repacker seems underconstrained and fully constrained would be simpler and easier to work with

[rs-dhow]

In the pb_type_annotations section of openfpga.xml, there is a property physical_pb_type_index_offset which can be used as follows.

An FLE may have a 2xLUT5 mode with 2 physical FFs. When it is in 1xLUT6 mode, only one FF may be available. This property indicates which FF is to be used.

This pins down the repacker and forces it to use a particular FF. This means it has no choice which hardware subblocks to use during repacking.

Why is a similar idea not used with the muxes? Muxes in the physical mode can have the following relationships to a logical mode mux visible to VPR's packer in the selected mode:

1. The physical mux is not on any path used in the logical mode. Its setting is don't-care.
2. The physical mux must be configured to a particular fixed setting in the logical mode. Its setting matters, but it's not alterable in this mode.
3. The physical mux's setting must follow the setting of a mux in the logical mode. The logical mux may have settings 0 and 1, selecting two different signals, and the physical mux may have settings 0, 1, 2, and 3, selecting four different signals. Say, when the logical is 0, the physical must be 2, and when the logical is 1, the physical must be 3. This mapping of mux selects is 0-->2, 1-->3, logical-->physical, in this particular logical mode.

Every logical mode would directly force some muxes via (2) (like logical modes can now force mode bits). The combination of a logical mode and that logical mode's mux selects would then force other muxes via (3).

This seems to entirely avoid any surprises from the repacker, like those reported in #1670, #1671, and #1672.

Thanks.

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