[llvm-dev] Splitting up Type.h: Good idea, bad idea?

[John McCall via llvm-dev]

On 7 Apr 2020, at 13:27, Reid Kleckner wrote:
> Hello Clang folks,

Why is this on llvm-dev instead of cfe-dev?

> I was using -ftime-trace to see where the compiler spends time parsing
> clang's own headers, and it pointed me to Type.h. Many AST headers 
> need
> QualType to be complete, but they do not need the full type class
> hierarchy. To improve compile time, I have attempted to create a new
> header, QualType.h, that defines only the QualType wrapper class. I 
> have
> started to transition popular clang headers (Decl.h, Expr.h, 
> ASTContext.h,
> etc) to just use QualType.h. I got pretty far with this, and pushed 
> the
> results to github:
> https://github.com/llvm/llvm-project/compare/master...rnk:split-qual-type-prune
>
> You can see it is pretty involved / invasive. Many inline methods had 
> to be
> sunk to cpp files, and this may affect performance. So far, I have 
> nothing
> to show for it in build time gains, because there are still too many 
> Type.h
> includes.

How many translation units actually don’t ultimately need the type
definitions?  Because this achieves nothing if almost every translation
unit ends up including Type.h anyway.

John.