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From: Joseph Myers <joseph@codesourcery.com>
To: Ties Klappe <tg.klappe@gmail.com>
Cc: <gcc@gcc.gnu.org>
Subject: Re: Unjustified optimization due to restricted struct members?
Date: Thu, 30 Nov 2023 17:16:53 +0000	[thread overview]
Message-ID: <ee19dc84-c842-e88-69fa-bbcb249f14ab@codesourcery.com> (raw)
In-Reply-To: <CAHSOcvA-Ko8a3f8G_4fHxcKh+Nd-T6JiXRzeBQhxHq1Tsx+MdA@mail.gmail.com>

On Thu, 30 Nov 2023, Ties Klappe via Gcc wrote:

> When reading section 6.7.3.1 of the C standard (quoted below) about
> the *restrict
> *type qualifier, the first section talks about *ordinary identifiers*.
> These are defined in section 6.2.3, and exclude members of structures.
> 
> Let D be a declaration of an ordinary identifier that provides a means of
> > designating an object P as a restrict-qualified pointer to type T.
> 
> 
> I would assume that this means that in the code excerpt below the function
> *h* cannot be optimized by substituting the load of *b.p *for *10*, as the
> standard does not specify what it means for a struct member to be restrict
> qualified. However, the code is still optimized by gcc (but not Clang), as
> can be seen here: https://godbolt.org/z/hEnKKoaae
> 
> struct bar {
> int* restrict p;
> int* restrict q;
> };
> 
> int h(struct bar b) {

In this code, 'b' is the ordinary identifier; "struct bar b" is the 
declaration D.  The declaration does not itself need to have pointer type.

-- 
Joseph S. Myers
joseph@codesourcery.com

      parent reply	other threads:[~2023-11-30 17:16 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-11-30 11:05 Ties Klappe
2023-11-30 12:12 ` Richard Biener
2023-11-30 12:50   ` Ties Klappe
2023-11-30 13:06     ` Richard Biener
2023-11-30 17:16 ` Joseph Myers [this message]

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