Solve the FP-arithmetic fpscr problem without lowering fpscr into IR. Add a
generic single-instruction shim, xj_interp_op(ctx, env, raw, addr), that decodes
and runs one instruction through the interpreter's execute() on the live
context/memory — bit-exact by construction. This lets a block containing FP math
still be JIT-compiled: the surrounding integer/memory/branch ops run as machine
code and only the FP op calls back into Rust (the technique production JITs use
for complex ops). Validated on a full boot+movie run (movie plays, clean exit):
checked 147.2M blocks, 73.48% native (108.2M, up from 39.82%!), MISMATCHES=0
The fpscr bookkeeping (rounding-mode-dependent rounding, sticky exception bits,
FPRF classification) was too intricate to emit correctly in IR; punting sidesteps
it entirely while still capturing the coverage.
is_fp_punt allowlist (all verified always-Continue, non-branch):
faddx/faddsx, fsubx/fsubsx, fmulx/fmulsx, fdivx/fdivsx, fmaddx/fmaddsx,
fmsubx/fmsubsx, fnmaddx/fnmaddsx, fnmsubx/fnmsubsx, frspx, fsqrtx, fresx,
frsqrtex, fselx, fnabsx, fcmpu, fcmpo.
covered() returns true for them; emit_op emits `call interp(ctx, env, raw,
addr)`. execute() advances pc by 4 (harmless — the block epilogue stamps end_pc
for non-branch blocks; cycle_count is bumped once per block, not by execute).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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