Adjust interpretation of SYNTH_KEEP_MODULES

[openroad-ci]

Only apply SYNTH_KEEP_MODULES when SYNTH_HIERARCHICAL=1. Also make this variable override the list of kept modules selected by the SYNTH_MINIMUM_KEEP_SIZE threshold. This is done to reduce user confusion.

[povik]

FYI @oharboe @jeffng-or

[povik]

We could go further and make setting SYNTH_KEEP_MODULES without enabling SYNTH_HIEARCHICAL an error as the user probably intended hierarchical synthesis.

[oharboe]

We could go further and make setting SYNTH_KEEP_MODULES without enabling SYNTH_HIEARCHICAL an error as the user probably intended hierarchical synthesis.

I use SYNTH_KEEP_MODULES to ensure they are not flattened together or without SYNTH_HIERACHICAL to automatically flatten small modules

Also, I do parallel abc builds in bazel where I use SYNTH_KEEP_MODULES as part of the mechanics. Parallel abc is MUCH faster on my design.

[oharboe]

So I may need infinite minimum keep size if I want to manually list keep modules?

Confusing.

Maybe SYNTH_HIERARCHICAL should be removed?

We either keep modules or not based on automatic and/or manual criteria?

[povik]

So I may need infinite minimum keep size if I want to manually list keep modules?

We either keep modules or not based on automatic and/or manual criteria?

The state after this change would be: SYNTH_HIEARCHICAL=1 implies we keep some modules. If SYNTH_KEEP_MODULES is defined, the selection is manual, otherwise we go by minimum keep size.

Let me know if this sounds good. This is in response to a seasoned user of ORFS being confused by how the options currently work (if you want manual selection, you need to define KEEP_MODULES and disable SYNTH_HIERARCHICAL)

[povik]

I use SYNTH_KEEP_MODULES to ensure they are not flattened together or without SYNTH_HIERACHICAL to automatically flatten small modules

@oharboe I see so with this change we won't be able to support your "together" use case.

[oharboe]

I use SYNTH_KEEP_MODULES to ensure they are not flattened together or without SYNTH_HIERACHICAL to automatically flatten small modules

@oharboe I see so with this change we won't be able to support your "together" use case.

Yes. We either keep modules or not and we have manual and/or automatic criteria.

The default minimum automatic keep size is a PDK specific default.

[povik]

We need a different change then.