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

[Gabor Greif]

On 1/20/15, Simon Marlow <marlowsd at gmail.com> wrote:
> No, it checks for and skips over complete zero words, not just trailing
> zeroes.

Sure, I mean the for-loops after your if-condition, in lines 291 and 304
https://git.haskell.org/ghc.git/blob/9894f6a5b4883ea87fd5f280a2eb4a8abfbd2a6b:/rts/sm/Scav.c#l291

Cheers,

    Gabor

>
> 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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>