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Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUI

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON
Date: Thu, 19 Jan 2017 14:58:48 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

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:

    struct { unsigned int _gl_verify_error_if_negative: (R) ? 1 : -1; }

   (!!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 { ... }.  */

# define _GL_VERIFY _Static_assert
# define _GL_VERIFY(R, DIAGNOSTIC)                                     \
    extern int (*_GL_GENSYM (_gl_verify_function) (void))              \

/* 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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