[commit: ghc] master: comments only (9894f6a)

[Gabor Greif]

Hi Simon,

JFTR, you seem to be after the trailing zeros in the code you commented.

If the bitmap is *really* that sparse then it might be profitable to
rewrite it in terms of
__builtin_ctz (when present). Some CPUs even have instructions for this.

http://hardwarebug.org/2010/01/14/beware-the-builtins/

Possibly one could even switch to checking *leading* zeros by
reformulating the algorithm and eliminate a few more instructions.

http://www.hackersdelight.org/ might be another source for inspiration.

Cheers,

    Gabor

On 1/20/15, git at git.haskell.org <git at git.haskell.org> wrote:
> Repository : ssh://git@git.haskell.org/ghc
>
> On branch  : master
> Link       :
> http://ghc.haskell.org/trac/ghc/changeset/9894f6a5b4883ea87fd5f280a2eb4a8abfbd2a6b/ghc
>
>>---------------------------------------------------------------
>
> commit 9894f6a5b4883ea87fd5f280a2eb4a8abfbd2a6b
> Author: Simon Marlow <marlowsd at gmail.com>
> Date:   Wed Jan 14 08:45:07 2015 +0000
>
>     comments only
>
>
>>---------------------------------------------------------------
>
> 9894f6a5b4883ea87fd5f280a2eb4a8abfbd2a6b
>  rts/sm/Scav.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/rts/sm/Scav.c b/rts/sm/Scav.c
> index 2ecb23b..781840c 100644
> --- a/rts/sm/Scav.c
> +++ b/rts/sm/Scav.c
> @@ -285,6 +285,8 @@ scavenge_large_srt_bitmap( StgLargeSRT *large_srt )
>
>      for (i = 0; i < size / BITS_IN(W_); i++) {
>          bitmap = large_srt->l.bitmap[i];
> +        // skip zero words: bitmaps can be very sparse, and this helps
> +        // performance a lot in some cases.
>          if (bitmap != 0) {
>              for (j = 0; j < BITS_IN(W_); j++) {
>                  if ((bitmap & 1) != 0) {
>
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