> > Hi, > > I'm in the process of changing the vectorizer to consider all > vector sizes as advertised by targetm.autovectorize_vector_sizes > and to decide which one to use based on its cost model. > > I expect that to make sense for example when choosing between > AVX128 and AVX256 since the latter may have penalties for > cross-lane operations which the former lacks. > > Now while the option -mprefer-* and the documentation (to > some extent) suggests that the user communicates a preference > the actual implementation in the x86 backend uses -mprefer-* > to disable the use of larger vector sizes. That also makes > targetm.preferred_simd_mode somewhat redundant. > > In the light of using the cost model to decide on the vector > width used how should we go forward here? > > I would suggest to more clearly document that -mprefer-* > will disable the use of larger vector sizes and that smaller > vector sizes might be used if costs tell us to do so (or loops > do not roll enough). Implementation-wise the size chosen > by targetm.preferred_simd_mode could be used as tie-breaker > in case equal costs are computed.
I suppose one useful meaning of -mpreffer would be to overwrite the decision heuristics to only consider the mode specified. (it is what we do at the moment anyway, right?) So one can use it in the case your heuristics makes wrong choice. But disabling only bigger vector sizes makes sense to me as well. Honza > > In the (distant?) future we might want to support mixed-size > vectorization which makes all this even more complicated. > Still having -mprefer-* disabling vector sizes makes sense > here? > > Thanks, > Richard.
