On Fri, 3 Dec 2010, Eygene Ryabinkin wrote:
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.
Ah, the functions actually return something :-).
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).
This is why amd64 and i386 use __extension when they use
statement-expressions. grep in -current shows 32 .h files under /sys
matching "({", and only 7 of these files use __extension. It is mostly
headers visible in userland that are careful. sparc64 and sun4v
atomic.h seem to be the only headers that both use ({ and are used in
userland. For .c files, statement-expressions are remarkably little-used
-- there are more line 32 lines total matching "({", and 0 lines
matching "__extension".
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?".
I guess it is just because they seemed to be simple enough to be macros.
Macros and inline have different technical advantages but I don't see
any important ones here.
Bruce
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