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Drew T
ce7780b91f
Item 1's remaining half. Oracle mechanized and reusable at tools/oracle/reg_renumber_swap.sh: break at reload entry (cc1 unstripped: reg_renumber @0x82d4330, reload @0x815d4d7), swap two hard regs across reg_renumber, finish the compile, re-score with masked_diff REUSED not reimplemented (R33). NEGATIVE CONTROL: a no-op swap (31<->31) reproduces exactly the baseline 13 mismatches, so the harness faithfully reproduces the pinned compile. RESULT: both contested swaps are far WORSE — <-> (17 pseudos) = 345 mismatches +1 insn; <-> (31 pseudos) = 97. Baseline 13. WHY, AND IT REFUTES THE FRAMING: reading .greg for cluster B's own insn shows (set (reg/v:SI 6 a2) (plus:SI (reg/v:SI 5 a1) (const_int 60))) — the destination is a HARD register, not a pseudo. reg_renumber only maps pseudos (>= FIRST_PSEUDO_REGISTER = 68), so that value is structurally unreachable by this oracle. The draft has NO register __asm__ pins (header says so, grep confirms), so is hard because it is an incoming PARAMETER register that local-alloc reused as a destination. VERDICT for func_80176734 (51,198 ins): the residual is NOT global-allocation 2-colouring. It is the LOCAL-alloc hard-reg reuse / tying class — combine_regs (2.7.2 local-alloc.c:1722) + qty_phys_copy_sugg, i.e. regalloc.md K8/RC-4, whose lever is C-level LIFETIME SHAPING, not the permuter and not reg_renumber. That also explains the flat permuter: it was mutating a dial that does not control this residual. MAP REFINEMENT OWED: §H presents the swap oracle as THE way to discriminate RC-6 from S3 in one gdb run. It has an unstated PRECONDITION — the contested registers must be held by PSEUDOS. Check .greg first; if they appear as (reg/v:SI N ...) with N < 68 they are already hard, and a coarse swap returns a large meaningless number (345 here) that looks like a verdict and is not one. Tree clean; nothing banked, nothing broken.