review for 7032963: StoreCM shouldn't participate in store elimination

[Tom Rodriguez]

On Apr 1, 2011, at 3:13 PM, Y. Srinivas Ramakrishna wrote:

> On 4/1/2011 2:55 PM, Tom Rodriguez wrote:
>> I could push this to hotspot-gc so it gets more CMS testing .
> 
> That would be a very good idea (CMS as well as G1 testing, actually)!
> 
> I can't review your changes, lacking sufficient bkgrd or
> familiarity with the code, but ...
> 
>> This actually
>>> eliminates duplicates that were previously missed so the code quality
>>> is slightly better.
> 
> wow! What more could one ask for --you fixed a correctness
> bug _and_ got us a bit more performance.

I have some other ideas about improving the code for barriers that occurred to me while looking at the conditional card marks code.  It would mainly help with the G1 code and might require a little extra work but it's simpler than the more extensive changes we've talked about before.

> Hmm, by chance does
> the fix also come with a free bottle of beer for all of us? ;-)

If you stop by my office I'd be happy to give you a beer. ;)

tom

> 
> -- ramki
> 
>> 
>> http://cr.openjdk.java.net/~never/7032963
>> 
>> 7032963: StoreCM shouldn't participate in store elimination
>> Reviewed-by:
>> 
>> StoreCM shouldn't participate in redundant store elimination since
>> that could violate the requirement that a StoreCM must be strictly
>> after a field update.  This results in a large number of redundant
>> StoreCMs being emitted for blocks of fields updates, so I added an
>> optimization to fold them up safely.  Previously the extra dependence
>> was converted into a precedence edge just before register allocation
>> but I moved this logic into final_graph_reshape.  I then added logic
>> to search through chains of StoreCMs to eliminate earlier redundant
>> ones and transfer their precedence edges to the one that is kept.
>> This ensures that they are scheduled properly.  This actually
>> eliminates duplicates that were previously missed so the code quality
>> is slightly better.  Tested by inspecting code generation with script
>> to identify duplicates.  Also ran CTW with -XX:+UseCondCardMark and
>> -XX:+UseG1GC.
>> 
>