w6c+selfhost: slice globals — 24B DATAW + (AX,BX,CX) load

Extend top-level mutable `let` to cover slices. Same shape as the
str work, with one more 8-byte field and the address holder CX
overwritten by the cap as the last load step:

  - emit_lets / emitletdataw: 24-byte zero DATAW for `let v: []u8;`
    (and the trivial `nil` init); no slice-literal syntax exists
    so the no-init path is the only supported shape;
  - cgident: LEAQ name(SB), CX → MOVQ (CX), AX → MOVQ 8(CX), BX →
    MOVQ 16(CX), CX, so the slice ABI triple lands in (AX, BX, CX);
  - cgdot: .cap delta 16 wired alongside .ptr / .len through the
    same &name(SB) base.

Slice reassignment (`v = some_slice;`) is still unsupported — slice
values don't yet flow as a full (AX, BX, CX) triple through general
expressions even for locals — so reads/`&` are the supported surface
today. Manual fill through `(&v): *u64` continues to work.

Tests 630 (10/10), 990, 994, 995 stay green; bootstrap fixed point
holds.

cmd/w6c/cgen.c

@@ -398,9 +398,9 @@ struct LetVar {
 static LetVar *letvars;
 
 /* Slot size for a top-level `let` of type t, or 0 if the type isn't
- * supported as a writable global yet. Floats (MOVSS/SD) and slice/
- * struct/tagged unions are deferred. enums route through their
- * storage type. Keep this tight — extending it requires the matching
+ * supported as a writable global yet. Floats (MOVSS/SD) and struct/
+ * tagged unions are deferred. enums route through their storage
+ * type. Keep this tight — extending it requires the matching
  * load/store code below. */
 static int
 let_emit_size(Type *t)
@@ -417,6 +417,8 @@ let_emit_size(Type *t)
 		return 8;
 	case TY_STR:
 		return 16;	/* {ptr, len}; literal-strlit init NYI. */
+	case TY_SLICE:
+		return 24;	/* {ptr, len, cap}; no init only. */
 	default:
 		return 0;
 	}
@@ -433,6 +435,16 @@ let_isstr(Type *t)
 	return u && u->kind == TY_STR;
 }
 
+/* Is the unwrapped type a slice? Slice globals flow as the (AX, BX,
+ * CX) triple — same as the local ABI. */
+static int
+let_isslice(Type *t)
+{
+	if (t == NULL) return 0;
+	Type *u = (t->kind == TY_NAMED) ? t->under : t;
+	return u && u->kind == TY_SLICE;
+}
+
 static int
 decl_has_ffisym(Node *d)
 {
@@ -764,13 +776,20 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				    areg(D_BX));
 				goto ident_done;
 			}
-			if (let_islet(n->str) && let_isstr(n->type)) {
-				/* Top-level str global: load both halves via
-				 * its address (the asm has no `name+8(SB)`
-				 * operand form). */
+			if (let_islet(n->str)
+			    && (let_isstr(n->type) || let_isslice(n->type))) {
+				/* Top-level str/slice global: load each half
+				 * via its address (the asm has no `name+8(SB)`
+				 * operand form). Slice has a third 8B (cap)
+				 * — the address holder CX gets overwritten by
+				 * the cap as the last step, after we no longer
+				 * need it. */
+				int is_slice = let_isslice(n->type);
 				ins2(c, A_LEAQ, masym(c, n->str), areg(D_CX));
 				ins2(c, A_MOVQ, amem(D_CX, 0), areg(D_AX));
 				ins2(c, A_MOVQ, amem(D_CX, 8), areg(D_BX));
+				if (is_slice)
+					ins2(c, A_MOVQ, amem(D_CX, 16), areg(D_CX));
 				goto ident_done;
 			}
 			ins2(c, A_MOVQ, masym(c, n->str), areg(D_AX));
@@ -3662,7 +3681,10 @@ emit_data_row_zero(FILE *out, const char *dir, const char *name, int sz)
  * str lets (16B): emit 16 zero bytes when there is no init (or
  * the init is `nil` / `""`). A non-empty strlit init would need a
  * compile-time .data → .text relocation (asm doesn't support that
- * yet); we silently skip and let the link fail loudly. */
+ * yet); we silently skip and let the link fail loudly.
+ *
+ * Slice lets (24B): no-init only — the slot is zero. There's no
+ * literal slice syntax to honour, so this is the natural shape. */
 static void
 emit_lets(Cg *c, FILE *out, Node *file)
 {
@@ -3687,10 +3709,11 @@ emit_lets(Cg *c, FILE *out, Node *file)
 			emit_data_row(out, "DATAW", mod_mangle(c, d->str), v);
 			continue;
 		}
-		/* Multi-word (str, 16B). Only zero-init shapes are
-		 * supported: no rhs, or `nil`, or `""` (which interns to
-		 * a strlit but we still emit a zero header — the program
-		 * has to assign a real strlit at runtime to use it). */
+		/* Multi-word (str=16, slice=24). Only zero-init shapes
+		 * are supported: no rhs, or `nil`, or `""` (which interns
+		 * to a strlit but we still emit a zero header — the
+		 * program has to assign a real strlit/slice at runtime
+		 * to use it). */
 		if (d->rhs != NULL) {
 			Node *r = d->rhs;
 			while (r != NULL && r->kind == N_CAST) r = r->lhs;

selfhost/cmd/w6c/main.combined.ww

@@ -6130,17 +6130,25 @@ fn cgident(c: *cgen, n: *node) void = {
 	};
 	// Top-level mutable `let` — RIP-relative load from its DATAW
 	// slot. Mirrors C cgen's catch-all `MOVQ masym(s), AX` for
-	// scalar lets, plus the (LEAQ, MOVQ, MOVQ) sequence for str
-	// globals so both ptr and len land in (AX, BX). Names that
-	// aren't lets either (typos, never-defined) drop through to
-	// the silent return.
+	// scalar lets, plus the (LEAQ, MOVQ, MOVQ[, MOVQ]) sequence
+	// for str / slice globals so the ABI pair / triple lands in
+	// (AX, BX[, CX]). Names that aren't lets either (typos,
+	// never-defined) drop through to the silent return.
 	if (isletvar(c, nm)) {
-		if (letvarisstr(c, nm)) {
+		let isstr: bool = letvarisstr(c, nm);
+		let issl: bool  = letvarisslice(c, nm);
+		if (isstr || issl) {
 			emitline("\tLEAQ\t");
 			emitsymname(c, nm);
 			emitline("(SB), CX\n");
 			emitline("\tMOVQ\t(CX), AX\n");
 			emitline("\tMOVQ\t8(CX), BX\n");
+			if (issl) {
+				// Overwrites the address holder with the
+				// cap as the last step — CX is no longer
+				// needed once both ptr/len are loaded.
+				emitline("\tMOVQ\t16(CX), CX\n");
+			};
 			return;
 		};
 		emitline("\tMOVQ\t");
@@ -6608,17 +6616,22 @@ fn cgdot(c: *cgen, n: *node) void = {
 			};
 		};
 	};
-	// Top-level str global field access — load .ptr / .len via
-	// &name(SB) into CX, then MOVQ delta(CX), AX. Without this
-	// the module-qualified fallback below would mis-emit
-	// `MOVQ <field>(SB), AX`.
+	// Top-level str/slice global field access — load .ptr / .len
+	// (and .cap for slices) via &name(SB) into CX, then MOVQ
+	// delta(CX), AX. Without this the module-qualified fallback
+	// below would mis-emit `MOVQ <field>(SB), AX`.
 	if (lhs != nil) {
 		if (lhs.kind == nkind.N_IDENT) {
 			if (isletvar(c, lhs.str)) {
-				if (letvarisstr(c, lhs.str)) {
+				let isstr: bool = letvarisstr(c, lhs.str);
+				let issl: bool  = letvarisslice(c, lhs.str);
+				if (isstr || issl) {
 					let delta: i32 = -1;
 					if (streq(fld, "ptr")) { delta = 0; };
 					if (streq(fld, "len")) { delta = 8; };
+					if (issl) {
+						if (streq(fld, "cap")) { delta = 16; };
+					};
 					if (delta >= 0) {
 						emitline("\tLEAQ\t");
 						emitsymname(c, lhs.str);
@@ -9036,11 +9049,13 @@ fn letscalarprim(nm: str) bool = {
 // aliases so byte output matches C cgen, which resolves Type kinds.
 //   8  → scalar (literal init supported)
 //   16 → str    (only zero-init / nil / "" supported)
+//   24 → slice  (only zero-init supported)
 fn letemitsize(c: *cgen, d: *node) i32 = {
 	if (d == nil) { return 0; };
 	let t: *node = d.lhs;
 	for (t != nil) {
 		if (t.kind == nkind.N_TPTR) { return 8; };
+		if (t.kind == nkind.N_TSLICE) { return 24; };
 		if (t.kind != nkind.N_TNAME) { return 0; };
 		let nm: str = t.str;
 		if (letscalarprim(nm)) { return 8; };
@@ -9106,6 +9121,24 @@ fn letvarisstr(c: *cgen, name: str) bool = {
 	return false;
 };
 
+// letvarisslice — is the named top-level let a slice global?
+// Slice headers are 24 bytes; the ABI flows as (AX, BX, CX) so the
+// load sequence ends with `MOVQ 16(CX), CX` (overwrites the
+// address holder with the cap). Mirrors C cgen's `let_isslice`.
+fn letvarisslice(c: *cgen, name: str) bool = {
+	let lv: *letvar = c.lets;
+	for (lv != nil) {
+		if (streq(lv.name, name)) {
+			let t: *node = lv.tnode;
+			if (t == nil) { return false; };
+			if (t.kind == nkind.N_TSLICE) { return true; };
+			return false;
+		};
+		lv = lv.lvnext;
+	};
+	return false;
+};
+
 // emitdatawbyte — write one byte of an asm string literal using
 // the same escape rules as emitdefconstants / emitdatasection.
 fn emitdatawbyte(b: u8) void = {
@@ -9215,6 +9248,35 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitline("\"\n");
 					};
 				};
+				if (sz == 24) {
+					// Slice: zero-init only (no slice-literal
+					// syntax to honour). Any rhs other than
+					// `nil` is skipped → undefined symbol at
+					// link.
+					let ok: bool = true;
+					if (d.rhs != nil) {
+						let r: *node = d.rhs;
+						for (r != nil) {
+							if (r.kind != nkind.N_CAST) { break; };
+							r = r.lhs;
+						};
+						ok = false;
+						if (r != nil) {
+							if (r.kind == nkind.N_NIL) { ok = true; };
+						};
+					};
+					if (ok) {
+						emitline("DATAW ");
+						emitsymname(c, nm);
+						emitline("(SB),\"");
+						let i: i32 = 0;
+						for (i < 24) {
+							emitdatawbyte(0u8);
+							i += 1;
+						};
+						emitline("\"\n");
+					};
+				};
 			};
 		};
 		d = d.next;

selfhost/cmd/wcc/cgen.ww

@@ -586,11 +586,13 @@ fn letscalarprim(nm: str) bool = {
 // aliases so byte output matches C cgen, which resolves Type kinds.
 //   8  → scalar (literal init supported)
 //   16 → str    (only zero-init / nil / "" supported)
+//   24 → slice  (only zero-init supported)
 fn letemitsize(c: *cgen, d: *node) i32 = {
 	if (d == nil) { return 0; };
 	let t: *node = d.lhs;
 	for (t != nil) {
 		if (t.kind == nkind.N_TPTR) { return 8; };
+		if (t.kind == nkind.N_TSLICE) { return 24; };
 		if (t.kind != nkind.N_TNAME) { return 0; };
 		let nm: str = t.str;
 		if (letscalarprim(nm)) { return 8; };
@@ -656,6 +658,24 @@ fn letvarisstr(c: *cgen, name: str) bool = {
 	return false;
 };
 
+// letvarisslice — is the named top-level let a slice global?
+// Slice headers are 24 bytes; the ABI flows as (AX, BX, CX) so the
+// load sequence ends with `MOVQ 16(CX), CX` (overwrites the
+// address holder with the cap). Mirrors C cgen's `let_isslice`.
+fn letvarisslice(c: *cgen, name: str) bool = {
+	let lv: *letvar = c.lets;
+	for (lv != nil) {
+		if (streq(lv.name, name)) {
+			let t: *node = lv.tnode;
+			if (t == nil) { return false; };
+			if (t.kind == nkind.N_TSLICE) { return true; };
+			return false;
+		};
+		lv = lv.lvnext;
+	};
+	return false;
+};
+
 // emitdatawbyte — write one byte of an asm string literal using
 // the same escape rules as emitdefconstants / emitdatasection.
 fn emitdatawbyte(b: u8) void = {
@@ -765,6 +785,35 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitline("\"\n");
 					};
 				};
+				if (sz == 24) {
+					// Slice: zero-init only (no slice-literal
+					// syntax to honour). Any rhs other than
+					// `nil` is skipped → undefined symbol at
+					// link.
+					let ok: bool = true;
+					if (d.rhs != nil) {
+						let r: *node = d.rhs;
+						for (r != nil) {
+							if (r.kind != nkind.N_CAST) { break; };
+							r = r.lhs;
+						};
+						ok = false;
+						if (r != nil) {
+							if (r.kind == nkind.N_NIL) { ok = true; };
+						};
+					};
+					if (ok) {
+						emitline("DATAW ");
+						emitsymname(c, nm);
+						emitline("(SB),\"");
+						let i: i32 = 0;
+						for (i < 24) {
+							emitdatawbyte(0u8);
+							i += 1;
+						};
+						emitline("\"\n");
+					};
+				};
 			};
 		};
 		d = d.next;

selfhost/cmd/wcc/cgenexpr.ww

@@ -402,17 +402,25 @@ fn cgident(c: *cgen, n: *node) void = {
 	};
 	// Top-level mutable `let` — RIP-relative load from its DATAW
 	// slot. Mirrors C cgen's catch-all `MOVQ masym(s), AX` for
-	// scalar lets, plus the (LEAQ, MOVQ, MOVQ) sequence for str
-	// globals so both ptr and len land in (AX, BX). Names that
-	// aren't lets either (typos, never-defined) drop through to
-	// the silent return.
+	// scalar lets, plus the (LEAQ, MOVQ, MOVQ[, MOVQ]) sequence
+	// for str / slice globals so the ABI pair / triple lands in
+	// (AX, BX[, CX]). Names that aren't lets either (typos,
+	// never-defined) drop through to the silent return.
 	if (isletvar(c, nm)) {
-		if (letvarisstr(c, nm)) {
+		let isstr: bool = letvarisstr(c, nm);
+		let issl: bool  = letvarisslice(c, nm);
+		if (isstr || issl) {
 			emitline("\tLEAQ\t");
 			emitsymname(c, nm);
 			emitline("(SB), CX\n");
 			emitline("\tMOVQ\t(CX), AX\n");
 			emitline("\tMOVQ\t8(CX), BX\n");
+			if (issl) {
+				// Overwrites the address holder with the
+				// cap as the last step — CX is no longer
+				// needed once both ptr/len are loaded.
+				emitline("\tMOVQ\t16(CX), CX\n");
+			};
 			return;
 		};
 		emitline("\tMOVQ\t");
@@ -880,17 +888,22 @@ fn cgdot(c: *cgen, n: *node) void = {
 			};
 		};
 	};
-	// Top-level str global field access — load .ptr / .len via
-	// &name(SB) into CX, then MOVQ delta(CX), AX. Without this
-	// the module-qualified fallback below would mis-emit
-	// `MOVQ <field>(SB), AX`.
+	// Top-level str/slice global field access — load .ptr / .len
+	// (and .cap for slices) via &name(SB) into CX, then MOVQ
+	// delta(CX), AX. Without this the module-qualified fallback
+	// below would mis-emit `MOVQ <field>(SB), AX`.
 	if (lhs != nil) {
 		if (lhs.kind == nkind.N_IDENT) {
 			if (isletvar(c, lhs.str)) {
-				if (letvarisstr(c, lhs.str)) {
+				let isstr: bool = letvarisstr(c, lhs.str);
+				let issl: bool  = letvarisslice(c, lhs.str);
+				if (isstr || issl) {
 					let delta: i32 = -1;
 					if (streq(fld, "ptr")) { delta = 0; };
 					if (streq(fld, "len")) { delta = 8; };
+					if (issl) {
+						if (streq(fld, "cap")) { delta = 16; };
+					};
 					if (delta >= 0) {
 						emitline("\tLEAQ\t");
 						emitsymname(c, lhs.str);

selfhost/cmd/wwdump/main.combined.ww

@@ -6130,17 +6130,25 @@ fn cgident(c: *cgen, n: *node) void = {
 	};
 	// Top-level mutable `let` — RIP-relative load from its DATAW
 	// slot. Mirrors C cgen's catch-all `MOVQ masym(s), AX` for
-	// scalar lets, plus the (LEAQ, MOVQ, MOVQ) sequence for str
-	// globals so both ptr and len land in (AX, BX). Names that
-	// aren't lets either (typos, never-defined) drop through to
-	// the silent return.
+	// scalar lets, plus the (LEAQ, MOVQ, MOVQ[, MOVQ]) sequence
+	// for str / slice globals so the ABI pair / triple lands in
+	// (AX, BX[, CX]). Names that aren't lets either (typos,
+	// never-defined) drop through to the silent return.
 	if (isletvar(c, nm)) {
-		if (letvarisstr(c, nm)) {
+		let isstr: bool = letvarisstr(c, nm);
+		let issl: bool  = letvarisslice(c, nm);
+		if (isstr || issl) {
 			emitline("\tLEAQ\t");
 			emitsymname(c, nm);
 			emitline("(SB), CX\n");
 			emitline("\tMOVQ\t(CX), AX\n");
 			emitline("\tMOVQ\t8(CX), BX\n");
+			if (issl) {
+				// Overwrites the address holder with the
+				// cap as the last step — CX is no longer
+				// needed once both ptr/len are loaded.
+				emitline("\tMOVQ\t16(CX), CX\n");
+			};
 			return;
 		};
 		emitline("\tMOVQ\t");
@@ -6608,17 +6616,22 @@ fn cgdot(c: *cgen, n: *node) void = {
 			};
 		};
 	};
-	// Top-level str global field access — load .ptr / .len via
-	// &name(SB) into CX, then MOVQ delta(CX), AX. Without this
-	// the module-qualified fallback below would mis-emit
-	// `MOVQ <field>(SB), AX`.
+	// Top-level str/slice global field access — load .ptr / .len
+	// (and .cap for slices) via &name(SB) into CX, then MOVQ
+	// delta(CX), AX. Without this the module-qualified fallback
+	// below would mis-emit `MOVQ <field>(SB), AX`.
 	if (lhs != nil) {
 		if (lhs.kind == nkind.N_IDENT) {
 			if (isletvar(c, lhs.str)) {
-				if (letvarisstr(c, lhs.str)) {
+				let isstr: bool = letvarisstr(c, lhs.str);
+				let issl: bool  = letvarisslice(c, lhs.str);
+				if (isstr || issl) {
 					let delta: i32 = -1;
 					if (streq(fld, "ptr")) { delta = 0; };
 					if (streq(fld, "len")) { delta = 8; };
+					if (issl) {
+						if (streq(fld, "cap")) { delta = 16; };
+					};
 					if (delta >= 0) {
 						emitline("\tLEAQ\t");
 						emitsymname(c, lhs.str);
@@ -9036,11 +9049,13 @@ fn letscalarprim(nm: str) bool = {
 // aliases so byte output matches C cgen, which resolves Type kinds.
 //   8  → scalar (literal init supported)
 //   16 → str    (only zero-init / nil / "" supported)
+//   24 → slice  (only zero-init supported)
 fn letemitsize(c: *cgen, d: *node) i32 = {
 	if (d == nil) { return 0; };
 	let t: *node = d.lhs;
 	for (t != nil) {
 		if (t.kind == nkind.N_TPTR) { return 8; };
+		if (t.kind == nkind.N_TSLICE) { return 24; };
 		if (t.kind != nkind.N_TNAME) { return 0; };
 		let nm: str = t.str;
 		if (letscalarprim(nm)) { return 8; };
@@ -9106,6 +9121,24 @@ fn letvarisstr(c: *cgen, name: str) bool = {
 	return false;
 };
 
+// letvarisslice — is the named top-level let a slice global?
+// Slice headers are 24 bytes; the ABI flows as (AX, BX, CX) so the
+// load sequence ends with `MOVQ 16(CX), CX` (overwrites the
+// address holder with the cap). Mirrors C cgen's `let_isslice`.
+fn letvarisslice(c: *cgen, name: str) bool = {
+	let lv: *letvar = c.lets;
+	for (lv != nil) {
+		if (streq(lv.name, name)) {
+			let t: *node = lv.tnode;
+			if (t == nil) { return false; };
+			if (t.kind == nkind.N_TSLICE) { return true; };
+			return false;
+		};
+		lv = lv.lvnext;
+	};
+	return false;
+};
+
 // emitdatawbyte — write one byte of an asm string literal using
 // the same escape rules as emitdefconstants / emitdatasection.
 fn emitdatawbyte(b: u8) void = {
@@ -9215,6 +9248,35 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitline("\"\n");
 					};
 				};
+				if (sz == 24) {
+					// Slice: zero-init only (no slice-literal
+					// syntax to honour). Any rhs other than
+					// `nil` is skipped → undefined symbol at
+					// link.
+					let ok: bool = true;
+					if (d.rhs != nil) {
+						let r: *node = d.rhs;
+						for (r != nil) {
+							if (r.kind != nkind.N_CAST) { break; };
+							r = r.lhs;
+						};
+						ok = false;
+						if (r != nil) {
+							if (r.kind == nkind.N_NIL) { ok = true; };
+						};
+					};
+					if (ok) {
+						emitline("DATAW ");
+						emitsymname(c, nm);
+						emitline("(SB),\"");
+						let i: i32 = 0;
+						for (i < 24) {
+							emitdatawbyte(0u8);
+							i += 1;
+						};
+						emitline("\"\n");
+					};
+				};
 			};
 		};
 		d = d.next;

test/wcc/630_let_global.c

@@ -113,6 +113,31 @@ static const struct fixture fixtures[] = {
 		"};\n",
 		7,
 	},
+	{
+		"slice-zeroinit",
+		/* Slice globals start as {nil, 0, 0}. .len and .cap both
+		 * read zero; .ptr is nil but we don't dereference it. */
+		"let buf: []u8;\n"
+		"fn main() i32 = {\n"
+		"\treturn (buf.len + buf.cap): i32;\n"
+		"};\n",
+		0,
+	},
+	{
+		"slice-cap-via-raw-pointer",
+		/* Drive the slice header through a raw u64 pointer cast
+		 * — the language has no slice-reassignment expression
+		 * yet, so this is the only way to fill the header from
+		 * ww source today. .cap reads back as the value we
+		 * wrote. */
+		"let buf: []u8;\n"
+		"fn main() i32 = {\n"
+		"\tlet p: *u64 = (&buf): *u64;\n"
+		"\tp[2] = 99u64;\n"
+		"\treturn buf.cap: i32;\n"
+		"};\n",
+		99,
+	},
 	{ NULL, NULL, 0 }
 };