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

[Eric Blake <eblake@redhat.com>]

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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).

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


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