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Message-ID: <20180923023234.GE17995@brightrain.aerifal.cx>
Date: Sat, 22 Sep 2018 22:32:34 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: un-UBify-strings
On Sun, Sep 23, 2018 at 02:11:42AM +0000, Pascal Cuoq wrote:
> Hello Rich,
>
> On 23 Sep 2018, at 02:35, Rich Felker <dalias@...c.org<mailto:dalias@...c.org>> wrote:
>
> I've had this patch sitting around since 2016, and just updated it to
> apply cleanly. Any objections?
>
> Your patch contains:
>
> ....
> size_t __attribute__((__may_alias__)) *wd;
> const size_t __attribute__((__may_alias__)) *ws;
> ....
>
> In my experience, this use of __may_alias__ does not do anything.
> See function f in the example below, which both GCC and Clang
> optimize as if the programmer had not used __may_alias__ at all:
> https://gcc.godbolt.org/z/Um4NU7
>
> You should use a typdef for the aliasing type, as shown for function
> g (in with GCC and Clang do not apply the optimization).
>
> The example in GCC's documentation for __may_alias__ also uses a
> typedef:
> https://gcc.gnu.org/onlinedocs/gcc-4.0.4/gcc/Type-Attributes.html
Thanks, this is very helpful. I'll prepare an updated version.
Rich
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