compiler thoughts

Friday July 27, 2007

GCC vs GCCfss vs Studio performance on SPARC

Here is a relative performance chart for several versions of plain GCC including the latest GCC 4.2, just released GCCfss 4.0.4, previous GCCfss 4.0.3, upcoming GCCfss 4.2 and Studio 12 with the most recent patches.

All runs were done on UltraSPARC 4+ box on SPEC CPU2006 FP base with C and C++ benchmarks.

GCC flags:

 -O3 -mcpu=ultrasparc3 -funroll-loops -ffast-math -fprefetch-loop-arrays 

GCCfss and Studio flags:

-fast -xipo=2 -xpagesize=4M -xprefetch_level=2 -xalias_level=std/layout/compatible

The runs are not SPEC conforming, since Fortran benchmarks were not run, but very representative of any floating point heavy application on SPARC. So consider them an 'estimate'.

Some observations:

• GCCfss 4.0.4 is using Studio 12 backend, so the difference between GCCfss 4.0.4 and Studio 12 can only be explained by differences in IR generated by front-ends.
• GCCfss 4.0.3 allows most of the gcc tree-ssa optimizers to run, whereas GCCfss 4.0.4 has completely different 'gimplification' process, so no tree-ssa passes can run, but outgoing IR is much better suited for Studio 12 backend to work on. As demonstrated by the jump from GCCfss 4.0.3 to 4.0.4
• GCCfss 4.2 is 'work in progress', which should be released by the end of this year