Class A compile-time fatal retirement — `return f()` from an sret
callee bailed both stages with "sret return-forwarding for >24B
struct not wired (task #23)" at every site, forcing every caller
into a `let r = f(); return r;` workaround that materialised an
intermediate >24B copy in outer's frame. Forwarding now elides the
copy: outer reloads its own @sretarg into RDI for the inner CALL
via `MOVQ @sretarg(BP), DI` (NOT `LEAQ <local>, DI`), inner writes
directly into outer's caller-prealloc dest, RAX (inner's returned
dest pointer per the sret discipline) is already outer's return
value.
Wires 2 sites × 2 stages (same triangle as #23): caller arg-shift
in cgcall/pushargsrev gains an RDI-source switch via
cg_sret_forward / c.sretforward; callee return-arm in cgreturn
replaces the fail-loud abort with cgexpr-into-cgcall + epilogue.
The @sretscr scratch slot is still pre-allocated on the forwarding
branch (unused) — eliding would need AST-walk awareness in
scanlocals; symmetric-allocate is the simpler path and keeps
byte-id with non-forwarding callers.
Latent surfaced and filed during probe (NOT in this commit's
scope): multi-sret-receive in a single fn diverges between stages
— cstage always allocates @sretscr on first sret CALL, wwstage
only when sretdestoff == 0. Bootstrap stays green because the
selfhost corpus has zero >1-sret-receive call sites.
Tests:
- 721_sret_struct_return gains 2 forwarding rows + a 4th asm-
presence sentinel: at the inner CALL site inside outer fn, the
RDI source must be `MOVQ -K(BP), DI` (reload of outer's saved
@sretarg) NOT `LEAQ -K(BP), DI` (a temporary local would write
inner's payload into outer's frame, not caller's dest).
- 925_sret_struct_return_run gains 3 forwarding rows: simple
quad forward, multi-arg inner (pair-by-value + scalar args
alongside the hidden RDI), and slice-payload (decoder
{ i64, []u8 } — the utf8 iterator shape, asserts ptr/len/cap
survive the @sretarg chain).
90/90 ok. 995_self_rebuild stays green (ww2==ww3==ww4 byte-id).
Hojun-Cho
dd274315a0