On Thu, 15 Dec 2011, Ed Schouten wrote:
Hello Bruce,
After reading through your email, I think it's best to fix things using
the following patch. Essentially it does the following:
- It makes sure __alignof() is always present, by defining it as a macro
for GCC < 2.95.
I tested this with a K&R compiler. It worked (since it is similar to
the fallback for offsetof() which always worked) after fixing 2 bugs
that prevented the fallback working with any compiler.
From your patch (quoted below):
| +#if !__GNUC_PREREQ__(2, 95)
| +#define __alignof(x) __offsetof(struct { char __a; e __b; }, __b)
| +#endif
The 2 bugs are:
1. Even __offsetof() is broken in the fallback case (that's all cases
where it uses a struct like the above and doesn't use a gccish builtin).
This is because __offsetof() uses size_t, but size_t is not necessarily
in declared. offsetof() in <stddef.h>, etc. works because size_t is
necessarily declared there. BTW, the ifdef for __offsetof in
<stddef.h> is bogus since <sys/cdefs.h> is always included so
__offsetof is always defined.
__size_t could be used, but that would give lots of pollution for
cdefs.h, and the file that declares __size_t, namely
<machine/types.h> is broken in fallback cases:
- recently broken for K&R since it now uses signed instead of __signed
- broken for longer for K&R and C90 since it uses long long. In working
versions, the __int64_t declarations were hacked for 32-bit machines
on to make them compile (but not work). Non-hacked versions should
simply not declare the 64-bit types if the compiler doesn't support
them.
2. The macro parameter x is now mispelled, so it is never used, and the
non-misspelled macro parameter e gives a syntax error when it is
used in the macro body.
Test program:
% #include <sys/cdefs.h>
%
% typedef unsigned size_t; /* XXX */
%
% #define __alignof(e) __offsetof(struct { char __a; e __b; }, __b)
%
% int c = __alignof(char);
% int s = __alignof(short);
% int i = __alignof(int);
% int l = __alignof(long);
% int f = __alignof(float);
% int d = __alignof(double);
Fixes:
1. Just a hack to make __offsetof() work. I don't know a good way to
declare __size_t. Of course, size_t should not be used.
2. Change the spelling back to e. I prefer x, but all the other
definitions use e.
- All the C1X macros can now be implemented using the ones we already
had (__dead2, etc). This means we don't need to repeat all the
compiler version specific checks.
- While there, add struct __hack to the _Static_assert, as it always
requires a semicolon.
Good.
| Index: sys/sys/cdefs.h
| ===================================================================
| --- sys/sys/cdefs.h (revision 228504)
| +++ sys/sys/cdefs.h (working copy)
| @@ -230,27 +230,24 @@
| #elif defined(__STDC_VERSION__) && __STDC_VERSION__ > 201000L
| /* Do nothing. They are language keywords. */
| #else
| -/* Not supported. Implement them manually. */
| -#ifdef __GNUC__
| -#define _Alignas(e) __attribute__((__aligned__(e)))
| -#define _Alignof(e) __alignof__(e)
| -#define _Noreturn __attribute__((__noreturn__))
| +/* Not supported. Implement them using our versions. */
It may be worth emphasizing that in some cases, "our version" doesn't
exist, and that this is intentional (we omit the definition when none
is possible, and this results in __aligned() sometimes not being defined
and __thread never being defined).
| +#define _Alignas(e) __aligned(e)
| +#define _Alignof(e) __alignof(e)
I prefer to always spell the parameter as x. Most older macros in this
file, and CTASSERT() in another file, spell their parameters (x, y, ...).
| +#define _Noreturn __dead2
| #define _Thread_local __thread
| -#else
| -#define _Alignas(e)
| -#define _Alignof(e) __offsetof(struct { char __a; e __b; },
__b)
| -#define _Noreturn
| -#define _Thread_local
| -#endif
| #ifdef __COUNTER__
| #define _Static_assert(e, s) __Static_assert(e, __COUNTER__)
| #define __Static_assert(e, c) ___Static_assert(e, c)
| #define ___Static_assert(e, c) typedef char __assert ## c[(e) ? 1 : -1]
| #else
| -#define _Static_assert(e, s)
| +#define _Static_assert(e, s) struct __hack
CTASSERT() has regressed for compilers that don't support __COUNTER__,
since it uses this. Previously:
- CTASSERT() never worked for K&R compilers, since it uses C90 token pasting
- CTASSERT() worked for all C90 and later compilers.
| #endif
| #endif
|
| +#if !__GNUC_PREREQ__(2, 95)
| +#define __alignof(x) __offsetof(struct { char __a; e __b; }, __b)
| +#endif
Needs the spelling fix.
| +
| #if __GNUC_PREREQ__(2, 96)
| #define __malloc_like __attribute__((__malloc__))
| #define __pure __attribute__((__pure__))
Check the sorting of the new macros, especially __alignof(). I think
the ordering should be mostly on the gcc and standards version, not on
the order in which the macros were added to cdefs.h. You sorted the 2.95
ifdef before a 2.96 ifdef, but this 2.96 ifdef is especially disordered
according to my rule. Even if the order is "logical" instead of historical,
it makes more sense to define basic macros like __alignof() before using
them.
Bruce
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