Date: Mon, 28 Jul 2003 14:54:49 -0400 From: Jake Burkholder <jake@locore.ca> 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: <20030728185449.GA396@locore.ca> In-Reply-To: <20030728015900.GB5628@crow.dom2ip.de> References: <7mwue3v6gf.wl@black.imgsrc.co.jp> <20030728015900.GB5628@crow.dom2ip.de>
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Apparently, On Mon, Jul 28, 2003 at 03:59:00AM +0200,
Thomas Moestl said words to the effect of;
> On Mon, 2003/07/28 at 09:30:08 +0900, Jun Kuriyama wrote:
> >
> > Is this caused by -oS option?
> >
> > ----- in making BOOTMFS in make release
> > cc -c -Os -pipe -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual -fformat-extensions -std=c99 -nostdinc -I- -I. -I/usr/src/sys -I/usr/src/sys/dev -I/usr/src/sys/contrib/dev/acpica -I/usr/src/sys/contrib/ipfilter -I/usr/src/sys/contrib/dev/ath -I/usr/src/sys/contrib/dev/ath/freebsd -D_KERNEL -include opt_global.h -fno-common -finline-limit=15000 -mno-align-long-strings -mpreferred-stack-boundary=2 -ffreestanding -Werror /usr/src/sys/geom/geom_dev.c
> > /usr/src/sys/geom/geom_dev.c: In function `g_dev_open':
> > /usr/src/sys/geom/geom_dev.c:198: warning: dereferencing type-punned pointer will break strict-aliasing rules
> > [...]
>
> 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
> casted pointer, which violates the C99 aliasing rules.
> An alternative is to type-pun via a union, which is also a bit ugly,
> but explicitly allowed by C99. Patch attached (but only superficially
> tested).
>
> - Thomas
>
...
Using a union sounds fine to me.
Jake
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