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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
Download Patch File Download Diff File Expand all files Collapse all files

25
cmd/w6c/cgen.c
View File

@@ -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;
}
/* arrays/slices left uninitialised — caller writes via index */
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 left uninitialised — caller writes via index */
break;
}
case N_RETURN:

10
lib/ww/parse/expr.ww
View File

@@ -68,14 +68,12 @@ fn parseprimary(p: *parser) *node = {
return newnode(p.a, nkind.N_VOIDLIT, pf, pl, pc);
};
if (p.curkind == tkind.TK_UNDER) {
// Bare `_` — valid only as a discard lvalue. Emit an nkind.N_IDENT
// with empty str; the checker rejects it outside lvalue
// Bare `_` — valid only as a discard lvalue. Emit an N_IDENT
// with empty str (newnode zeroes the node, so str.len is
// already 0); the checker rejects it outside lvalue
// positions.
advance(p);
let n: *node = newnode(p.a, nkind.N_IDENT, pf, pl, pc);
let empty: str;
n.str = empty;
return n;
return newnode(p.a, nkind.N_IDENT, pf, pl, pc);
};
if (p.curkind == tkind.TK_LBRACK) {
// Array literal `[a, b, c]` or `[v, w...]` (repeat suffix).

3
lib/ww/parse/parse.ww
View File

@@ -92,8 +92,7 @@ fn expectident(p: *parser, into: *str) bool = {
// scope_define for the binding.
fn expectbindname(p: *parser, into: *str) bool = {
if (p.curkind == tkind.TK_UNDER) {
let empty: str;
*into = empty;
*into = "";
advance(p);
return true;
};

8
selfhost/CLAUDE.md
View File

@@ -64,5 +64,13 @@ Fixed (no workaround needed):
selfhost/cmd/wcc/cgenutil.ww slotsize N_TARRAY follows
`aliaslookup` on a TNAME element, and `aliaslookup` strips a
`pkg.` prefix so cross-module references resolve.
- Bare `let x: T;` (no rhs) of a multi-word composite — str (16B),
slice (24B), tuple, struct, tagged — now zero-inits the slot.
Previously only 8B-primitive slots were zeroed; larger slots
read whatever the stack held, so `let empty: str; *into = empty;`
copied stack garbage into the caller's slot. `[N]T` arrays still
follow the per-index-write contract and stay uninit. See
cmd/w6c/cgen.c N_LET (else branch, sz > 8 && !TY_ARRAY) and
selfhost/cmd/wcc/cgenstmt.ww cglet no-rhs branch.
If a port "should work" but the binary is wrong, suspect these first.

53
selfhost/cmd/w6c/main.combined.ww
View File

@@ -2730,14 +2730,12 @@ fn parseprimary(p: *parser) *node = {
return newnode(p.a, nkind.N_VOIDLIT, pf, pl, pc);
};
if (p.curkind == tkind.TK_UNDER) {
// Bare `_` — valid only as a discard lvalue. Emit an nkind.N_IDENT
// with empty str; the checker rejects it outside lvalue
// Bare `_` — valid only as a discard lvalue. Emit an N_IDENT
// with empty str (newnode zeroes the node, so str.len is
// already 0); the checker rejects it outside lvalue
// positions.
advance(p);
let n: *node = newnode(p.a, nkind.N_IDENT, pf, pl, pc);
let empty: str;
n.str = empty;
return n;
return newnode(p.a, nkind.N_IDENT, pf, pl, pc);
};
if (p.curkind == tkind.TK_LBRACK) {
// Array literal `[a, b, c]` or `[v, w...]` (repeat suffix).
@@ -3799,8 +3797,7 @@ fn expectident(p: *parser, into: *str) bool = {
// scope_define for the binding.
fn expectbindname(p: *parser, into: *str) bool = {
if (p.curkind == tkind.TK_UNDER) {
let empty: str;
*into = empty;
*into = "";
advance(p);
return true;
};
@@ -11788,16 +11785,46 @@ fn cglet(c: *cgen, n: *node) void = {
};
} else {
// Bare `let x: T;` with no initializer. C cgen
// (cmd/w6c/cgen.c:3317) zero-inits whenever the raw type size
// is 8: scalar primitives, pointers, fn/chan handles, plus 8B
// composites like `[8]bool`, `[2]i32`, `[4]i16`, `[1]i64`.
// Larger composites and `[N]T` with size != 8 are left for
// per-field writes.
// (cmd/w6c/cgen.c N_LET no-rhs branch) zero-inits in two
// shapes:
// - 8B primitives (scalar/ptr/fn/chan/`[8]bool` etc.):
// single `MOVQ $0, off(BP)`.
// - multi-word composites (str/slice/tuple/struct/tagged):
// `XORQ AX,AX` + a run of `MOVQ AX, ...` over the slot
// so reads after the bare let see {0...} rather than
// stack garbage.
// `[N]T` arrays of size != 8 keep the per-index-write
// contract — they're left uninit.
let isarr: bool = false;
if (n.lhs != nil) {
if (n.lhs.kind == nkind.N_TARRAY) { isarr = true; };
};
if (typeis8byteprimitive(c, n.lhs)) {
emitline("\tMOVQ\t$0, ");
emitoff(off: i64);
emitline("(BP)\n");
} else { if (!isarr) { if (sz > 8) {
emitline("\tXORQ\tAX, AX\n");
let zi: i32 = 0;
for (zi + 8 <= sz) {
emitline("\tMOVQ\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 8;
};
for (zi + 4 <= sz) {
emitline("\tMOVL\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 4;
};
for (zi < sz) {
emitline("\tMOVB\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 1;
};
}; }; };
};
c.lastwasreturn = 0;
return;

40
selfhost/cmd/wcc/cgenstmt.ww
View File

@@ -544,16 +544,46 @@ fn cglet(c: *cgen, n: *node) void = {
};
} else {
// Bare `let x: T;` with no initializer. C cgen
// (cmd/w6c/cgen.c:3317) zero-inits whenever the raw type size
// is 8: scalar primitives, pointers, fn/chan handles, plus 8B
// composites like `[8]bool`, `[2]i32`, `[4]i16`, `[1]i64`.
// Larger composites and `[N]T` with size != 8 are left for
// per-field writes.
// (cmd/w6c/cgen.c N_LET no-rhs branch) zero-inits in two
// shapes:
// - 8B primitives (scalar/ptr/fn/chan/`[8]bool` etc.):
// single `MOVQ $0, off(BP)`.
// - multi-word composites (str/slice/tuple/struct/tagged):
// `XORQ AX,AX` + a run of `MOVQ AX, ...` over the slot
// so reads after the bare let see {0...} rather than
// stack garbage.
// `[N]T` arrays of size != 8 keep the per-index-write
// contract — they're left uninit.
let isarr: bool = false;
if (n.lhs != nil) {
if (n.lhs.kind == nkind.N_TARRAY) { isarr = true; };
};
if (typeis8byteprimitive(c, n.lhs)) {
emitline("\tMOVQ\t$0, ");
emitoff(off: i64);
emitline("(BP)\n");
} else { if (!isarr) { if (sz > 8) {
emitline("\tXORQ\tAX, AX\n");
let zi: i32 = 0;
for (zi + 8 <= sz) {
emitline("\tMOVQ\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 8;
};
for (zi + 4 <= sz) {
emitline("\tMOVL\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 4;
};
for (zi < sz) {
emitline("\tMOVB\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 1;
};
}; }; };
};
c.lastwasreturn = 0;
return;

53
selfhost/cmd/wwdump/main.combined.ww
View File

@@ -2730,14 +2730,12 @@ fn parseprimary(p: *parser) *node = {
return newnode(p.a, nkind.N_VOIDLIT, pf, pl, pc);
};
if (p.curkind == tkind.TK_UNDER) {
// Bare `_` — valid only as a discard lvalue. Emit an nkind.N_IDENT
// with empty str; the checker rejects it outside lvalue
// Bare `_` — valid only as a discard lvalue. Emit an N_IDENT
// with empty str (newnode zeroes the node, so str.len is
// already 0); the checker rejects it outside lvalue
// positions.
advance(p);
let n: *node = newnode(p.a, nkind.N_IDENT, pf, pl, pc);
let empty: str;
n.str = empty;
return n;
return newnode(p.a, nkind.N_IDENT, pf, pl, pc);
};
if (p.curkind == tkind.TK_LBRACK) {
// Array literal `[a, b, c]` or `[v, w...]` (repeat suffix).
@@ -3799,8 +3797,7 @@ fn expectident(p: *parser, into: *str) bool = {
// scope_define for the binding.
fn expectbindname(p: *parser, into: *str) bool = {
if (p.curkind == tkind.TK_UNDER) {
let empty: str;
*into = empty;
*into = "";
advance(p);
return true;
};
@@ -11788,16 +11785,46 @@ fn cglet(c: *cgen, n: *node) void = {
};
} else {
// Bare `let x: T;` with no initializer. C cgen
// (cmd/w6c/cgen.c:3317) zero-inits whenever the raw type size
// is 8: scalar primitives, pointers, fn/chan handles, plus 8B
// composites like `[8]bool`, `[2]i32`, `[4]i16`, `[1]i64`.
// Larger composites and `[N]T` with size != 8 are left for
// per-field writes.
// (cmd/w6c/cgen.c N_LET no-rhs branch) zero-inits in two
// shapes:
// - 8B primitives (scalar/ptr/fn/chan/`[8]bool` etc.):
// single `MOVQ $0, off(BP)`.
// - multi-word composites (str/slice/tuple/struct/tagged):
// `XORQ AX,AX` + a run of `MOVQ AX, ...` over the slot
// so reads after the bare let see {0...} rather than
// stack garbage.
// `[N]T` arrays of size != 8 keep the per-index-write
// contract — they're left uninit.
let isarr: bool = false;
if (n.lhs != nil) {
if (n.lhs.kind == nkind.N_TARRAY) { isarr = true; };
};
if (typeis8byteprimitive(c, n.lhs)) {
emitline("\tMOVQ\t$0, ");
emitoff(off: i64);
emitline("(BP)\n");
} else { if (!isarr) { if (sz > 8) {
emitline("\tXORQ\tAX, AX\n");
let zi: i32 = 0;
for (zi + 8 <= sz) {
emitline("\tMOVQ\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 8;
};
for (zi + 4 <= sz) {
emitline("\tMOVL\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 4;
};
for (zi < sz) {
emitline("\tMOVB\tAX, ");
emitoff((off + zi): i64);
emitline("(BP)\n");
zi += 1;
};
}; }; };
};
c.lastwasreturn = 0;
return;