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

[Joe Perches]

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@xxxxxxxxxxx> 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.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/