From 186b7095eeb84bec527d6d7ac9320776a8b78734 Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Fri, 26 Apr 2024 18:31:18 -0700
Subject: Use hlt to implement noreturn

---
 cc.h                   |  1 +
 decl.c                 |  2 ++
 qbe.c                  | 29 ++++++++++++++++++++++++-----
 test/func-noreturn.c   | 13 +++++++++++++
 test/func-noreturn.qbe | 28 ++++++++++++++++++++++++++++
 5 files changed, 68 insertions(+), 5 deletions(-)
 create mode 100644 test/func-noreturn.c
 create mode 100644 test/func-noreturn.qbe

diff --git a/cc.h b/cc.h
index 1cd1b32..ea237c5 100644
--- a/cc.h
+++ b/cc.h
@@ -556,6 +556,7 @@ struct value *funcexpr(struct func *, struct expr *);
 void funcjmp(struct func *, struct block *);
 void funcjnz(struct func *, struct value *, struct type *, struct block *, struct block *);
 void funcret(struct func *, struct value *);
+void funchlt(struct func *);
 struct gotolabel *funcgoto(struct func *, char *);
 void funcswitch(struct func *, struct value *, struct switchcases *, struct block *);
 void funcinit(struct func *, struct decl *, struct init *, bool);
diff --git a/decl.c b/decl.c
index 0749017..39c272d 100644
--- a/decl.c
+++ b/decl.c
@@ -1066,6 +1066,8 @@ decl(struct scope *s, struct func *f)
 				s = funcscope;
 				f = mkfunc(d, name, t, s);
 				stmt(f, s);
+				if (d->u.func.isnoreturn)
+					funchlt(f);
 				/* XXX: need to keep track of function in case a later declaration specifies extern */
 				if (!d->u.func.inlinedefn)
 					emitfunc(f, d->linkage == LINKEXTERN);
diff --git a/qbe.c b/qbe.c
index a6781dd..5c76bb2 100644
--- a/qbe.c
+++ b/qbe.c
@@ -62,6 +62,7 @@ struct jump {
 		JUMP_JMP,
 		JUMP_JNZ,
 		JUMP_RET,
+		JUMP_HLT,
 	} kind;
 	struct value *arg;
 	struct block *blk[2];
@@ -598,6 +599,15 @@ funcret(struct func *f, struct value *v)
 	}
 }
 
+void
+funchlt(struct func *f)
+{
+	struct block *b = f->end;
+
+	if (!b->jump.kind)
+		b->jump.kind = JUMP_HLT;
+}
+
 struct gotolabel *
 funcgoto(struct func *f, char *name)
 {
@@ -631,7 +641,7 @@ funclval(struct func *f, struct expr *e)
 	case EXPRIDENT:
 		d = e->u.ident.decl;
 		if (d->kind != DECLOBJECT && d->kind != DECLFUNC)
-			error(&tok.loc, "identifier is not an object or function");  /* XXX: fix location, var name */
+			error(&tok.loc, "identifier '%s' is not an object or function", d->name);
 		if (d == f->namedecl) {
 			fputs("data ", stdout);
 			emitname(d->value);
@@ -727,10 +737,12 @@ funcexpr(struct func *f, struct expr *e)
 			t = arg->type;
 			funcinst(f, IARG, qbetype(t).base, argvals[i], t->value);
 		}
-		/*
-		if (functype->func.isnoreturn)
-			funcret(f, NULL);
-		*/
+		e = e->base;
+		if (e->kind == EXPRUNARY && e->op == TBAND) {
+			e = e->base;
+			if (e->kind == EXPRIDENT && e->u.ident.decl->u.func.isnoreturn)
+				funchlt(f);
+		}
 		return v;
 	case EXPRUNARY:
 		switch (e->op) {
@@ -1187,6 +1199,8 @@ static void
 emitjump(struct jump *j)
 {
 	switch (j->kind) {
+	case JUMP_NONE:
+		break;
 	case JUMP_RET:
 		fputs("\tret", stdout);
 		if (j->arg) {
@@ -1209,6 +1223,11 @@ emitjump(struct jump *j)
 		emitname(&j->blk[1]->label);
 		putchar('\n');
 		break;
+	case JUMP_HLT:
+		fputs("\thlt\n", stdout);
+		break;
+	default:
+		assert(0);
 	}
 }
 
diff --git a/test/func-noreturn.c b/test/func-noreturn.c
new file mode 100644
index 0000000..0cd7f7a
--- /dev/null
+++ b/test/func-noreturn.c
@@ -0,0 +1,13 @@
+_Noreturn void exit(int);
+int puts(const char *);
+void _Noreturn f(void) {
+	static int c;
+	while (c)
+		puts("loop");
+}
+int main(void) {
+	exit(0);
+	(*f)();
+	(***f)();
+	return 1;
+}
diff --git a/test/func-noreturn.qbe b/test/func-noreturn.qbe
new file mode 100644
index 0000000..d1771dc
--- /dev/null
+++ b/test/func-noreturn.qbe
@@ -0,0 +1,28 @@
+data $.Lc.2 = align 4 { z 4 }
+data $.Lstring.3 = align 1 { b "loop\000", }
+export
+function $f() {
+@start.1
+@body.2
+@while_cond.3
+	%.1 =w loadw $.Lc.2
+	jnz %.1, @while_body.4, @while_join.5
+@while_body.4
+	%.2 =w call $puts(l $.Lstring.3)
+	jmp @while_cond.3
+@while_join.5
+	hlt
+}
+export
+function w $main() {
+@start.6
+@body.7
+	call $exit(w 0)
+	hlt
+@dead.8
+	call $f()
+	hlt
+@dead.9
+	call $f()
+	hlt
+}
-- 
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