Re: [GIT] Networking

Andy Lutomirski
2015-11-07T00:49:41Z

On Fri, Nov 6, 2015 at 7:27 AM, David Laight <David.Laight@aculab.com> wrote:
>> From: Linus Torvalds
>> Sent: 03 November 2015 20:45
>> On Tue, Nov 3, 2015 at 12:05 PM, Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>> > result = add_overflow(
>> > mul_overflow(sec, SEC_CONVERSION, &overflow),
>> > mul_overflow(nsec, NSEC_CONVERSION, &overflow),
>> > &overflow);
>> >
>> > return overflow ? MAX_JIFFIES : result;
>>
>> Thinking more about this example, I think the gcc interface for
>> multiplication overflow is fine.
>>
>> It would end up something like
>>
>> if (mul_overflow(sec, SEC_CONVERSION, &sec))
>> return MAX_JIFFY_OFFSET;
>> if (mul_overflow(nsec, NSEC_CONVERSION, &nsec))
>> return MAX_JIFFY_OFFSET;
>> sum = sec + nsec;
>> if (sum < sec || sum > MAX_JIFFY_OFFSET)
>> return MAX_JIFFY_OFFSET;
>> return sum;
>>
>> and that doesn't look horribly ugly to me.
>
> If mul_overflow() is a real function you've just forced some of the
> values out to memory, generating a 'clobber' for all memory
> (unless 'strict-aliasing' is enabled) and making a mess of other
> optimisations.
> (If it is a static inline that might not happen.)

I doubt anyone would ever make it a real function. On new gcc, it
would be an inline backed by an intrinsic. On old gcc it would be a
normal inline or perhaps an inline with inline asm in it.

--Andy