opt_shift: add -expand-keep-arith to keep a variable barrel off an add tree

[akashlevy]

Summary

-expand rewrites (a OP b) << c into (a << c) OP (b << c) for every OP it
handles, without looking at what the amount costs.

• A constant amount is free rewiring, so splitting it in two costs nothing.
• A bitwise OP is one gate level, so the amount's cone barely notices moving ahead of it.
• A variable amount on $add/$sub is a barrel. Expanding duplicates it and
  lands it ahead of the carry chain, so the amount's own cone gains the adder's whole
  depth instead of bypassing it. On an N-operand add tree the rewrite cascades down
  every link, ending with N barrels in front of the tree rather than one behind it.

-expand-keep-arith restricts -expand to reject exactly that last case. It
defaults off, so -expand is bit-exact unchanged without it and this PR is a
no-op to merge on its own.

Motivating shape

A gated accumulate under a variable barrel: six enabled terms sum into a 40-bit
accumulator, then the sum is shifted by bw + bh. -expand fired six times down the
tree, so the shift-amount input drove six barrels and then the entire carry-save tree
and its 39-bit final adder, instead of driving one barrel hanging off the adder output.

Measured in Preqorsor on that design: lol 29.0 -> 28.25, clk 449.3 -> 425.0,
area 295.0 -> 168.7 (the six duplicated barrels were most of the area).

Test plan

• tests/silimate/opt_expand_shifts.ys — 5 new cases, each equiv_opt -assert:
  • variable amount on $add is held (1 $add, 1 $shl)
  • variable amount on $sub is held
  • constant amount still expands
  • bitwise ops untouched by the guard (still 2 $shl)
  • four-operand tree comes out with 1 barrel, not 4
• The pre-existing cases in that file all use plain -expand and still pass,
  including "SHL across ADD" asserting 2 $shl — the default really is unchanged.
• Siblings green: opt_combine_shifts, opt_chain_shifts, opt_descale_shifts,
  opt_fuse_shifts, opt_sink_shifts, opt_expand.
• Preqorsor A/B over its 38 shift/arithmetic regressions with the flag on: 36
  bit-identical, 2 small area wins, 0 regressions.

Preqorsor side that turns it on: Silimate/preqorsor#2284.

Made with Cursor

[greptile-apps]

Greptile Summary

The PR adds an opt-in restriction that prevents variable shifts from being expanded across addition and subtraction while preserving existing default behavior.

• Adds and documents the -expand-keep-arith option.
• Propagates the option into the expansion pattern after each PMG setup.
• Rejects variable-amount $add/$sub expansion while retaining constant and bitwise expansion.
• Adds equivalence and structural tests for addition, subtraction, constants, bitwise operations, and add trees.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness, security, or compatibility failures identified.

The option defaults off, its state is assigned after each PMG reset, and the guarded pattern preserves the documented constant, bitwise, addition, and subtraction behavior with focused equivalence tests.

Important Files Changed

Filename	Overview
passes/silimate/opt_shift.cc	Adds option help, parsing, and correctly restores the PMG udata flag after each setup.
passes/silimate/peepopt_expand_shifts.pmg	Adds a narrowly scoped guard that retains variable shifts after $add and $sub cells.
tests/silimate/opt_expand_shifts.ys	Adds equivalence and cell-count coverage for all principal branches of the new option.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Match shifted binary operation] --> B{keep_arith enabled?}
    B -- No --> E[Expand shift across operation]
    B -- Yes --> C{Operation is add or sub?}
    C -- No --> E
    C -- Yes --> D{Shift amount fully constant?}
    D -- Yes --> E
    D -- No --> F[Reject rewrite and retain one output shifter]

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