------- Additional Comments From roger at eyesopen dot com 2005-04-14 17:37 ------- You'll notice in loop.c that everywhere that we currently set all_reduced to zero, we also set ignore to true. This change is to avoid wasting CPU cycles, if we know that an IV can't be eliminated, there's no point in trying to modify any more instructions that use it. At best, this incurs wasted CPU cycles, at worst we'll end up substituting in some places and not others, which will result in requiring both the original IV *and* the replacement IV which will increase register pressure in the loop.
As your (Alex's) testing showed, I'm not sure that its strictly required for correctness, it's mainly to preserve consistency with the exisiting all_reduced invariants by using the same idiom as used elsewhere, but also as a potential compile-time/run-time micro-optimization. However for 4.0, I thought it best to reuse/copy the existing idiom, rather than risk clearing all_reduced without setting ignore (which may have potentially exposed code paths not seen before). We still need the 4.1 variant to be tested/committed to mainline. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20126
