Vectorizer size preferences (-mprefer-* on x86)

[Richard Earnshaw (lists)]

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.