svn commit: r216143 - in head: share/man/man9 sys/amd64/include sys/arm/include sys/i386/include sys/ia64/include sys/mips/include sys/pc98/include sys/powerpc/include sys/sparc64/include sys/sun4v...

[Eygene Ryabinkin]

Sat, Dec 04, 2010 at 05:01:55AM +1100, Bruce Evans wrote:
> On Fri, 3 Dec 2010, Bruce Cran wrote:
> > Log:
> >  Revert r216134. This checkin broke platforms where bus_space are macros:
> >  they need to be a single statement, and do { } while (0) doesn't
> >  work in this situation so revert until a solution can be devised.
> 
> Surprising that do-while doesn't work.

Prior to the revert, something like "a = bus_space_read_multi_1(...)"
will generate improper code like "a = KASSERT(); __bs_nonsigle(XXX);"
and making "do { KASSERT(); __bs_nonsingle(XXX); } while(0)" won't
help either, since we can't generally assign the compound statement
to the lvalue.

> > Modified: head/sys/arm/include/bus.h
> > ==============================================================================
> > --- head/sys/arm/include/bus.h	Fri Dec  3 07:01:07 2010	(r216142)
> > +++ head/sys/arm/include/bus.h	Fri Dec  3 07:09:23 2010	(r216143)
> > ...
> > @@ -321,29 +318,21 @@ struct bus_space {
> >  * Bus read multiple operations.
> >  */
> > #define	bus_space_read_multi_1(t, h, o, a, c)				\
> > -	KASSERT(c != 0, ("bus_space_read_multi_1: count == 0"));	\
> > 	__bs_nonsingle(rm,1,(t),(h),(o),(a),(c))
> 
> I just noticed the following possibly more serious problems for the macro
> versions:
> 
> - the `c' arg is missing parentheses in the KASSERT()
> - the `c' arg is now evaluated twice.  This turns safe macros into unsafe
>    ones.

Perhaps we can define the macros as
{{{
#define	bus_space_read_multi_1(t, h, o, a, c)				({\
	size_t count = (c);						\
	KASSERT(count != 0, ("bus_space_read_multi_1: count == 0"));	\
	__bs_nonsingle(rm,1,(t),(h),(o),(a),count);			\
	})
}}}

This will both allow to avoid unsafety and will make this statement
to be the correct assignment for any compiler that supports the
"braced-groups within expressions" GNU extension.  GNU C, Clang
and Intel C both support it (but not with -pedantic -ansi -Werror
flag combo).

But, probably, the inline function will be better here from the
portability point of view, since it is supported by the C standard
and braced-groups -- aren't.

So, the question is "why these statements were made to be
macros at some platforms?".
-- 
Eygene Ryabinkin                                        ,,,^..^,,,
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