[FSM] Eliminate mutually exclusive transition guards (#3577)#10754
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[FSM] Eliminate mutually exclusive transition guards (#3577)#10754arefinaa wants to merge 1 commit into
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When a state has exactly two outgoing transitions whose guards are logical complements of one another (e.g. `cond` and `!cond`), the second transition is only reached when the first guard has already failed, which implies the second guard necessarily holds. The second guard is therefore redundant. This adds a canonicalization to `StateOp::canonicalize` that detects this case and strips the second transition's guard, turning it into an unconditional default transition. The complement is recognized in both the comb (`comb.xor %cond, %true`) and arith (`arith.xori %cond, %true`) spellings; the now-dead complement op is removed by dead-code elimination. This removes redundant logic that the Calyx -> FSM lowering emits verbatim and that would otherwise propagate all the way to emitted SystemVerilog. The pattern is conservatively scoped to states with exactly two transitions to avoid the non-determinism that affected earlier unreachable-state work. Adds lit tests covering the comb and arith positive cases and negative cases (non-complementary guards, and a three-transition state). Assisted-by: Claude Code:claude-opus-4-8 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What does this PR do?
Adds a canonicalization to
fsm.statethat detects when a state has exactlytwo outgoing transitions whose guards are logical complements of one another
(e.g.
condand!cond) and strips the second transition's guard, turning itinto an unconditional default transition. Since the second transition is only
reached when the first guard has already failed, the second guard necessarily
holds and is redundant. The complement is recognized in both the comb
(
comb.xor %cond, %true) and arith (arith.xori %cond, %true) spellings.Why was this PR needed?
The Calyx → FSM lowering emits these mutually exclusive guard pairs verbatim,
and the redundant logic propagates all the way into the emitted SystemVerilog.
The pattern is conservatively scoped to states with exactly two transitions to
avoid the non-determinism that affected earlier unreachable-state work on this
issue.
What are the relevant issue numbers?
Fixes #3577
Testing
test/Dialect/FSM/canonicalize.mlir: positive casesfor both comb and arith complement spellings, and negative cases
(non-complementary guards, three-transition state)
Assisted-by: Claude Code:claude-opus-4-8