Faster filling of arrays of holes.

Faster filling of arrays of holes.

Committed: http://code.google.com/p/v8/source/detail?r=3977

[antonm, 2010-02-25 20:14:18]

Søren,

may you have a look?

There might be even faster implementations, but let's start with the basic one.

If overall inline asm is ok, I'd port it under VS.  For ARM the loop might be
the best approach (at least I am not aware of any better approach.)

[Søren Thygesen Gjesse, 2010-02-25 21:40:14]

LGTM

You could look at the code generated for the loop on ARM - it probably cannot be
improved.

Are there other placed we can use this. I am thinking filling hash tables with
undefined.

http://codereview.chromium.org/661105/diff/1/2
File src/heap.cc (right):

http://codereview.chromium.org/661105/diff/1/2#newcode2981
src/heap.cc:2981: // update RSets.
Please keep the assert.

http://codereview.chromium.org/661105/diff/1/3
File src/utils.h (right):

http://codereview.chromium.org/661105/diff/1/3#newcode584
src/utils.h:584: asm("cld\n\trep\n\t" STOS
Please break this into one instruction per line for better redability.

http://codereview.chromium.org/661105/diff/1/3#newcode587
src/utils.h:587: :);
Maybe make a comment why the clobber list is empty.

http://codereview.chromium.org/661105/diff/1/3#newcode594
src/utils.h:594: #undef HAS_STOS
HAS_STOS -> STOS

[antonm, 2010-02-26 11:54:39]

> You could look at the code generated for the loop on ARM - it probably cannot
be improved.

The problem is gcc with current defaults generates plain loop for ia32.  I
googled a bit, but all the memset32 implementations I found seems to use loop
(with some unrolling).

> Are there other placed we can use this. I am thinking filling hash tables with
undefined.

Yes, undefined and I suspect that stub which creates Array for new Array doesn't
use stos as well.  Going to audit the code.

Thanks a lot for review, Søren, submitting.

http://codereview.chromium.org/661105/diff/1/2
File src/heap.cc (right):

http://codereview.chromium.org/661105/diff/1/2#newcode2981
src/heap.cc:2981: // update RSets.
On 2010/02/25 21:40:15, Søren Gjesse wrote:
> Please keep the assert.

Done.

http://codereview.chromium.org/661105/diff/1/3
File src/utils.h (right):

http://codereview.chromium.org/661105/diff/1/3#newcode584
src/utils.h:584: asm("cld\n\trep\n\t" STOS
On 2010/02/25 21:40:15, Søren Gjesse wrote:
> Please break this into one instruction per line for better redability.

Done.

http://codereview.chromium.org/661105/diff/1/3#newcode587
src/utils.h:587: :);
On 2010/02/25 21:40:15, Søren Gjesse wrote:
> Maybe make a comment why the clobber list is empty.

Done.

http://codereview.chromium.org/661105/diff/1/3#newcode594
src/utils.h:594: #undef HAS_STOS
On 2010/02/25 21:40:15, Søren Gjesse wrote:
> HAS_STOS -> STOS

Done.