Issue #8399 has been updated by dbussink (Dirkjan Bussink).


ko1 (Koichi Sasada) wrote:
> (2013/05/15 14:38), dbussink (Dirkjan Bussink) wrote:
>  > If we're changing something anyway, it makes far more sense to change to either RARRAY_AREF or rb_ary_entry in extensions. I also discussed this on irc (#ruby-core) with ko1 and he also agrees with me in that respect. 
>  
>  My thoughts are:
>  
>    (3) I think RARRAY_PTR() is needed in a few cases for performance and
>        usability with C functions.
>        I want to introduce RARRAY_PTR_USE(ary, ptr, expr).
>          /* example code */
>          RARRAY_PTR_USE(ary, ptr, {
>            memset(ptr, 0, RARRAY_LEN(ary));
>          });
>        In this macro, we can observe that `ptr' is live only in `expr'.

How does this guarantee that the expression doesn't for example calls something that GC's? In that case, having to scan the whole array for pointers for a writer barrier would likely outweigh the benefit of doing this. Or am I missing something in this reasoning? Also this example doesn't remove the need for having to scan the array afterwards for pointers to young objects in a generational GC.
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Bug #8399: Remove usage of RARRAY_PTR in C extensions when not needed
https://bugs.ruby-lang.org/issues/8399#change-39352

Author: dbussink (Dirkjan Bussink)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 
ruby -v: trunk
Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN


Rubinius uses quite a few C extensions directly from MRI. Some of these use functionality such as RARRAY_PTR which is not necessary. For compatibility reasons, RARRAY_PTR works on Rubinius but suffers from a heavy performance penalty. Take for example the test of the parser gem (http://github.com/whitequark/parser). These run over 10x faster with the patch applied to Racc that is submitted here:

https://gist.github.com/dbussink/57c32c08fb21c7a41719

Consider issue #8339 where there is work being done on generational GC, I think it is also beneficial to remove usage of internal structures such as RARRAY_PTR where there is the problem of going around the write barrier. In Rubinius, an array is treated special if RARRAY_PTR is used on it in the C-API, so I can imagine MRI being able to optimize the GC better if extensions don't do this. There are functions available for both getting and setting elements in an array and they work fine.

I have only make a patch against Racc here as a showcase, I also want to update all the other extensions to remove RARRAY_PTR. Please consider this change to MRI since in my opinion it has benefits also for MRI and so Rubinius can keep using these extensions directly without having to maintain custom versions just for the considerations described here. I'm also already actively checking C extension gems and sending pull requests for updating this.



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