Date: Mon, 28 Jul 2003 04:05:35 +0200 From: Thomas Moestl <t.moestl@tu-bs.de> To: Jun Kuriyama <kuriyama@imgsrc.co.jp>, Current <freebsd-current@freebsd.org> Subject: Re: dereferencing type-punned pointer will break strict-aliasing rules Message-ID: <20030728020534.GA678@crow.dom2ip.de> In-Reply-To: <20030728015900.GB5628@crow.dom2ip.de> References: <7mwue3v6gf.wl@black.imgsrc.co.jp> <20030728015900.GB5628@crow.dom2ip.de>
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On Mon, 2003/07/28 at 03:59:00 +0200, Thomas Moestl wrote:
> Yes, by implying -fstrict-aliasing, so using -fno-strict-aliasing is a
> workaround. The problem is caused by the i386 PCPU_GET/PCPU_SET
> implementation:
>
> #define __PCPU_GET(name) ({ \
> __pcpu_type(name) __result; \
> \
> [...]
> } else if (sizeof(__result) == 4) { \
> u_int __i; \
> __asm __volatile("movl %%fs:%1,%0" \
> : "=r" (__i) \
> : "m" (*(u_int *)(__pcpu_offset(name)))); \
> __result = *(__pcpu_type(name) *)&__i; \
> [...]
>
> In this case, the PCPU_GET is used to retrieve curthread, causing
> sizeof(__result) to be 4, so the cast at the end of the code snippet
> is from a u_int * to struct thread *, and __i is accessed through the
^^^^^^^^^^^^^^^ struct thread **, of course.
> casted pointer, which violates the C99 aliasing rules.
- Thomas
--
Thomas Moestl <t.moestl@tu-bs.de> http://www.tu-bs.de/~y0015675/
<tmm@FreeBSD.org> http://people.FreeBSD.org/~tmm/
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