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Message-ID: <84144f0205082113123049afe2@mail.gmail.com>
Date: Sun, 21 Aug 2005 23:12:06 +0300
From: Pekka Enberg <penberg@gmail.com>
To: Adrian Bunk <bunk@stusta.de>
Subject: Re: [RFC: -mm patch] kcalloc(): INT_MAX -> ULONG_MAX
Cc: linux-kernel@vger.kernel.org
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On Sun, Aug 21, 2005 at 10:47:13PM +0300, Pekka Enberg wrote:
> > You'll probably get even better code if you change the above to:
> >
> >     if (size != 0 && n > ULONG_MAX / size)
> >
> > Reason being that size is virtually always a constant so the compiler
> > can evaluate the division at compile-time.

On 8/21/05, Adrian Bunk <bunk@stusta.de> wrote:
> I doubt this would make any difference.
> 
> And besides, except in some rare cases, the second argument is a
> sizeof(foo) whose size is already known at compile time.

Yes, that's my point. The second argument (size) is virtually always
sizeof() whereas the first one (n) is sometimes a variable. GCC
currently does not optimize away the division when n is not a
constant.

Looking at 2.6.13-rc6-mm1, we have roughly 15 callers with the first
parameter being a variable. The compiler would be able to get rid of
one comparison and division instruction for each of these so looks
like we could shave off some more bytes...

                            Pekka