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Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUI
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON |
Date: |
Thu, 19 Jan 2017 23:02:58 +0200 |
On Thu, Jan 19, 2017 at 02:58:48PM -0600, Eric Blake wrote:
> 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
>
I coded up something similar independently.
Will post in a minute.
--
MST
- Re: [Qemu-devel] [PATCH v2 2/4] compiler: drop ; after BUILD_BUG_ON, (continued)
Re: [Qemu-devel] [PATCH v2 2/4] compiler: drop ; after BUILD_BUG_ON, Markus Armbruster, 2017/01/19
[Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Michael S. Tsirkin, 2017/01/18
Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON, Michael S. Tsirkin, 2017/01/19
[Qemu-devel] [PATCH v2 4/4] ARRAY_SIZE: check that argument is an array, Michael S. Tsirkin, 2017/01/18
Re: [Qemu-devel] [PATCH v2 0/4] virtio: ARRAY_SIZE fixups, no-reply, 2017/01/18