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cgen: str chained-*struct field read -> 3-word -- Phase 2 C4.6 caseB (both stages)

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The chained N_DOT path (o.p.f, depth>=2, base AX) still loaded a str-typed field as 2 words, dropping cap -- the C4.6 sibling deferred to caseB. Fold the str case onto the adjacent 3-word slice-field arm (widen kind-gate: cstage type_isstr, ww typeisstr; never size==24). Emits len->BX+8, cap->CX+16, ptr->AX+0 LAST (AX is the base). cstage==wwstage byte-identical; the slice arm is unchanged for slices.

test/wcc/934: table-driven runtime .cap-survives over the chained read; the row interposes a CX-clobbering call so a 2-word read cannot coincidentally pass on stale CX (per the 933 discriminator lesson). Verified fail-before/pass-after on both drivers.

main.combined.ww regenerated via the canonical make path (md5-stable).
This commit is contained in:
Hojun-Cho
2026-05-24 13:57:20 +09:00
parent 90deeb3821
commit 634cbefc22
6 changed files with 199 additions and 52 deletions
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19
cmd/w6c/cgen.c
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@@ -6061,20 +6061,13 @@ cgexpr(Cg *c, Node *n, Local *locals)
Type *ft = f->type;
Type *fu = (ft && ft->kind == TY_NAMED)
? ft->under : ft;
/* str field: load (ptr, len) into (AX, BX). */
if (fu && fu->kind == TY_STR) {
ins2(c, A_MOVQ,
amem(D_AX, (int)f->offset + 8),
areg(D_BX));
ins2(c, A_MOVQ,
amem(D_AX, (int)f->offset + 0),
areg(D_AX));
goto dot_done;
}
/* slice field: load (ptr, len, cap) into
/* str IS []u8 — same 3-word {ptr,len,cap} as a
* slice field: load (ptr, len, cap) into
* (AX, BX, CX). AX is the *struct base, so
* load .ptr (which targets AX) LAST. */
if (fu && fu->kind == TY_SLICE) {
* load .ptr (which targets AX) LAST. str folds
* onto the slice arm (#1/Phase 3 collapse). */
if ((fu && fu->kind == TY_SLICE) ||
type_isstr(ft)) {
ins2(c, A_MOVQ,
amem(D_AX, (int)f->offset + 8),
areg(D_BX));