On 2014-06-16, 10:14 AM, Ajit Kumar Agarwal wrote:
Hello All:
I have worked on the Open64 compiler where the Register Pressure Guided Unroll
and Jam gave a good amount of performance improvement for the C and C++ Spec
Benchmark and also Fortran benchmarks.
The Unroll and Jam increases the register pressure in the Unrolled Loop leading
to increase in the Spill and Fetch degrading the performance of the Unrolled
Loop. The Performance of Cache locality achieved through Unroll and Jam is
degraded with the presence of Spilling instruction due to increases in register
pressure Its better to do the decision of Unrolled Factor of the Loop based on
the Performance model of the register pressure.
Most of the Loop Optimization Like Unroll and Jam is implemented in the High
Level IR. The register pressure based Unroll and Jam requires the calculation
of register pressure in the High Level IR which will be similar to register
pressure we calculate on Register Allocation. This makes the implementation
complex.
To overcome this, the Open64 compiler does the decision of Unrolling to both
High Level IR and also at the Code Generation Level. Some of the decisions way
at the end of the Code Generation . The advantage of using this approach like
Open64 helps in using the register pressure information calculated by the
Register Allocator. This helps the implementation much simpler and less complex.
Can we have this approach in GCC of the Decisions of Unroll and Jam in the High
Level IR and also to defer some of the decision at the Code Generation Level
like Open64?
Please let me know what do you think.
Most loop optimizations are a good target for register pressure
sensitive algorithms as loops are usually program hot spots and any
pressure decrease there would be harmful as any RA can not undo such
complex transformations.
So I guess your proposal could work. Right now we have only
pressure-sensitive modulo scheduling (SMS) and loop-invariant motion (as
I remember switching from loop-invariant motion based on some very
inaccurate register-pressure evaluation to one based on RA pressure
evaluation gave a nice improvement about 1% for SPECFP2000 on some
targets).
