Re: [PATCH] constexprify some tree variables

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jan 26, 2023 at 03:51:07PM +0100, Jakub Jelinek via Gcc-patches wrote:
> On Thu, Jan 26, 2023 at 09:45:35AM -0500, Patrick Palka via Gcc-patches wrote:
> > > -extern const unsigned char tree_code_length[];
> > > +
> > > +#define DEFTREECODE(SYM, NAME, TYPE, LENGTH) LENGTH,
> > > +#define END_OF_BASE_TREE_CODES 0,
> > > +constexpr unsigned char tree_code_length[] = {
> > > +#include "all-tree.def"
> > > +};
> > > +
> > > +#undef DEFTREECODE
> > > +#undef END_OF_BASE_TREE_CODES
> > 
> > IIUC defining these globals as non-inline constexpr gives them internal
> > linkage, and so each TU contains its own unique copy of these globals.
> > This bloats cc1plus by a tiny bit and is technically an ODR violation
> > because some inline functions such as tree_class_check also ODR-use
> > these variables and so each defn of tree_class_check will refer to a
> > "different" tree_code_class.  Since inline variables are a C++17
> > feature, I guess we could fix this by defining the globals the old way
> > before C++17 and as inline constexpr otherwise?
> 
> Agreed, just use
> __cpp_inline_variables >= 201606L
> to select between the old and new ways.

And I'd argue with the tiny bit.
In my x86_64-linux cc1plus from today, I see 193 _ZL16tree_code_length vars,
374 bytes each, and 324 _ZL14tree_code_type vars, 1496 bytes each.
So, that means waste of 555016 .rodata bytes, plus being highly non-cache
friendly.

	Jakub