http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39326
--- Comment #45 from Steven Bosscher <steven at gcc dot gnu.org> 2013-03-11 09:40:18 UTC --- Patches posted: * Restrict GIMPLE loop invariant code motion of loop-invariant loads and stores to loops with fewer memory references than a certain maximum that is controlled with --param loops-max-datarefs-for-datadeps" from the command line. http://gcc.gnu.org/ml/gcc-patches/2013-03/msg00380.html * Do not create new pseudo-registers for load-after-store transformations in RTL dead store elimination. This reduces the memory foot print after DSE by ~2 percent, and avoids the compile time and memory usage explosion in combine because it gets presented fewer single-def/single-use register moves that are really just register copies. http://gcc.gnu.org/ml/gcc-patches/2013-03/msg00379.html * Make gcse.c respect -fno-gcse-lm. For the RTL PRE problem, this means compile time is reasonable with -fno-gcse-lm. A follow-up patch will implement some mechanism to disable load motion automatically on extreme test cases like the one from this PR. http://gcc.gnu.org/ml/gcc-patches/2013-03/msg00386.html The remaining compile time bottlenecks are: - RTL dead store limination in its analysis phase. This is mostly time spent in dependence tests in alias analysis for instructions in a single basic block, so it's only a problem for test cases where there is a huge number of loads and stores in each basic block. I don't think it is worth speeding up DSE for such extreme cases. - Post-reload CSE because it is in the worst-case quadratic in the number of instructions in a basic block. In most practical cases, post-reload CSE scales linearly with the number of instructions in a basic block, but with a large constant bound. It looks up and down through the instruction chain to see if a reg is not clobbered between a use and a def. Because it only has to do so with hard registers the typical bound is closer to "number of insns in basic block" * "number of hard registers". This is fine, I am not going to try and improve this.
