opt_signif: carry operand ranges across module boundaries

[akashlevy]

Summary

opt_signif analysed one module at a time, so an instance was an opaque driver whose output port contributed its full declared width. A dot-product lane behind a module boundary therefore had its accumulate tree priced at the whole datapath bus, even though the lane's exponent range can only reach a fraction of it.

That is the root cause of the sawtooth we have been chasing in a customer's block-scaled MX dot products. The dead fraction scales with how narrow the format is, so the error was strongly family-dependent: on a 64-bit bus an e2m1 lane occupies 11 bits and over-counted by about 2x, while e4m3 occupies 39 bits and over-counted by about 1.2x. Adjacent points in their sweep alternate between families, which is what produced teeth rather than a uniform offset.

Four changes:

• Hierarchical propagation. HierCache computes and caches output-port ranges of instantiated modules; sig_range consults it for instances. Driver gained cell/port/offset so built-in cells and instances share one path instead of special-casing ports named Y.
• -summarize. Flows that blackbox modules before narrowing them have no body left to read. This stamps ranges onto output ports as signif_range_signed / signif_range_unsigned while the bodies still exist, and a later run reads them back off the blackbox. -no-cross-hier disables the feature entirely.
• Unsigned range fit. cell_range judged every range by its signed width, discarding any non-negative range that reaches into the top half of its word. A 6-bit word holding [0,45] is a block-scaled lane's exponent sum, and it was being thrown away. sig_range still re-checks per reader, so a signed reader of such a word falls back to the declared range.
• -min-bits <n>. Skips narrowings saving fewer than n bits. arith_tree only chains a cell whose output is read whole, so shaving one bit off a parent's operand port can drop a sum below carry_save_min_width and cost a whole compressor level. This was a real regression on tile_mul_shift, where a 29-bit adder stopped forming a 30-bit compressor.

Measured effect

Reproduced the customer's sweep locally (50 designs, asap7). Comparing predicted area against their reported real area, with -no-cross-hier standing in for the shipped behaviour:

configuration	median pred/real	spread	rank corr	mean err
without hierarchical propagation	1.93	7.93x	0.389	157%
with it	0.97	1.80x	0.961	10%

Flattening the design reaches the same place (0.96 / 1.88x / 0.961), which is a useful workaround for users today but costs runtime and defeats parallel batching on large designs.

Test plan

• tests/opt/opt_signif.ys passes; new groups cover single-level and two-level hierarchy, unbounded submodule inputs as a negative control, blackboxes with narrow and wide ports, repeated instances of one module, an instance with a port named Y, the attribute round-trip including stale-attribute removal, wide constant words under -cross-flops, and the -min-bits threshold
• Formal equivalence proof of hierarchical narrowing via the gold/gate flatten idiom
• Rebuilt and re-ran against current main (42 commits past the original base)
• Downstream Preqorsor regression suite green: 354 passed, 8 xfailed

Made with Cursor

[greptile-apps]

Greptile Summary

This PR extends opt_signif range analysis across module boundaries and preserves summaries through blackboxing, while adding controls for disabling hierarchy traversal and filtering low-value narrowings.

• Caches signed and unsigned output-port ranges for instantiated modules.
• Adds -summarize, -no-cross-hier, and -min-bits options.
• Improves handling of wide constants and non-negative ranges reaching the top half of a word.
• Adds hierarchy, blackbox round-trip, wide-constant, threshold, and equivalence tests.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

Hierarchical ranges are computed with unconstrained child inputs, selected by reader signedness, checked against consumed widths, and conservatively fall back to declared ranges when analysis is unavailable; the added tests cover the principal new execution paths.

Important Files Changed

Filename	Overview
passes/opt/opt_signif.cc	Adds conservative hierarchical range propagation, persisted blackbox summaries, wide-constant handling, and a minimum-savings threshold; no changed-code defect was established.
tests/opt/opt_signif.ys	Expands regression coverage for hierarchy traversal, summary attributes, repeated instances, boundary constants, narrowing thresholds, and formal equivalence.

Sequence Diagram

sequenceDiagram
    participant Parent as Parent SignifWorker
    participant Cache as HierCache
    participant Child as Child SignifWorker
    Parent->>Cache: Request instance output range
    alt Summary not cached
        Cache->>Child: Analyze output port with unconstrained inputs
        Child-->>Cache: Signed and unsigned ranges
        Cache->>Cache: Cache ranges by module and port
    end
    Cache-->>Parent: Range for reader signedness
    Parent->>Parent: Validate range fits consumed bits
    Parent->>Parent: Narrow eligible operand if savings meet min-bits

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[akashlevy]

Closing for now, as we need a better test case before we can continue here