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

[Y. Srinivas Ramakrishna]

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

-- 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.
>