Re: [RFC] saturate check_*_overflow() output?

From: Segher Boessenkool
Date: Wed Aug 05 2020 - 19:22:41 EST

Hi Rasmus,

On Wed, Aug 05, 2020 at 01:38:58PM +0200, Rasmus Villemoes wrote:
> I'm guessing gcc has some internal very early simplification that
> replaces single-expression statement-exprs with just that expression,
> and the warn-unused-result triggers later. But as soon as the
> statement-expr becomes a little non-trivial (e.g. above), my guess is
> that the whole thing gets assigned to some internal "variable"
> representing the result, and that assignment then counts as a use of the
> return value from must_check_overflow() - cc'ing Segher, as he usually
> knows these details.

A statement expression is not a statement (it's an expression), which
turns half of the world upside down. This GCC extension often has weird
(or at least non-intuitive) side effects, together with other extensions
(like attributes), etc.

This may be a convoluted way of saying "I don't know, look at c/c-decl.c
(and maybe c/c-parser.c) to see if you can find out" ;-)

> Anyway, we don't need to apply it to the last expression inside ({}), we
> can just pass the whole ({}) to must_check_overflow() as in


Yes, much nicer :-) Crisis averted, etc.