Re: [PATCH 16/16] fix handling of integer constant expressions

[Al Viro <viro@ftp.linux.org.uk>]

On Wed, Jun 27, 2007 at 09:34:55AM -0700, Josh Triplett wrote:

> > is actually nice code for something like the kernel, but it turns out that 
> > in order to make this work, you have to do it as
> > 
> > 	#define htons(x) (__builtin_constant_p(x) ? constant_htons(x) : __htons(x))

That's not quite right.  In principle, __builtin_choose_expr() could be
used for that kind of stuff and builtins can change the rules.
 
> Also agreed.  Same goes for other short-circuiting operations like &&,
> ||, and ?: without the center argument; if you can determine at
> compilation time that it does not need to evaluate part of the
> expression at all, go ahead and ignore that part of the expression even
> if it does not constitute an integer constant expression.  If you want
> to optionally check for this case and issue a diagnostic, put it under
> -Wstrict-constant-expressions or similar.

That actually means extra work for evaluate_expression().  Unfortunately.

The thing is, we want to typecheck all branches, even ones not taken.
_However_, we don't want to expand all of them.  Having extra places
where we have to do expansion means extra work.