Tobias Grosser wrote
Hi graphities,
graphite consists of four flags "-floop-block", "-floop-interchange",
"-floop-stripmine" and "-fgraphite".
If any of these flags is set, we enable the graphite pass and we search
for SCoPs.
For every SCoP we try to apply transformations specified with
"-floop-block", "-floop-interchange" or "-floop-stripmine". But only if
we could apply a transformation, we replace the SCoP with new code
generated from the GRAPHITE data structures. Otherwise we do not touch
the GIMPLE representation.
If we only specify "-fgraphite", we never generate code for any SCoP, as
we never tried any transformation. So just using "-fgraphite" is
useless.
What is missing is a way to make GRAPHITE always generate code, even if
we did not apply any transformation.
This has two benefits:
- We can check the overhead the GIMPLE -> GRAPHITE -> GIMPLE
transformation costs.
- Wider test coverage, as we are able to run the code generator for
every SCoP, not only the SCoPs, we are able to transform.
Now:
----
-fgraphite: Do nothing.
-floop-block, -floop-interchange, -floop-stripmine: Try these
transformations.
Only "-fgraphite": Do nothing
Only "-floop-block": Only loop blocked SCoPs are changed.
"-fgraphite -floop-block": Only loop blocked SCoPs are changed.
One solution: Reuse -fgraphite
------------------------------
-fgraphite: Enable always code generation
-floop-block, -floop-interchange, -floop-stripmine: Try these
transformations.
Only "-fgraphite": The identity transformation for all SCoPs.
Only "-floop-block": Only loop blocked SCoPs are changed.
"-fgraphite -floop-block": All SCoPs are are changed. Some SCoPs are
loop blocked, others just the identity
transformation.
This allows us to get all the benefits but (mis)uses the -fgraphite
flag. At the moment it has no other meaning, but I could think of it
containing an automatic optimizer, that applies all available graphite
transformations or selects them automatically.
The other solution is: Use -fgraphite-identity
----------------------------------------------
-fgraphite: Enable always code generation
-floop-block, -floop-interchange, -floop-stripmine, -fgraphite-identity:
Try these transformations.
Only "-fgraphite": Do nothing. Free for later use.
Only "-floop-block": Only loop blocked SCoPs are changed.
Only "-fgraphite-identity": The identity transformation for all SCoPs.
"-fgraphite-identity
-floop-block": All SCoPs are are changed. Some SCoPs are
loop blocked, others just the identity
transformation.
This makes sense, as we only generate code for applied and enabled
transformations. These are loop-blocking, interchange, stripmine and
- may be - the new and very easy to write identity transformation, that
does nothing, but enable code generation.
What du you think about these different solutions?
Hi Tobias,
I am in favor of prefixing all transformation passes with -fgraphite-
leading to -fgraphite-block -fgraphite-stripmine -fgraphite-identity and
the new ones to come. These flags should probably disappear at some
point, as a more unified optimization heuristic should arise that
combines all transformations, with some parameters to influence the
profitability heuristic.
I agree -fgraphite should be reserved for later use, but not mandatory
to use any of the new -fgraphite-xxx options.
This is just a suggestion with a partial understanding of the option
naming scheme in GCC only!
Albert
