isl.py's UNIT_ARG decides whether a built-in's slot-4 operand prints as a unit name
or a raw number. It was inferred statistically from operand ranges and, by its own
comment, listed a slot "only when the ratio stayed below 1.0" -- conservative.
The vtable base makes it a lookup instead: every unit-taking built-in's implementation
opens with lwz 324(phase) / lwz 4(argbase) / rlwinm 2,0,29 / lwzx / lwz 4(rec). Read
directly for all 147:
implementation indexes [phase+324] by an argument 55
of the statistical set's 31, confirmed 31 (zero false positives)
UNIT_ARG claims a unit, implementation does not 0
implementation says unit, UNIT_ARG missed it 24
The 24 include builtin80, group_ratio_pct, is_engaged, set_unit_flags,
squadron_trace, wait_units_ready and deploy_and_wait. Hand-verified by reading
builtin7, 16, 80, 105, 117 and 136.
Recorded because it nearly passed: my FIRST control -- whether the additions' operands
resolve to a symbol-table-2 index -- is worthless. The additions score 100.0%, but so
do the 31 baseline (100.0%) AND the 92 built-ins in neither set (99.3%). Symtab 2 is
dense enough that almost any small integer lands in it. A control the negative class
also passes is not evidence.
The control that discriminates is the tag word: a symbol operand is a two-word pair
whose first word is the constant 1, so slot0 == 1 exactly when slot 4 is a unit --
100.0% (13677 calls) / 100.0% (140) / 2.5% (2903). A 40x separation.
Artefacts: isl-stage02.txt and -phase-ends.txt regenerate byte-identical; -conditions
changes on 28 sites, every diff line pairing, each a raw number becoming a unit name.
Left unnamed on purpose: all 24. builtin80 returns a small enum (tested 0..4 in a
switch) but its body past the liveness check is unread; builtin103 is a predicate over
[phase+10152]/[phase+10156]; builtin105 tests a unit record's +16 against 4.
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