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w6c+selfhost+lib: zero-init multi-word no-rhs lets

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`let x: T;` for str/slice/tuple/struct/tagged previously left the slot
holding stack garbage — only 8B-primitive slots were zeroed. This bit
`expectbindname` in lib/ww/parse: `let empty: str; *into = empty;` was
copying stack bytes (often a recently-vacated str descriptor) into the
caller's `id`, so wwstage emitted `_` discard nodes carrying random
text instead of "". Both stages now zero the full slot on no-rhs lets;
`[N]T` arrays keep the per-index-write contract.

Also tightens the two known buggy sites: parse.ww `expectbindname`
writes `*into = ""` directly, expr.ww `_` primary returns the bare
newnode (amalloc already zeroes).
This commit is contained in:
Hojun-Cho
2026-05-13 12:46:02 +09:00
parent b6cf68f2b8
commit 956a20701b
7 changed files with 155 additions and 43 deletions
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25
cmd/w6c/cgen.c
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@@ -4158,8 +4158,31 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
}
} else if (sz == 8) {
ins2(c, A_MOVQ, aimm(0), amem(D_BP, off));
} else if (!n->rhs && sz > 8 && lu && lu->kind != TY_ARRAY) {
/* `let x: T;` with no rhs for a multi-word composite
* (str/slice/tuple/struct/tagged). Zero the slot so
* reads after the bare let see {0...} rather than
* whatever the stack already held. Arrays keep the
* per-index-write contract — leave them uninit. */
ins2(c, A_XORQ, areg(D_AX), areg(D_AX));
int zi = 0;
while (zi + 8 <= sz) {
ins2(c, A_MOVQ, areg(D_AX),
amem(D_BP, off + zi));
zi += 8;
}
while (zi + 4 <= sz) {
ins2(c, A_MOVL, areg(D_AX),
amem(D_BP, off + zi));
zi += 4;
}
while (zi < sz) {
ins2(c, A_MOVB, areg(D_AX),
amem(D_BP, off + zi));
zi += 1;
}
}
/* arrays/slices left uninitialised — caller writes via index */
/* arrays left uninitialised — caller writes via index */
break;
}
case N_RETURN: