Before this patch the machine scheduler was unable to reorder sequences like the following:
%vreg42:sub0 = ... %vreg42:sub1 = ...
This patch series enables this. It is however disabled by default. Targets that benefit from this (probably R600) can enable it by change the "shouldTrackLaneMasks" flag in the SchedulingPolicy/Strategy.
About the patch:
Tracking lanemasks allows the scheduler to differentiate between
different subregister defs and schedule them independently even when
they go into the same vreg.
For this to work we have to take care that the subregister defs count as
register pressure neutral except for the first one. We also have to attach the
reads-undef marker to the first subregister def and remove it from all