Closes the ambiguity 05d9bf6 raised. At 0x3a8060:
263200c0 addiu r17, r18, 0x00c0 r17 = r18 + 0xc0
263000d0 addiu r17, r16, 0x00d0 r17 = r16 + 0xd0
262400e0 addiu r17, r4, 0x00e0 r17 = r4 + 0xe0
3c013f80 lui r0, r1, 0x3f80 the actual float constant: 1.0f
0xc0 is an address offset, not a float constant. The float constant is in the same
function and is 0x3f80, where it belongs -- so the ambiguity does not arise here,
and the reading of 0xc0 as moby_classes.offset survives.
The three offsets are 0xc0, 0xd0, 0xe0: sixteen bytes apart. The function walks a
table of fixed-stride structures and indexes the field at offset 0xc0, which is
what a class-table lookup looks like.
WHAT THIS DOES NOT SETTLE
That 0x3a8060 is the moby class loader. It indexes a 16-byte-stride table at
offset 0xc0, which is consistent with a moby class walk, with a different
structure that also lives at 0xc0, and with ordinary field access on an object
handed in by the caller.
The entry code sets a 64-byte frame and saves r16 through r20, and the three
indexes come off three different registers -- r18, r16 and r4. So the walk is over
arguments, not over a global. That is informative in itself: a loader would more
plausibly be handed the chunk by its caller than re-derive core_data + 0xc0 on
every call. Which also means the chunk reference is in the *caller*, and that is
where the next step is.
NEXT STEP, narrow and concrete: trace where r18 and r4 come from, and find the
call sites of this function. If a caller does `addiu r, base, 0xc0` and passes it
in, that caller is doing the chunk lookup and this function is the per-class work
inside it.
Worth recording how long the ambiguity took to close. It was raised one commit ago
and resolved by reading twelve instructions. Five commits before that the same
constant was reported as a "real shortlist", and two before that as a size
artifact. Every one of those states was arithmetically correct; only this one was
the right question, and it took looking at the operands rather than counting them
to arrive at it.
284 passing with the disc present (23 + 19 + 64 + 156 + 22; disc binary 35.00s),
clippy clean.
ReaperOfVeriod
cee17c95b1