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w6c+cgen: read .cap of an indexed str/slice array element (fix #13, #20 read-sibling)

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`t[i].cap` (t a `[N][]u8` / `[N]str`) miscompiled in BOTH stages,
divergently — the read-side sibling of #20's store fix. cgexpr on the
indexed element leaves the full {ptr,len,cap} header (AX/BX/CX via
cgslicehdr), but the `.cap` field-selector never shuffled CX→AX:
cstage's typed pseudo-field else-branch handled only .ptr/.len, so
`.cap` fell through returning AX=.ptr; wwstage's cgdot non-ident
catch-all likewise handled only .ptr/.len, emitting no read (stale AX).
`t[i].len` already worked (BX→AX shuffle) — only `.cap` was missing.

Fix mirrors the .len shuffle: add the .cap CX→AX arm in both stages.
The shuffle fires ONLY for a typed slice/str base (TY_SLICE/TY_STR
after NAMED-chase); an untyped str literal (`"abc".cap`) leaves only
AX=ptr/BX=len and must return AX unshuffled — keeping the wwstage
catch-all byte-identical with cstage, whose cap-shuffle lives in the
typed branch, not the untyped catch-all.

Validated direct `t[i].cap` (slice + str, elements 0/1) against the
whole-element-copy oracle (`let q=t[i]; q.cap`, made correct by #20),
plus .len-after-index regression pins, in test 683; dual-stage runtime
+ byte-id (36/36 ok). combined.ww regenerated.
This commit is contained in:
Hojun-Cho
2026-06-03 17:50:54 +09:00
parent b3d4d2df32
commit 84ed2ab15a
5 changed files with 126 additions and 9 deletions
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20
selfhost/cmd/wcc/cgenexpr.ww
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@@ -2887,6 +2887,26 @@ fn cgdot(c: *cgen, n: *node) void = {
emitline("\tMOVQ\tBX, AX\n");
return;
};
// .cap on a non-ident base (indexed element `t[i].cap`, call,
// dot-slice): cgexpr leaves the full {ptr,len,cap} header via
// cgslicehdr — shuffle CX→AX. The shuffle fires ONLY for a TYPED
// slice/str base (kind TY_SLICE/TY_STR after NAMED-chase); an untyped
// str literal (`"abc".cap`) leaves only AX=ptr/BX=len and must return
// AX unshuffled. Mirrors cstage cgen.c: the cap-shuffle lives in the
// typed pseudo-field branch (`u->kind == TY_SLICE/TY_STR`), never the
// untyped catch-all. #13 read-fix, sibling of the #20 store.
if (streq(fld, "cap")) {
cgexpr(c, lhs);
if (lhs != nil) {
let lu: *tinfo = lhs.type_: *tinfo;
for (lu != nil && lu.kind == tykind.TY_NAMED) { lu = lu.under; };
if (lu != nil && (lu.kind == tykind.TY_SLICE
|| lu.kind == tykind.TY_STR)) {
emitline("\tMOVQ\tCX, AX\n");
};
};
return;
};
// Chained struct-field-via-ptr-via-ptr access:
// r.sym.val where r: *lrel, .sym: *lsym, .val: u64
// Inner DOT (`r.sym`) returns a *struct (a pointer-to-struct