linux-kernel - Re: [PATCH] lib: One less subtraction in binary search iterations.

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Message-ID: <1373343147.9458.25.camel@joe-AO722>
Date:	Mon, 08 Jul 2013 21:12:27 -0700
From:	Joe Perches <joe@...ches.com>
To:	Wedson Almeida Filho <wedsonaf@...il.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Tim Abbott <tabbott@...lice.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib: One less subtraction in binary search iterations.

On Mon, 2013-07-08 at 20:51 -0700, Wedson Almeida Filho wrote:
> On Sat, Jul 6, 2013 at 9:59 PM, Joe Perches <joe@...ches.com> wrote:
> >
> > Not correct.
> >
> >>       while (start < end) {
> >> -             size_t mid = start + (end - start) / 2;
> >> +             size_t mid = (start + end) / 2;
> >
> >         size_t start = 0x80000000;
> >         size_t end   = 0x80000001;
> 
> Good point, they aren't equivalent in all cases.
> 
> For the overflow to happen though, we need an array with at least
> N/2+1 entries, where N is the address space size. The array wouldn't
> fit in addressable memory if the element size is greater than 1, so
> this can only really happen when the element size is 1. Even then, it
> would require the kernel range to be greater than half of all
> addressable memory, and allow an allocation taking that much memory. I
> don't know all architectures where linux runs, but I don't think such
> configuration is likely to exist.

Nor do I but that wasn't what you wrote.

> There is no functional change, but this change eliminates a subtraction that
> the compiler doesn't optimize out (as of gcc 4.7.3).

That's flatly incorrect.

I don't mind if you change it, for just the reason
you wrote, but you still have to now say under what
conditions the test works and when it doesn't.


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