On 01/19/2017 01:25 PM, Michael S. Tsirkin wrote:
>>>>> +#define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(int[(x) ? -1 : 1]) -
>>>>> sizeof(int))
>>>
>>> Linux here uses:
>>>
>>> #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
>>>
>>> and the issue is that sizeof(int[(x) ? -1 : 1]) could be
>>> runtime-evaluated (the type is a variable-length array).
Use of the struct with an int bitfield forces 'e' to be a non-zero
compile-time constant to compile; while [(x) ? -1 : 1] could indeed
compile even if not a compile-time constant. So I agree that we want to
prefer something that forces x to be a compile-time constant.
>>
>> Let's copy both macros from Linux.
>
> I prefer our variant, I don't think it's portable to assume that
> sizeof(struct {int:0}) is 0. Besides, Linux code is GPLv2 only and this
> file is 2 or later.
I agree that using the Linux version is problematic from a licensing
perspective, as well as the portability of getting sizeof() to produce
0. But at least it uses bitfields to force a compile time constant.
Here's a third approach: gnulib has a LGPLv2+ version that does:
====
# define _GL_VERIFY_TYPE(R, DIAGNOSTIC) \
struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }
#define _GL_VERIFY_TRUE(R, DIAGNOSTIC) \
(!!sizeof (_GL_VERIFY_TYPE (R, DIAGNOSTIC)))
/* Verify requirement R at compile-time, as a declaration without a
trailing ';'. If R is false, fail at compile-time, preferably
with a diagnostic that includes the string-literal DIAGNOSTIC.
Unfortunately, unlike C11, this implementation must appear as an
ordinary declaration, and cannot appear inside struct { ... }. */
#ifdef _GL_HAVE__STATIC_ASSERT
# define _GL_VERIFY _Static_assert
#else
# define _GL_VERIFY(R, DIAGNOSTIC) \
extern int (*_GL_GENSYM (_gl_verify_function) (void)) \
[_GL_VERIFY_TRUE (R, DIAGNOSTIC)]
#endif
/* Verify requirement R at compile-time. Return the value of the
expression E. */
#define verify_expr(R, E) \
(_GL_VERIFY_TRUE (R, "verify_expr (" #R ", " #E ")") ? (E) : (E))
/* Verify requirement R at compile-time, as a declaration without a
trailing ';'. */
#define verify(R) _GL_VERIFY (R, "verify (" #R ")")
====
where BUILD_BUG_ON_ZERO(e) pretty much maps to verify_expr(!(e), 0), if
you want to copy any of those ideas.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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