Byte-proven on md_SC07_003/func_801A293C (313 ins, 6 -> 0 in 23 compiles). $t0 there is
reload's scratch (regalloc.md L80: first wholly-unused call-clobbered reg), not an
allocation: a launder output goes to $v0 via local-alloc, and any $8 pin evicts every
mfhi/reload to $t1 (measured 3x). The reach is §385's single-instruction non-volatile
asm with the bare symbol as an INPUT.
Also: spelling an addPrim RMW as a libgpu P_TAG 24-bit addr bitfield makes
store_fixed_bit_field mask the value first, hoisting 0xFFFFFF before 0xFF000000 and
closing both the lui/ori and lw-swap clusters at once (6 -> 3); and an in-place `and $s0`
needs `s0 = s0 & 0xFFFFFF` written out, not the compound form.
The law: before spending iterations on a register you cannot get, ask whether it belongs
to regalloc at all - reload runs after every dial C gives you.
Drew T
a10613fa4a