Vectorizer size preferences (-mprefer-* on x86)

[Richard Biener <rguenther@suse.de>]

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.