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Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v2] osdep: Make MIN/MAX evaluate arguments only once |
Date: |
Mon, 7 Jan 2019 15:07:38 +0000 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
* Eric Blake (address@hidden) wrote:
> On 1/7/19 3:49 AM, Dr. David Alan Gilbert wrote:
> > * Eric Blake (address@hidden) wrote:
> >> Add the macro QEMU_TYPEOF() to access __auto_type in new enough
> >> compilers, while falling back to typeof on older compilers (the
> >> fallback doesn't handle variable length arrays, but we don't use
> >> those; it also expands to more text).
> >>
> >> 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(),
>
>
> >> +#undef MIN
> >> +#define MIN(a, b) \
> >> + ({ \
> >> + QEMU_TYPEOF((a) + 0) _a = (a) + 0; \
> >> + QEMU_TYPEOF((b) + 0) _b = (b) + 0; \
> >> + _a < _b ? _a : _b; \
> >> + })
> >> +#define MIN_CONST(a, b) \
> >> + __builtin_choose_expr( \
> >> + __builtin_constant_p(a) && __builtin_constant_p(b), \
> >> + (a) < (b) ? (a) : (b), \
> >> + __builtin_unreachable())
> >
> > Why do these need to be separate macros? Can't you just put the
> > non-constant code in what you have as the 'builtin_unreachable' side of
> > the choose_expr:
> >
> > #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; \
> > }))
>
> 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)];
int a=5;
int b=6;
fprintf(stderr, "sizeof(anarray)=%zd DMIN(a,b)=%d\n", sizeof(anarray),
DMIN(a++,b++));
fprintf(stderr, "a=%d b=%d\n", a,b);
}
Dave
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc. +1-919-301-3226
> Virtualization: qemu.org | libvirt.org
>
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK