20260514 tricky cse assign

[prozacchiwawa]

Because this would have kept programs from compiling, this does not need to be feature gated.

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Note

Medium Risk
Changes compiler optimization semantics for assign/CSE interaction; incorrect ceiling logic could skip valid CSE or still miscompile, though new tests lock in the reported breakage case.

Overview
Fixes common subexpression elimination (CSE) incorrectly hoisting shared code above assign / let bindings when one occurrence lives in a binding that depends on other names in that form (e.g. B uses A while (+ A 1) is also duplicated in the assign body).

detect_common_cse_root now takes an optional ceiling path and returns Option: it refuses a root above a containing assign body that still provides names the subexpression needs, or when the only common root would sit above that ceiling. cse_optimize_bodyform computes that ceiling via new match_bindings (names in the duplicated expr ∩ names bound by enclosing lets) and skips the CSE rewrite when no safe root exists.

Adds regression coverage: unit test for ceiling rejection, cse-complex-2.clsp, and test_cse_breakage_example_lift_outside_bindings (compile + run expects 1).

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