opt_argmax pass

[akashlevy]

If your work is part of a larger effort, please discuss your general plans on Discourse first to align your vision with maintainers.

What are the reasons/motivation for this change?

Explain how this is achieved.

Make sure your change comes with tests. If not possible, share how a reviewer might evaluate it.

These template prompts can be deleted when you're done responding to them.

[greptile-apps]

Greptile Summary

This PR adds opt_argmax, a new Yosys optimization pass that detects combinational masked-argmax loops (priority selection with validity bits, value lookup, and a strict-< update condition) and replaces the serial loop-carried cone with a balanced binary compare tree of {valid, value, index} records. Detection is guarded by structural shape checks (bmux + lt presence, power-of-two width 4–64, matching index width) followed by a behavioral fingerprint via ConstEval over a set of covering test vectors.

• Core pass (opt_argmax.cc): builds a driver map and output cone, enumerates (valid, index, values) port triples, validates candidates with fingerprinting, then emits a balanced tree of $not / $lt / $and / $or / $mux / $bmux cells and disconnects the old output cone.
• Tests (opt_argmax.sv / opt_argmax.ys): 15 test cases including SAT-based equivalence proofs for several widths and value-width combinations, plus negative tests that confirm non-matching patterns are left untouched.

Confidence Score: 4/5

The pass is safe to merge for typical use; the rewrite logic and fingerprint tests are thorough, and the non-matching cases are well-covered.

The core detection and tree-emission logic is correct and guarded by a behavioral fingerprint over covering test vectors. Existing negative tests confirm the pass leaves non-matching designs untouched. The findings are all quality or documentation issues rather than incorrect transformations, and they do not affect correctness of the emitted netlist.

passes/opt/opt_argmax.cc — the dead cell member, the early addWire in disconnect_old_output, and the undocumented value_width > 62 limit are worth a second look before this ships to a broader audience.

Important Files Changed

Filename	Overview
passes/opt/opt_argmax.cc	New 775-line pass implementing fingerprint-based detection and balanced-tree rewrite of masked argmax loops; contains a dead struct member cell, unconditional allocation of a dangling wire in disconnect_old_output, and missing comments on non-obvious limits (value_width > 62, is_sequential exclusion list).
passes/opt/Makefile.inc	Adds opt_argmax.o to the build and reorders opt_addcin.o to be alphabetically consistent with neighbouring entries.
tests/silimate/opt_argmax.sv	Comprehensive set of 14 SystemVerilog test modules covering basic, scaled, flat-bus, multi-region, shared-consumer, and negative (non-power-of-two, bad index width, tie, non-zero default, min-select) cases for the new pass.
tests/silimate/opt_argmax.ys	Yosys script with 15 test cases including SAT-based equivalence proofs and structural cell-count assertions; all tests rely on Verific (verific -import / verific -cfg), so the suite cannot run without a Verific-enabled build.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[OptArgmaxPass::execute] --> B[For each selected module]
    B --> C[OptArgmaxWorker: build_indexes\nbuild bit_to_driver map\ncollect input_port_bits]
    C --> D[run: enumerate output wires]
    D --> E{out_width >= 2?}
    E -- No --> D
    E -- Yes --> F[get_cone: BFS from output\nup to max_cone_cells / max_leaf_bits]
    F --> G{cone has $bmux?}
    G -- No --> D
    G -- Yes --> H[For each valid input wire\nwidth == 2^out_width]
    H --> I[Collect index_buses / values_buses\nincl. split buses]
    I --> J[check_candidate]
    J --> K{width in range?\npower-of-two?\nindex_width matches?\nvalue_width in range?}
    K -- No --> H
    K -- Yes --> L{cone_has_required_shape?\nleaves_are_candidate_inputs?\nfind_anchor_driver?}
    L -- No --> H
    L -- Yes --> M[fingerprint via ConstEval\nover covering test vectors]
    M --> N{Behavioral match?}
    N -- No --> H
    N -- Yes --> O[Record Candidate\nclaimed_outputs.insert out]
    O --> D
    D --> P[For each accepted Candidate]
    P --> Q[emit_argmax:\nbmux leaves + balanced tree\nof not/lt/and/or/mux]
    Q --> R[disconnect_old_output]
    R --> S[module->connect out_wire to new output]
    S --> T[run_pass: clean -purge]

Loading

_{Reviews (1): Last reviewed commit: "opt_argmax pass" | Re-trigger Greptile}