svn commit: r304555 - head/sys/compat/cloudabi

[Bruce Evans]

On Sun, 21 Aug 2016, Slawa Olhovchenkov wrote:

> On Sun, Aug 21, 2016 at 07:41:11AM +0000, Ed Schouten wrote:
>> ...
>> Log:
>>   Use memcpy() to copy 64-bit timestamps into the syscall return values.
>>
>>   On 32-bit platforms, our 64-bit timestamps need to be split up across
>>   two registers. A simple assignment to td_retval[0] will cause the top 32
>>   bits to get lost. By using memcpy(), we will automatically either use 1
>>   or 2 registers depending on the size of register_t.
>> ..
>> Modified: head/sys/compat/cloudabi/cloudabi_clock.c
>> ==============================================================================
>> --- head/sys/compat/cloudabi/cloudabi_clock.c	Sun Aug 21 07:28:38 2016	(r304554)
>> +++ head/sys/compat/cloudabi/cloudabi_clock.c	Sun Aug 21 07:41:11 2016	(r304555)
>> @@ -117,7 +117,7 @@ cloudabi_sys_clock_res_get(struct thread
>>  	error = cloudabi_convert_timespec(&ts, &cts);
>>  	if (error != 0)
>>  		return (error);
>> -	td->td_retval[0] = cts;
>> +	memcpy(td->td_retval, &cts, sizeof(cts));
>>  	return (0);
>>  }
>>
>> @@ -129,6 +129,6 @@ cloudabi_sys_clock_time_get(struct threa
>>  	int error;
>>
>>  	error = cloudabi_clock_time_get(td, uap->clock_id, &ts);
>> -	td->td_retval[0] = ts;
>> +	memcpy(td->td_retval, &ts, sizeof(ts));
>>  	return (error);
>
> Why do not use more simple solution:

Because it doesn't work.

> *(cloudabi_timestamp_t *)td->td_retval = cts;
>
> This is eliminate call to memcpy and allow compiler to use most
> effeicient way.

memcpy() is the most efficient way (except for the kernel pessimization
of losing builtin memcpy with -ffreestanding 15 years ago and not bringing
it back).

*(foo_t *)asks for alignment bugs.  We have already fixed lots of these
bugs for copying struct timevals in places like ping.c.  Compilers warn
about misalignment when certain warnings are enabled, but only on arches
where misalignment is more than a pessimization.  There is no reason why
td_retval would be always aligned on these arches.  Alignment of 64-bit
types on 32-bit arches is usually so unimportant that even int32_t is
not required to be aligned by the ABI, and there is no point in
aligning td_retval specially unless you also do it for a large fraction
of 64-bit integers in the kernel, and there are negative points for
doing that.

Bruce