Re: [PATCH] Fix incorrect int->float conversions caught by clang -Wimplicit-int-float-conversion

[Fangrui Song]

On 2019-11-20, Juan Quintela wrote:
> Markus Armbruster <address@hidden> wrote:
> > Fangrui Song <address@hidden> writes:
> >
> > > The warning will be enabled by default in clang 10. It is not
> > > available for clang <= 9.
> > >
> > > qemu/migration/migration.c:2038:24: error: implicit conversion from
> > > 'long' to 'double' changes value from 9223372036854775807 to
> > > 9223372036854775808 [-Werror,-Wimplicit-int-float-conversion]
> > > ...
> > > qemu/util/cutils.c:245:23: error: implicit conversion from 'unsigned
> > > long' to 'double' changes value from 18446744073709550592 to
> > > 18446744073709551616 [-Werror,-Wimplicit-int-float-conversion]
> > >
> > > Signed-off-by: Fangrui Song <address@hidden>
> > > ---
> > >   migration/migration.c | 4 ++--
> > >   util/cutils.c         | 4 ++--
> > >   2 files changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/migration/migration.c b/migration/migration.c
> > > index 354ad072fa..ac3ea2934a 100644
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -53,6 +53,7 @@
> > >   #include "monitor/monitor.h"
> > >   #include "net/announce.h"
> > >   #include "qemu/queue.h"
> > > +#include <math.h>
> > >
> > >   #define MAX_THROTTLE  (32 << 20)      /* Migration transfer speed throttling 
> > > */
> > >
> > > @@ -2035,11 +2036,10 @@ void qmp_migrate_set_downtime(double value, Error 
> > > **errp)
> >         if (value < 0 || value > MAX_MIGRATE_DOWNTIME_SECONDS) {
> >             error_setg(errp, "Parameter 'downtime_limit' expects an integer in "
> >                              "the range of 0 to %d seconds",
> >                              MAX_MIGRATE_DOWNTIME_SECONDS);
> >             return;
> > >       }
> >
> > @value is now in [0,2000].
> >
> > >       value *= 1000; /* Convert to milliseconds */
> >
> > @value is in [0,2000000]
> >
> > > -    value = MAX(0, MIN(INT64_MAX, value));
> >
> > This does nothing.
> >
> > >       MigrateSetParameters p = {
> > >           .has_downtime_limit = true,
> > > -        .downtime_limit = value,
> > > +        .downtime_limit = (int64_t)fmin(value, nextafter(0x1p63, 0)),
> >
> > This does nothing and is hard to read :)
> >
> > Can we simply drop the offending line statement instead?
>
> Agreed aboutdropping the whole bussines for migration.
>
>
> > >       };
> > >
> > >       qmp_migrate_set_parameters(&p, errp);
> > > diff --git a/util/cutils.c b/util/cutils.c
> > > index fd591cadf0..2b4484c015 100644
> > > --- a/util/cutils.c
> > > +++ b/util/cutils.c
> > > @@ -239,10 +239,10 @@ static int do_strtosz(const char *nptr, const char **end,
> > >           goto out;
> > >       }
> > >       /*
> > > -     * Values >= 0xfffffffffffffc00 overflow uint64_t after their trip
> > > +     * Values > nextafter(0x1p64, 0) overflow uint64_t after their trip
> > >        * through double (53 bits of precision).
> > >        */
> > > -    if ((val * mul >= 0xfffffffffffffc00) || val < 0) {
> > > +    if ((val * mul > nextafter(0x1p64, 0)) || val < 0) {
> > >           retval = -ERANGE;
> > >           goto out;
> > >       }
>
> This comment was really bad (it says the same that the code).
> On the other hand, I can *kind of* understand what does 0xffff<more
> f's here>.
>
> But I am at a complete loss about what value is:
>
> nextafter(0x1p64, 0).
>
> Can we put what value is that instead?

It is a C99 hexadecimal floating-point literal.
0x1p64 represents hex fraction 1.0 scaled by 2**64, that is 2**64.

We can write this as `val * mul > 0xfffffffffffff800p0`, but I feel that
counting the number of f's is error-prone and is not fun.

(We cannot use val * mul >= 0x1p64.
If FLT_EVAL_METHOD == 2, the intermediate computation val * mul will be
performed at long double precision, val * mul may not by representable
by a double and will overflow as (double)0x1p64.)