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

[Juan Quintela <quintela@redhat.com>]

Markus Armbruster <armbru@redhat.com> wrote:
> Fangrui Song <i@maskray.me> 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 <i@maskray.me>
>> ---
>>   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?

Thanks, Juan.