linux-kernel - Re: [GIT] Networking

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Date:	Fri, 6 Nov 2015 16:49:19 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	David Laight <David.Laight@...lab.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andy Lutomirski <luto@...nel.org>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [GIT] Networking

On Fri, Nov 6, 2015 at 7:27 AM, David Laight <David.Laight@...lab.com> wrote:
>> From: Linus Torvalds
>> Sent: 03 November 2015 20:45
>> On Tue, Nov 3, 2015 at 12:05 PM, Linus Torvalds
>> <torvalds@...ux-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
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