Read the encoding off the interpreter rather than guessing: instruction is a
big-endian u32 whose LOW byte is the opcode (25 of them, table 0x822635FC),
byte[2] is the instruction length -- every handler advances the pc by it -- and
bytes[0..1] are operand kinds. Op 12 is a jump whose operand is relative to the
code base [phase+232], which settles that offsets are code-base-relative for
this opcode. Op 19 is the built-in call: id in word@+4, and word@+8 is a
monotonically increasing STATEMENT id (0x245, 0x248, 0x24A, ...).
Confirmed by disassembling Stage02.ssb: the stream decodes cleanly from the code
base and routines terminate on ret exactly where expected.
Scanning the code region on the call encoding: 2846 call sites, 73 of the 147
built-ins used. The phase-control ones are located -- built-in 6 (end phase) at
12 sites, 62 at 3, 39 (mark last phase) at 8 -- so a phase has several exit
paths, as a mission with win and lose branches should.
New tool tools/re-capture/isl.py with --calls and --to (resync-into-target,
needed because instructions are variable-length so you cannot walk backwards).
Not settled: the 147 built-ins are uncharacterised, so this is structure without
meaning -- we can see THAT a phase ends, not WHAT was tested.
