muxpack usage

[ben-j-c]

Hello,

I'm trying to use muxpack from the shell and it doesnt seem to be doing anything when I have three muxes that I expect to be merged to one pmux.

Here is my working directory:
https://github.com/ben-j-c/riscv_v2f_optimized/tree/ffb8102c85fee99f8051914f9d7adf1ff09a9e75/testbench/divider

Here is show after read_json riscv_divider_rtl.json

When I execute

read_json riscv_divider_rtl.json
muxpack
show

I get the following result and the image above.

 /----------------------------------------------------------------------------\
 |  yosys -- Yosys Open SYnthesis Suite                                       |
 |  Copyright (C) 2012 - 2025  Claire Xenia Wolf <[email protected]>         |
 |  Distributed under an ISC-like license, type "license" to see terms        |
 \----------------------------------------------------------------------------/
 Yosys 0.52 (git sha1 fee39a328, g++ 13.3.0-6ubuntu2~24.04 -fPIC -O3)

yosys> read_json riscv_divider_rtl.json 

1. Executing JSON frontend.
Importing module v2f_xor from JSON tree.
Importing module v2f_sub from JSON tree.
Importing module v2f_sshr from JSON tree.
Importing module v2f_sshl from JSON tree.
Importing module v2f_shr from JSON tree.
Importing module v2f_shl from JSON tree.
Importing module v2f_reduce_or from JSON tree.
Importing module v2f_reduce_and from JSON tree.
Importing module v2f_pow from JSON tree.
Importing module v2f_pmux from JSON tree.
Importing module v2f_or from JSON tree.
Importing module v2f_not from JSON tree.
Importing module v2f_neg from JSON tree.
Importing module v2f_ne from JSON tree.
Importing module v2f_mux from JSON tree.
Importing module v2f_mul from JSON tree.
Importing module v2f_mod from JSON tree.
Importing module v2f_lt from JSON tree.
Importing module v2f_le from JSON tree.
Importing module v2f_gt from JSON tree.
Importing module v2f_ge from JSON tree.
Importing module v2f_eq from JSON tree.
Importing module v2f_div from JSON tree.
Importing module v2f_and from JSON tree.
Importing module v2f_add from JSON tree.
Importing module riscv_divider from JSON tree.

yosys> muxpack

2. Executing MUXPACK pass ($mux cell cascades to $pmux).
Converted 0 (p)mux cells into 0 pmux cells.
<suppressed ~3 debug messages>

yosys> show

I would expect that that red mux should be merged with its two fanin muxes.

Am I using this command correctly? Is there a reason it doesn't apply here?

Thanks,
Ben

[ben-j-c]

This optimisation is conservative --- it will only pack $mux or $pmux cells
whose select lines are driven by '$eq' cells with other such cells if it can be
certain that their select inputs are mutually exclusive.

Hmm, is there any way that I can get a non-conservative behaviour? I would like to have those two bits on the selects to decode into a 1 hot and then feed that into a $pmux.

Doing that would let me do further optimizations later.