fix: beta reduction no longer duplicates a non-trivial argument (#167) - #191

Fixes #167.

Problem

betaReduce rewrote App (Abs (ParamNamed x) body) r by substituting r into every occurrence of x, without looking at the shape of r or the number of occurrences. In a strict language that duplicates evaluation: (\x -> ... x ... x ...) (g y) became ... (g y) ... (g y) ..., so the work of g y ran once per occurrence.

The path is live. The inliner puts a lambda into call-head position for used-once bindings and for @inline always, so the better the inliner gets, the more of these redexes betaReduce sees.

Fix

Apply GHC's inlining rule, the same one inlineLocalBinding already uses:

• Substitute directly when the argument is trivial (a Ref, a literal, or @inline always, where re-evaluation is free) or the parameter is used at most once.
• Otherwise rewrite the redex to let x = r in body and leave the inline-or-not decision to inlineLocalBindings.

The guard in betaReduce is deliberately the mirror of the one in inlineLocalBinding (isInlinableExpr r || occurrences <= 1). That is what makes the rewrite converge in one bottom-up pass: the Let that betaReduce produces is exactly the one inlineLocalBindings then declines to inline, so nothing ping-pongs. Both sites reference a shared Note [Beta reduction and local inlining share an inlining guard] that spells this out.

The zero-occurrence case keeps today's behaviour: countFreeRef x body <= 1 covers it, so an unused non-trivial argument is still discarded. The Exception-in-a-dropped-argument edge follows the evaluation-order policy the pipeline already declares for let-bound values (the FloatIn note: discardable and reorderable once nothing observes them).

Tests

TDD, in IR/Optimizer/Spec.hs under a new beta reduction does not duplicate work (#167) group, all driven through optimizedExpression:

• The regression case: a non-trivial argument (g 1) with the parameter used twice. Before the fix it optimised to Eq (g 1) (g 1) (the duplication); it now optimises to let x = g 1 in Eq x x. I confirmed this case is red before the fix and green after.
• Three pins that are green both before and after, guarding the substitute path: a trivial Ref used twice still substitutes; a non-trivial argument used exactly once still substitutes; a non-trivial argument never used is still discarded.

Golden churn

Structural goldens (golden.ir / golden.lua) move in the expected direction: repeatedly-used non-trivial arguments (dictionary expressions) are now shared through a local binding instead of pasted at each use site. The hand-verified eval/golden.txt oracles are unchanged, so runtime behaviour is preserved.