linux-kernel - Re: [PATCH 1/1] vsprintf: optimize decimal conversion (again)

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Message-ID: <op.wbt93pu4pw9a85@mpn-glaptop>
Date:	Tue, 27 Mar 2012 19:12:51 +0200
From:	"Michal Nazarewicz" <mpn@...gle.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Denys Vlasenko" <vda.linux@...glemail.com>,
	linux-kernel@...r.kernel.org,
	"Douglas W Jones" <jones@...uiowa.edu>,
	"Michal Nazarewicz" <mnazarewicz@...gle.com>
Subject: Re: [PATCH 1/1] vsprintf: optimize decimal conversion (again)

On Mon, 26 Mar 2012 22:20:52 +0200, H. Peter Anvin <hpa@...or.com> wrote:

> On 03/26/2012 01:13 PM, Andrew Morton wrote:
>> On Mon, 26 Mar 2012 21:56:38 +0200
>> Denys Vlasenko <vda.linux@...glemail.com> wrote:
>>
>>>>> +#if BITS_PER_LONG != 32 || (~(0ULL)>>1) != ((1ULL<<63)-1)
>>>>
>>>> What's this for?
>>>
>>> The second check should be just BITS_PER_LONG_LONG != 64,
>>> but we don't have BITS_PER_LONG_LONG.
>>
>> So let's add BITS_PER_LONG_LONG rather than hacking around its absence!
>
> First of all, the #if is wrong: the preprocessor doesn't support data
> types and does all arithmetic at (u)intmax_t precision.

Maybe a regular “if” instead of macro would suffice here?  Compiler should
be smart enough to optimise out the dead path.

> As far as BITS_PER_LONG_LONG, there are tons of places in the kernel
> which already require that long long is exactly 64 bits.  That may or
> may not be a good thing, but for right now one could simply:
>
> #define BITS_PER_LONG_LONG 64

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
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