Software is Crap

Updated C compiler benchmarks: llvm-2.7, libfirm-1.18.1


Since I did my previous C compiler benchmark, there have been a few new software releases, including Gcc 4.5, llvm 2.7 (and 2.6) and libfirm 1.18 (and 1.17). Here are results from benchmarking these compilers:

Most important things: Gcc version is now 4.4.4, though this hasn’t made much difference to the results. llvm-2.7 is benchmarked both with the “native” clang frontend, and the gcc-4.2 based frontend; note the difference in bm1, 3 and 6.

In the case of LLVM, results have improved significantly in bm2 and bm5.

For LibFirm, results have improved dramatically for bm5, but are also significantly worse for bm3.

Gcc is still (subjectively) the overall winner, though it does get beaten in a few of the benchmarks (bm2,6,7) by LLVM and even in a couple (bm5,7) by LibFirm.

Gcc 4.5 has been released but has yet to be benchmarked. Edit 13/7/2010: Gcc 4.5 benchmarked, and the results are so similar to those for Gcc 4.4.4 that it’s not worth reporting them separately.

There are three new tests:

bm8 tests the compiler’s ability to remove trivial loops (loops with empty bodies, but a large number of iterations). The loops are implemented with “goto” rather than a C loop structure, and there is an inner and outer loop. No tested compiler manages to  remove the loops.

bm9 tests the compiler’s ability to break up a loop into two separate loops, when doing so avoids an efficiency problem caused by a potential aliasing pointer. The function in the benchmark takes two arguments: an int array and an int pointer “p”. The inner part of the loop uses the value pointed at by “p” as part of an expression assigned to each array element in turn. Because “p” might alias one of the array elements, the stored value must be re-loaded each iteration, unless the compiler breaks the loop into two. None of the tested compilers perform this optimization.

bm10 tests that a shift-left of a value inside a large loop can be reduced to a constant (due to having shifted all the bits out). None of the tested compilers manage this.

It’s disappointing that some optimizations which potentially yield huge benefit are not implemented in any of the tested free compilers; however, GCC 4.5 has been released and is yet to be tested (edit 13/7/2010: tested, no surprises). Also, GCC includes some options for loop optimization which are not turned on at any optimization level, due to them requiring external libraries at build time; it would be interesting to see whether these would make any difference in any of the tests (13/7/2010. They don’t).

Further update 4/8/2010: I’ve just tested with gcc 4.5.1. No very noticable differences were seen from the performance of 4.4.4, however, I am running a new kernel and this seems to slightly affect the numbers for 4.4.4; for instance, bm3 comes down to about the 0.162 mark, more-or-less even with LLVM.