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cstage+selfhost+test: System V AMD64 sret discipline for >24B struct return (#23)

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Class B shared miscompile pre-fix: cstage skipped the CALL emit at the
receive site (frame collapsed, exit 11); wwstage emitted CALL but
truncated 32B return to AX only (slice payload garbage, segfault on
g.b[0]). Both stages now lower plain TY_STRUCT > 24B through the SysV
sret discipline: caller pre-allocates dest, passes &dest in RDI as a
hidden first-arg (user args shift to SI/DX/CX/R8/R9/+stack), callee
saves RDI to @sretarg at the prologue and writes through it, returns
RDI in RAX. Surfaced by lib/encoding/utf8 pre-flight when the
Hoehrmann decoder (32B) hit 698_cgreturn_struct.c's OUT-OF-SCOPE
marker.

Scope: plain TY_STRUCT > 24B only — tagged unions, tuples, str, slice
keep their existing register-return ABIs. `return f()` forwarding
from a sret callee is fail-loud-not-wired (compile-time error in
both stages, follow-up filed); the workaround `let r = f(); return
r;` is wired and byte-identical. Discard-context calls (`f();` of an
sret-returning function) share a per-fn single-slot @sretscr;
consecutive discards reuse the same slot.

698_cgreturn_struct.c's OUT-OF-SCOPE marker retired in the same
commit; three positive rows (32B quad, 32B decoder, 40B five) now
assert the sret discipline across both stages via byte-id diff.

Tests:
  - 721_sret_struct_return pins three asm-presence sentinels per
    row: (a) LEAQ -K(BP), DI immediately before CALL at the receive
    site, (b) MOVQ -K(BP), AX before RET in the callee (sret return-
    the-pointer), (c) negative-assert no MOVQ AX, -K(BP) capture for
    return type >8B. Three rows × both stages × cmp -s byte-id.
  - 925_sret_struct_return_run runtime-pins 7 rows × 2 stages
    including the collision row (25B+ struct BOTH returned AND passed
    by-value as arg — catches arg-shift, sister site to #11), nested
    struct payload, slice payload, reassign-receive, N_IDENT return
    rhs.

89/89 ok. 995_self_rebuild stays green (ww2==ww3==ww4 byte-id).
This commit is contained in:
Hojun-Cho
2026-05-17 23:17:03 +09:00
parent 6ab865d933
commit 7e0c280691
12 changed files with 1723 additions and 42 deletions
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12
Makefile
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@@ -246,6 +246,8 @@ TESTS = $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
$(BIN)/test_signed_data_emit_run \
$(BIN)/test_tagged_call_arg \
$(BIN)/test_tagged_call_arg_run \
$(BIN)/test_sret_struct_return \
$(BIN)/test_sret_struct_return_run \
$(BIN)/test_param_shadow_mod \
$(BIN)/test_localoff_scope \
$(BIN)/test_cast_enum_movl \
@@ -531,6 +533,16 @@ $(BIN)/test_tagged_call_arg_run: test/wcc/924_tagged_call_arg_run.c \
$(LIB)/libwwrt.a | $(BIN)
$(CC) $(CFLAGS) -o $@ $<
$(BIN)/test_sret_struct_return: test/wcc/721_sret_struct_return.c \
$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
$(CC) $(CFLAGS) -o $@ $<
$(BIN)/test_sret_struct_return_run: test/wcc/925_sret_struct_return_run.c \
$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
$(LIB)/libwwrt.a | $(BIN)
$(CC) $(CFLAGS) -o $@ $<
$(BIN)/test_use_promote_alias: test/wcc/699_use_promote_alias.c \
$(BIN)/ww $(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
$(LIB)/libwwrt.a | $(BIN)