Messages in this thread | | | From | Rasmus Villemoes <> | Subject | Re: [PATCH v2 1/3] lib: find_*_bit reimplementation | Date | Mon, 02 Feb 2015 13:56:27 +0100 |
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On Mon, Feb 02 2015, "George Spelvin" <linux@horizon.com> wrote:
> Rasmus Villemoes <linux@rasmusvillemoes.dk> wrote: >> ... and this be part of _find_next_bit? Can find_next_bit not be simply >> 'return _find_next_bit(addr, size, offset, 1);', and similarly for >> find_next_zero_bit? Btw., passing true and false for the boolean >> parameter may be a little clearer. > > Looking at the generated code, it would be better to replace the boolean > parameter with 0ul or ~0ul and XOR with it. The same number of registers, > and saves a conditional branch.
Nice trick. When I compiled it, gcc inlined _find_next_bit into both its callers, making the conditional go away completely. That was with gcc 4.7. When I try with 5.0, I do see _find_next_bit being compiled separately.
With the proposed change, 4.7 also makes find_next{,_zero}_bit wrappers for _find_next_bit, further reducing the total size, which is a good thing. And, if some other version decides to still inline it, it should then know how to optimize the xor with 0ul or ~0ul just as well as when the conditional was folded away.
Yury, please also incorporate this in the next round.
Rasmus
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