On Mon, 2014-06-16 at 14:14 +0000, 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.
I have been working on calculating something analogous to register pressure using a count of the number of live SSA values during the ipa-inline pass. I've been working on steering inlining (especially in LTO) away from decisions that explode the register pressure downstream, with a similar goal of avoiding situations that cause a lot of spill code. I have been working in a branch if you want to take a look: gcc/branches/lto-pressure Aaron > > Thanks & Regards > Ajit > -- Aaron Sawdey, Ph.D. acsaw...@linux.vnet.ibm.com 050-2/C113 (507) 253-7520 home: 507/263-0782 IBM Linux Technology Center - PPC Toolchain
