Re: find_new_bit bloat from x86 tree...

Alexander van Heukelum
2008-04-28T11:41:52Z

On Sat, Apr 26, 2008 at 10:07:26PM -0700, David Miller wrote:
> Ingo, what the heck is this?
>
> commit 64970b68d2b3ed32b964b0b30b1b98518fde388e
> Author: Alexander van Heukelum <heukelum@mailshack.com>
> Date: Tue Mar 11 16:17:19 2008 +0100
>
> x86, generic: optimize find_next_(zero_)bit for small constant-size bitmaps
>
> Thanks for bloating up the inline expansion of this thing on every
> architecture that doesn't do __ffs() in a simple sequence of a few
> instructions like x86 does.
>
> Now every call that matches your tests gets this turd inline:
>
> static inline unsigned long __ffs(unsigned long word)
> {
> int num = 0;
>
> #if BITS_PER_LONG == 64
> if ((word & 0xffffffff) == 0) {
> num += 32;
> word >>= 32;
> }
> #endif
> if ((word & 0xffff) == 0) {
> num += 16;
> word >>= 16;
> }
> if ((word & 0xff) == 0) {
> num += 8;
> word >>= 8;
> }
> if ((word & 0xf) == 0) {
> num += 4;
> word >>= 4;
> }
> if ((word & 0x3) == 0) {
> num += 2;
> word >>= 2;
> }
> if ((word & 0x1) == 0)
> num += 1;
> return num;
> }
>
> as well as all of that address formation, bit shifting, and masking.
>
> Please revert or make this conditional on something architectures can
> opt-in for.

Alternatively, implement __ffs out of line? Like:

static inline unsigned long __ffs(unsigned long word)
{
return generic___ffs(word);
}

And a generic___ffs implemented in lib/ffs.c?

If __ffs is too big to be inlined it should not be inlined. That
is a generic problem and has nothing to do with this patch, IMHO.

Greetings,
Alexander

> The version actually applied was posted only on linux-kernel, instead
> of also CC:'ing linux-arch as previous versions had been. Nobody
> commented on this version other than you Ingo.