Date: Thu, 15 Dec 2011 02:18:59 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Ed Schouten <ed@FreeBSD.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org Subject: Re: svn commit: r228495 - head/sys/sys Message-ID: <20111214234615.B3839@besplex.bde.org> In-Reply-To: <201112140909.pBE99bS3090646@svn.freebsd.org> References: <201112140909.pBE99bS3090646@svn.freebsd.org>
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On Wed, 14 Dec 2011, Ed Schouten wrote:
> Log:
> Slightly alter the C1X definitions in in cdefs.h:
>
> - Add _Alignas(). Unfortunately this macro is only partially functional.
> The C1X standard will allow both an integer and a type name to be
> passed to this macro, while this macro only allows an integer. To be
> portable, one must use _Alignas(_Alignof(double)) to use type names.
This is still quite broken.
> - Don't do _Static_assert() when __COUNTER__ is not supported. We'd
> better keep this implementation robust and allow it to be used in
> header files, without mysteriously breaking older compilers.
Not sure about this.
> Modified: head/sys/sys/cdefs.h
> ==============================================================================
> --- head/sys/sys/cdefs.h Wed Dec 14 08:52:27 2011 (r228494)
> +++ head/sys/sys/cdefs.h Wed Dec 14 09:09:37 2011 (r228495)
> @@ -222,6 +222,7 @@
> * Keywords added in C1X.
> */
> #if defined(__cplusplus) && __cplusplus >= 201103L
> +#define _Alignas(e) alignas(e)
> #define _Alignof(e) alignof(e)
> #define _Noreturn [[noreturn]]
> #define _Static_assert(e, s) static_assert(e, s)
> @@ -231,21 +232,23 @@
> #else
> /* Not supported. Implement them manually. */
> #ifdef __GNUC__
> +#define _Alignas(e) __attribute__((__aligned__(e)))
This breaks versions of gcc that don't have __attribute__ or __aligned__.
Elsewhere in the same file, use of __attribute__ is restricted
starting with the condition:
#if __GNUC__ == 2 && __GNUC_MINOR__ >= 5 && __GNUC_MINOR__ < 7 && \
!defined(__INTEL_COMPILER)
(this actually gives the starting point for 2 attributes, with many
necessary and unnecessary convolutions -- see below).
and use of __aligned__ is carefully restricted by the condition
#if __GNUC_PREREQ__(2, 7) || defined(__INTEL_COMPILER)
(now it starts with gcc-2.7 where the above starts with gcc-2.5,
since __aligned__ apparently wasn't in the original set of attributes).
The other places give a definition of __aligned() that can almost be
used here. There is a bogus one for lint and a duplicate one for
__INTEL_COMPILER. But there is none for
!__GNUC_PREREQ__(2, 7) && !__INTEL_COMPILER. That is, all cases
where the compiler is either gcc-before-2.6, or non-gcc other than
lint and icc, are already broken.
> #define _Alignof(e) __alignof__(e)
This one mainly has a style bug. __alignof__ would be be spelled
__alignof if this were FreeBSD code. gcc has accepted all of alignof(),
__alignof() and __alignof__() for a long time. I checked this for some
gccs back to gcc-2.95.4. It might even work for gcc-1.
FreeBSD uses the extra trailing underscores for __attribute__(()) and
individual attributes because unlike for __alignof() and __typeof(),
gcc used to require them and changing this now would give style bugs.
gcc-4.2.1 accepts the __aligned__ attribute, but gcc-3.3.3 doesn't.
I don't know when __attribute(()) started working. I checked it for
some gccs back to gcc-2.95.4, and all accepted it.
> #define _Noreturn __attribute__((__noreturn__))
Elsewhere in this file, the __noreturn__ attribute is handled like
the __aligned__ attribute, but more carefully, with many style bugs
but no bugs as far as I can see:
- For lint, the macro that uses it (__dead2) is bogusly defined as
nothing. I thought that lint bogusly defines __GNUC__, but can't
find this now. Lint should not define __GNUC__ since it only supports
a tiny subsets of gcc features like __attribute__(), even with -g.
If lint defined __GNUC__, then the extra ifdef for it would be be
justified for some of the macros including __dead2/_Noreturn, since
__dead2 has no semantic effect so it is safe to make it null for
compilers that don't support it. If lint doesn't define __GNUC__
then for __dead2 and now hopefully for _Noreturn, an extra ifdef is
not needed since lint is given a null definition in the section for
nondescript non-gcc compilers.
- For non-gcc, or gcc < 2.5, and not icc, __dead2 is defined as nothing
under the condition:
#if !__GNUC_PREREQ__(2, 5) && !defined(__INTEL_COMPILER)
- For gcc-2.5 and gcc-2.6, and icc, __attribute__((__noreturn__)) is
used to define __dead2 under the condition:
#if __GNUC__ == 2 && __GNUC_MINOR__ >= 5 && __GNUC_MINOR__ < 7 && \
!defined(__INTEL_COMPILER)
Note the style bug in this condition (it doesn't use GNUC_PREREQ()).
- For gcc >= 2.7, the definition of __dead2 is duplicated under the
condition:
#if __GNUC_PREREQ(2, 7)
- For icc, the definition of __dead2 is duplicated yet again, under the
condition:
#if defined(__INTEL_COMPILER)
Note the style bug in this (defined() when ifdef would do).
There are 2 other definitions under the gcc-2.5, gcc-2.6, or icc ifdef.
Only the one for __unused belongs there, since its attribute is new
in gcc-2.7. The others should be define without duplication for gcc
>= 2.5.
Under the the gcc >= 2.7 and separate icc condition, there are definitions
to use 7 attributes. All of these have style bugs or worse:
- __noreturn__ attribute. Shouldn't be duplicated.
- __const__ attribute. Shouldn't be duplicated.
- __unused__ attribute. May actually be correct.
- __used__ attribute. Doesn't belong here, since it is much newer than
gcc-2.7. It is in gcc-3.3.3 but not in gcc-3.2.2.
- __packed__ attribute. Not defined for older gcc.
- __aligned__ attribute. Not defined for older gcc.
- __section attribute. Not defined for older gcc.
These definitions have lots more of bugs:
- random order
- all duplicated verbatim for icc.
> #define _Thread_local __thread
This has similar problems. __thread doesn't work for at least gcc-2.95.4
and gcc-3.2.2, but works (compiles at least) for gcc-3.3.3.
> #else
> +#define _Alignas(e)
> #define _Alignof(e) __offsetof(struct { char __a; e __b; }, __b)
> #define _Noreturn
> #define _Thread_local
> #endif
_Alignas() can be defined in terms of __align() for all cases without
repeating the ifdefs, after fixing old bugs for __align().
_Noreturn can't be handled so simply. Its use has syntactic differences.
I have already complained about using it in <stdlib.h> breaking portability
in a critical header. All these messy ifdefs here are just obfuscations
if they don't work for <stdlib.h>. After writing the above and running some
more tests, the limits of the breakage are clearer:
- the __noreturn__ attribute is apparently new in gcc-2.5. Before that,
the old __pure macro presumably worked. __pure was implemented as
__volatile, and has the same syntax as C1x _Noreturn (it must be before
the function name).
- the __noreturn__ attribute (and presumably other attributes) is a
syntax errors if it is used before a function name (presumably the
same for before a variable name) in gcc-2.95.4. It works before a
function name in gcc-3.2.2.
Thus to work with all compilers that it used to work with, <stdlib.h>
must either use only __dead2 after function names like it used to, or
if it wants to use _Noreturn then it must use both _Noreturn before
function names (to support newer compilers) and __dead2 after function
names (to support older compilers), and _Noreturn must be more carefully
ifdefed here. There are the following compilers and ranges of compilers:
- not gcc and not icc and not clang: both macros null
- icc: hopefully like newer gcc
- clang: hopefully like newer gcc
- gcc-1: like gcc-2.early
- gcc-2.early: define __dead2 as null; define _Noreturn as either
__volatile (to bring back the special support for gcc-1 that I
deprecated in 1996 and removed in 1998) or define _Noreturn as
null (to preserve the removal, so that gcc-2.early is treated
as a nondescript compiler). _Noreturn restores the syntax
differences that made the gcc-2.early support touch much more than
<sys/cdefs.h>. Now we have the syntax differences in another
way, so restoring the gcc-2.early support only takes 1 line.
- gcc-2.5 through gcc-3.2.2 or a little earlier: must define _Noreturn
as null, and to restore functionality in <stdlib.h>, restore the
uses of __dead2 there. Don't forget to fix the new style bugs
there (see the 4.4BSD version for normal style with directives
before function names).
- post gcc-3.2.2: can define _Noreturn as non-null and __dead2 as
null, or vice versa, or both non-null.
Bruce
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