Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only once

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eric Blake (eblake@redhat.com) wrote:
> On 1/7/19 9:07 AM, Dr. David Alan Gilbert wrote:
> 
> >>>> Then use that macro to make MIN/MAX only evaluate their argument
> >>>> once; this uses type promotion (by adding to 0) to work around
> >>>> the fact that typeof(bitfield) won't compile.  However, we are
> >>>> unable to work around gcc refusing to compile ({}) in a constant
> >>>> context, even when only used in the dead branch of a
> >>>> __builtin_choose_expr(),
> 
> >> Because it doesn't work - gcc treats ({}) as a syntax error inside
> >> constant expressions, even in dead code (although 'info gcc' said that
> >> might change in the future, we can't wait for that change).  I also
> >> tried it as documented here:
> >> https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00715.html
> >> hence my mention in the commit message.
> > 
> > Ah, I didn't understand the context in your message;  you say 'even in
> > the dead branch of a __builtin_choose_expr()' but the following works
> > for me (on f29 and rhel7):
> > 
> > #include <stdio.h>
> > 
> > # define QEMU_TYPEOF(a) typeof(a)
> > 
> > #define DMIN(a,b) __builtin_choose_expr(                  \
> >     __builtin_constant_p(a) && __builtin_constant_p(b),   \
> >     (a) < (b) ? (a) : (b),                                \
> >     ({                                       \
> >         QEMU_TYPEOF((a) + 0) _a = (a) + 0;   \
> >         QEMU_TYPEOF((b) + 0) _b = (b) + 0;   \
> >         _a < _b ? _a : _b;                   \
> >     }))
> > 
> > 
> > int main(int argc, char *argv[])
> > {
> >     int anarray[DMIN(5, 10)];
> 
> Not a constant context. As written, you have declared a variable-length
> array, determined at runtime (even if the array is not actually
> variable-length because you always provide the same length).  Hoist the
> declaration anarray outside of main() to see the difference.  Or try:
> 
> struct foo {
>     int bar : DMIN(5, 10);
> };
> 
> for another example of a constant context (again, outside of a function).

Ah OK, yes that does fail as you say.  Fair enough.

Dave

> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
> 



--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK