Current --keep-hierarchy status

[Muxianesty]

Greetings everyone.

I've picked up yosys-slang quite recently and now I want to know what is being done when --keep-hierarchy is used.

I've noticed while processing "standard" designs like PicoRV32 from Yosys (yes, it's in Verilog but it shouldn't be a problem for the plugin) that even when using --keep-hierarchy along with --top the RTL is still elaborated and flattened.

In the help message I've noticed the following:

--keep-hierarchy                                      Keep hierarchy (experimental; may crash)
--best-effort-hierarchy                             Keep hierarchy in a 'best effort' mode

(This "may crash" is also bugging me.)
(I am also not sure what is meant by "best effort" while preserving the module hierarchy.)

Am I correct to assume that this flattening is by design and there is no current way (or need) to keep the module hierarchy for RTLIL as is?

[povik]

Hi @Muxianesty,

the help for these options needs updating. With --keep-hierarchy the frontend attempts to preserve the hierachy and will error out if it's unable to do so (mainly due to hierarchical references or due to non-modport interface port connections), --best-effort-hierarchy is similar but allows for selective flattening in cases which would otherwise error out.

Am I correct to assume that this flattening is by design and there is no current way (or need) to keep the module hierarchy for RTLIL as is?

No, --keep-hierarchy disables flattening. See e.g. this Ibex hierarchy as pulled from the CI:

2.2. Analyzing design hierarchy..
Top module:  \ibex_core
Used module:     \ibex_wb_stage$ibex_core.wb_stage_i
Used module:     \ibex_load_store_unit$ibex_core.load_store_unit_i
Used module:     \ibex_cs_registers$ibex_core.cs_registers_i
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_cpuctrlsts_part_csr
Used module:         \ibex_counter$ibex_core.cs_registers_i.minstret_counter_i
Used module:         \ibex_counter$ibex_core.cs_registers_i.mcycle_counter_i
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mstack_cause_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mstack_epc_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mstack_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_dscratch1_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_dscratch0_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_depc_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_dcsr_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mtvec_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mtval_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mcause_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mscratch_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mie_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mepc_csr
Used module:         \ibex_csr$ibex_core.cs_registers_i.u_mstatus_csr
Used module:     \ibex_ex_block$ibex_core.ex_block_i
Used module:         \ibex_multdiv_fast$ibex_core.ex_block_i.gen_multdiv_fast.multdiv_i
Used module:         \ibex_alu$ibex_core.ex_block_i.alu_i
Used module:     \ibex_id_stage$ibex_core.id_stage_i
Used module:         \ibex_controller$ibex_core.id_stage_i.controller_i
Used module:         \ibex_decoder$ibex_core.id_stage_i.decoder_i
Used module:     \ibex_if_stage$ibex_core.if_stage_i
Used module:         \ibex_compressed_decoder$ibex_core.if_stage_i.compressed_decoder_i
Used module:         \ibex_prefetch_buffer$ibex_core.if_stage_i.gen_prefetch_buffer.prefetch_buffer_i
Used module:             \ibex_fetch_fifo$ibex_core.if_stage_i.gen_prefetch_buffer.prefetch_buffer_i.fifo_i
Removed 0 unused modules.

What does you script look like when you're not seeing the hierarchy preserved?

[Muxianesty]

I've found the mistake on my side. Sorry for the inconvenience.

But the options' descriptions were helpful nonetheless.
Thank you very much.

[povik]

It's no problem. The ping to have the descriptions updated is good.