumentLocInfo.

[Haojian Wu via Phabricator via cfe-commits]

hokein added a comment.

> Would it be possible to compile some big file in LLVM (probably doesn't matter much which, Sema something?) and observe if there's a significant change in overall ASTContext size?

~3% saving (measuring the `ASTContext::.getASTAllocatedMemory`)

|              | Before   | After   |
| SemaDecl.cpp | 255.5 MB | 247.5MB |
| SemaExpr.cpp | 293.5 MB | 283.5MB |



================
Comment at: clang/include/clang/AST/TemplateBase.h:43
+// the dependency.
+template <> struct PointerLikeTypeTraits<clang::Expr *> {
+  static inline void *getAsVoidPointer(clang::Expr *P) { return P; }
----------------
sammccall wrote:
> At first glance this is unsafe: you could have two different definitions of the same specialization in different files.
> 
> In fact it's OK, the default one can now never be instantiated, because Expr.h includes this file and so anyone that can see the definition can see this specialization.
> 
> But this is subtle/fragile: at least it needs to be spelled out explicitly in the comment.
> I'd also suggest adding a static assert below the definition of Expr that it is compatible with this specialization (because it is sufficiently aligned).
> 
> (I can't think of a better alternative - use of PointerUnion is a win, partly *because* it validates the alignment)
yes, exactly. 

do you have a better idea on how the static_assert should look like? The way I can think of is to add a new flag in this specialization, and use `static_assert(PointerLikeTypeTraits<clang::Expr *>::flag, "...")`.


Repository:
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  https://reviews.llvm.org/D87080/new/

https://reviews.llvm.org/D87080