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Date: Wed, 20 May 2020 09:31:53 +0200 (CEST)
From: Richard Biener <rguenther@suse.de>
To: Joseph Myers <joseph@codesourcery.com>
cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH][C] c/95141 - fix bogus integer overflow warning
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On Tue, 19 May 2020, Joseph Myers wrote:

> On Tue, 19 May 2020, Richard Biener wrote:
> 
> > This fixes an integer overflow warning that ultimatively happens because
> > of TREE_OVERFLOW propagating through transforms and the existing guard
> > against this,
> > 
> > 375           if (TREE_OVERFLOW_P (ret)
> > 376               && !TREE_OVERFLOW_P (op0)
> > 377               && !TREE_OVERFLOW_P (op1))
> > 378             overflow_warning (EXPR_LOC_OR_LOC (expr, input_location,
> > 
> > being insufficient.  Rather than trying to use sth like walk_tree to
> > exhaustively walk operands (with the possibility of introducing
> > quadraticness when folding larger expressions recursively) the
> > following amends the above with an ad-hoc test for a binary op0
> > with a possibly constant op1.
> > 
> > Bootstrapped and tested on x86_64-unknown-linux-gnu, OK?
> 
> OK.
> 
> The test in bug 32643 looks vaguely similar, but that's an older 
> regression, do I take it this patch doesn't help with that one?

Yes, it doesn't help with that older bug.  That is, it does not
change in any way where we set TREE_OVERFLOW, it just avoids
emitting an overflow warning in the above spot when overflow
was already present on original operands and thus likely(!)
propagated to the result.

Pushed.

Richard.