First implementation step of Stream B1 (fibers). Make the inert abi(.pure)
ABI carry an is_pure flag through lowering, with LLVM emission deliberately
bailing loudly until B1.0b — the lock half of the lock->green cadence.
- IR Function.is_pure, set from fd.abi == .pure at both declareFunction
decl sites.
- funcWantsImplicitCtx skips .pure (no synthetic __sx_ctx, mirroring the
.c skip): a pure fn reads args from ABI registers, an implicit ctx would
occupy a register slot the asm doesn't expect.
- both body-lowering paths bypass lowerValueBody for .pure: lower the asm
body as statements + cap with unreachable. A pure body has no sx return
(the asm rets itself), so the implicit-return diagnostic must not fire.
- emit_llvm Pass 2 bails loudly when func.is_pure (build-gating nonzero
exit) rather than emit a framed body, whose epilogue would corrupt a
context switch's deliberate SP-in != SP-out.
examples/1800-concurrency-pure-asm.sx: one host example (no .build pin --
the bail fires before instruction selection, so it is host-independent),
locked to the bail snapshot. B1.0b flips emit to LLVM's naked attribute +
asm-only body and pins the example per-arch.
The sx-facing name is "pure" throughout (field, diagnostic); LLVM's naked
attribute is only the B1.0b lowering mechanism. Suite green (722/0).
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