Re: [PATCH] unshare expressions in attribute arguments

[Jakub Jelinek <jakub@redhat.com>]

On Mon, Nov 23, 2020 at 10:03:44AM -0700, Martin Sebor wrote:
> > If the most significant bound is lost, why don't you save in the attribute
> > early only the most significant bound before it is lost
> 
> The other bounds are a part of the type so saving them in the attribute
> isn't essential.  I save all of them because it simplifies their lookup.
> With only the most significant bound in the attribute argument, looking
> up the other bounds (e.g., when checking function redeclarations for
> mismatches) will, in addition to doing what's done for the most
> significant bound, involve scanning the declarations' argument lists,
> extracting the bounds from the SAVE_EXPRs, before comparing them.
> As an example, in
> 
>   void f (int A[m][n]);
> 
> the attribute has the chain (VAR_DECL(m), VAR_DECL(n)) as arguments
> and comparing them with another declaration of f is as simple as
> traversing the chain and comparing each node value.
> 
> With the change you suggest, the attribute will only have VAR_DECL(m)
> and the least significant bound will have to be extracted from A[m]'s
> type's size which is:
> 
>   MULT (NOP (bitsizetype, NOP (sizetype, SAVE (VAR (n)))), 32)
> 
> It's possible to do but not without some additional complexity and
> cost.

I don't think it would be significant complication, on the other side you
avoid wasting compile time memory on that (GC one, which means it will be
wasted until GC collection if there is one ever).  Plus all the issues from
having the same information in multiple different places.

	Jakub