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cstage+selfhost+test: zero unused ABI words in cgreturn variant-widen (#18)

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cgreturn's variant-widen arm only filled the registers each variant's
payload needed: scalar variants left CX and R8 stale; str variant
left R8 stale. The receiver (cg_widen_tagged_store non-N_IDENT
branch) writes all four ABI words to the dst slot unconditionally,
so caller-side residue in CX/R8 (the array-index IMULQ being the
canonical primer) landed at slot+16 and slot+24.

Worker-fmtparser surfaced this through fprintf's loop body where
array indexing primed CX and a 24B-return helper failed to clear
it; bug isn't loop-specific — straight-line repro at /tmp/wcrs_repro/
repro8.ww confirms.

Patch: emit `MOVQ $0, CX` after the variant's register shuffle when
the slot exceeds 16B and the variant doesn't fill CX; same for R8
when the slot exceeds 24B. Symmetric across cstage cgen.c and
wwstage cgenstmt.ww. Order: zero-MOVQs precede `MOVQ $tag, AX` so
AX-as-staging stays safe. Inline comment at cg_widen_tagged_store
non-N_IDENT branch documents the producer-zero contract.

Test 707 (cgreturn_variant_zero): 6 rows × both stages = 12 fixtures.
Covers scalar/bool/str returns after array-index priming in
straight-line / single-loop body / nested-loop body / mixed-variant
loop. Asm byte-identity check intentionally omitted; wwstage
taggedvariantindex divergence on str/bool N_IDENT is filed as task
#20. 995_self_rebuild covers the broader cross-stage drift surface.

ww2 == ww3 == ww4 byte-identical post-fix. 67/67 green.

Deferred (followups filed): #20 wwstage taggedvariantindex,
#21 [N]str/[N]bool array-literal non-pointer-half writes, #22
consolidate variant-widen into uniform scratch-slot path.
This commit is contained in:
Hojun-Cho
2026-05-16 01:37:15 +09:00
parent 6b6d7dd283
commit 166431a2da
6 changed files with 383 additions and 1 deletions
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7
Makefile
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@@ -233,6 +233,7 @@ TESTS = $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
$(BIN)/test_dot_structlit \
$(BIN)/test_nested_call_rhs \
$(BIN)/test_fnlabel_mangle \
$(BIN)/test_cgreturn_variant_zero \
$(BIN)/test_use_promote_alias \
$(BIN)/test_field_signed $(BIN)/test_frame_argcount \
$(BIN)/test_selfhost $(BIN)/test_w6a_ww $(BIN)/test_w6l_ww \
@@ -416,6 +417,12 @@ $(BIN)/test_fnlabel_mangle: test/wcc/706_fnlabel_mangle.c \
$(LIB)/libwwrt.a | $(BIN)
$(CC) $(CFLAGS) -o $@ $<
$(BIN)/test_cgreturn_variant_zero: test/wcc/707_cgreturn_variant_zero.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)