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

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Date:	Wed, 27 Jun 2007 18:25:42 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Josh Triplett <josht@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Neil Booth <neil@...kokuya.co.uk>,
	Josh Triplett <josh@...edesktop.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, linux-sparse@...r.kernel.org
Subject: Re: [PATCH 16/16] fix handling of integer constant expressions

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.
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