Re: dereferencing type-punned pointer will break strict-aliasing rules

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

-- 
Thomas Moestl <t.moestl_at_tu-bs.de>	http://www.tu-bs.de/~y0015675/
              <tmm_at_FreeBSD.org>		http://people.FreeBSD.org/~tmm/
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