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

[Simon Marlow]

No, it checks for and skips over complete zero words, not just trailing 
zeroes.

The bitmaps are really that sparse sometimes.  Like hundreds of zero 
words with a few non-zero bits at either end.  The change I made 
improved perf enough that this isn't a blocking issue for me any more, 
and I suspect that any further optimisation won't result in significant 
wins.  The real problem is that the code generator generates huge SRT 
bitmaps, and I bet there are bigger wins to be had there.

Cheers,
Simon

On 20/01/2015 13:45, Gabor Greif wrote:
> 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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