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From Naveen Neelakantam <neela...@illinois.edu>
Subject Re: Fwd: [jira] Created: (HARMONY-6007) [drlvm][jit][abcd] classic abcd pass fixes
Date Tue, 02 Dec 2008 08:30:54 GMT
Sorry for the delayed response.  Clearly this is not my "day" job.  :-)

Responses are inline.

Egor Pasko wrote:
> On the 0x4F2 day of Apache Harmony Naveen Neelakantam wrote:
>> Would someone who understands the ABCD pass be willing to check my patch?
>> Egor if you still follow the list, could you please take a look?
> Naveen, again thanks for the great work!
> though I have some concerns about your patch:
> 1. changes in the solver are very unwanted because we want to make the
>    solver as readable as possible (given that the concept is really
>    complicated). Once we want equivalent Pi operands to behave like a
>    single Pi operand, we should better link them together in the
>    inequality graph (zero-edges, both higher and lower graphs). The
>    solver goes through zero chains just fine.
I agree with the spirit of your comment, however in this case your 
suggestion is infeasible.  There are two alternatives as I see it, which 
are dictated by the specifics of the algorithm.  Either:
a) The upper bound and lower bound problems are solved using separate 
e-SSA graphs, or
b) The solver is modified to adopt the notion of pi-equivalence when 
checking for termination conditions (and _only_ when checking for 
termination conditions).

As a thought experiment, consider the implications of allowing zero 
edges between pi-renamed variables.  It would essentially convey 
constraints granted onto pi-renamed variables upon their sources.  This 
would make it possible for sources to appear constrained by inequalities 
that do _not_ dominate them.  This is clearly incorrect.

In my opinion something must be done here, or ABCD should simply be 
disabled as it is otherwise crippled.

Clearly this is a complicated problem, and I welcome argument. :-)
> 2. "hvn,simplify,dce,uce,memopt,dce,uce" -- hmm.. this is big change,
>    may affect a lot. Does anybody run benchmarks these days to ensure
>    we do not affect performance with this?
Can someone please comment on this?  I too am very concerned about the 
effect this might have on compilation speed.
> 3. "memopt" contains "hvn", is there really a reason to run it twice?
Probably not.

I will try removing hvn from the mix and see what happens.  Thanks for 
the heads up
>> Thanks,
>> Naveen
>> Naveen Neelakantam (JIRA) wrote:
>>> [drlvm][jit][abcd] classic abcd pass fixes
>>> ------------------------------------------
>>>                  Key: HARMONY-6007
>>>                  URL: https://issues.apache.org/jira/browse/HARMONY-6007
>>>              Project: Harmony
>>>           Issue Type: Bug
>>>          Environment: RHEL4, 32-bit x86, gcc 4.1.2
>>>             Reporter: Naveen Neelakantam
>>> The ABCD pass has effectively been crippled by changes that have been made to
DRLVM over the past year (I haven't been able to narrow down when the change happened).
>>> The good news is that with two simple fixes, ABCD works again.  The attached
patch adds the necessary fixes.
>>> To see the problem however you can try running the bi-directional bubble sort
from HARMONY-1564.  It is well-known that ABCD should be able to eliminate all the bounds
checks from the sort algorithm.  However, if you run drlvm without this patch, none of the
bounds checks are eliminated.  You can examine the logs by doing the following:
>>>> java -XX:jit.p.filter=BidirectionalBubbleSort.sort -XX:jit.p.arg.log=ct,irdump
-Xem:server_static BidirectionalBubbleSort
>>> After applying the patch, all the bounds checks will once again be eliminated.
 There were two separate problems to blame, which this patch addresses:
>>> 1) Harmony now inserts multiple "arraylen" operations when translating from bytecode,
even if the operation is redundant (because it post-dominates an earlier "arraylen" operation
for the same array).  This redundancy must be eliminated before running ABCD otherwise the
inequality graph that ABCD generates for its proofs will have unnecessary discontinuity. 
Thankfully, solving the problem is as simple as running the following passes before ABCD:
>>> 2) Our implementation of ABCD includes a novel design by Egor Pasko where both
the upper-bound and lower-bound problems can be solved using the same inequality graph.  In
addition the implementation adds more constraints to the graph than the original ABCD authors
specified.  Both improvements are correct, except that they can prevent the ABCD solver from
finding valid solutions because pi-renamed variables appear to be different.  The patch modifies
the solver so that it treats pi-renamed variables as equivalent when checking for termination
>>> I've verified that HARMONY-2141, HARMONY-2144, and HARMONY-2147 still pass with
these changes.

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