selfhost: port switch — N_SWITCH parser + cgen + scratch slot

selfhost/cmd/wcc/cgenstmt.ww

@@ -34,6 +34,8 @@ fn cgstmt(c: *cgen, n: *node) void = {
 
 	if (k == nkind.N_FOR) { cgfor(c, n); return; };
 
+	if (k == nkind.N_SWITCH) { cgswitch(c, n); return; };
+
 	if (k == nkind.N_MASSIGN) { cgmassign(c, n); return; };
 
 	if (k == nkind.N_MLET) { cgmlet(c, n); return; };
@@ -716,6 +718,65 @@ fn cgmlet(c: *cgen, n: *node) void = {
 	return;
 };
 
+// cgswitch — lower `switch (e) { case 1, 2: ...; case: default; }` to
+// a chain of compares against the scrutinee. Scrutinee lands in a
+// fresh 8B local slot so case bodies can spill SP without losing it.
+// Cases are tried top-to-bottom; the `case:` arm with no exprs is the
+// default and runs after all named arms fail. Mirrors cmd/w6c/cgen.c
+// N_SWITCH: same labelseq consumption order so labels match byte-for-
+// byte.
+fn cgswitch(c: *cgen, n: *node) void = {
+	let swname: str = mkscratchname(c, "sw");
+	let sloff: i32 = localalloc(c, swname, 8, nil);
+
+	if (n.lhs != nil) { cgexpr(c, n.lhs); };
+	emitline("\tMOVQ\tAX, ");
+	emitoff(sloff: i64);
+	emitline("(BP)\n");
+
+	let endl: str = mklabel(c, "swend");
+	let defcase: *node = nil;
+
+	let cs: *node = n.list;
+	for (cs != nil) {
+		if (cs.list == nil) {
+			defcase = cs;
+			cs = cs.next;
+			continue;
+		};
+		let body: str = mklabel(c, "swcase");
+		let nxt: str = mklabel(c, "swnext");
+		let e: *node = cs.list;
+		for (e != nil) {
+			cgexpr(c, e);
+			emitline("\tMOVQ\t");
+			emitoff(sloff: i64);
+			emitline("(BP), BX\n");
+			emitline("\tCMPQ\tBX, AX\n");
+			emitline("\tJE\t");
+			emitline(body);
+			emitline("\n");
+			e = e.next;
+		};
+		emitline("\tJMP\t");
+		emitline(nxt);
+		emitline("\n");
+		emitlabel(body);
+		if (cs.body != nil) { cgstmt(c, cs.body); };
+		emitline("\tJMP\t");
+		emitline(endl);
+		emitline("\n");
+		emitlabel(nxt);
+		cs = cs.next;
+	};
+	if (defcase != nil) {
+		if (defcase.body != nil) { cgstmt(c, defcase.body); };
+	};
+	emitlabel(endl);
+	c.lastwasreturn = 0;
+	return;
+};
+
 fn cgbreak(c: *cgen, n: *node) void = {
 	if (c.looptop > 0) {
 		let lbl: str = c.loopendbuf[c.looptop - 1];