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Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only once |
Date: |
Mon, 7 Jan 2019 10:16:02 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1 |
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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