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.
