On 15/06/18 12:48, Richard Biener wrote: > > 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. > > 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. >
Could you approximate here what we've done for Arm -mfpu= - add an 'auto' option and make it the default. Over time you then deprecate other settings and eventually drop them, leaving only the default. R.
