expr: Skip codegen for unused expression in conditional with constant expression

This compiles `0 ? e1 : e2` as `e2`, and `1 ? e1 : e2` as `e1`
(while still adjusting the type as necessary).

5 files changed, 71 insertions, 44 deletions

diff --git a/expr.c b/expr.c
index 2bf711a..182972e 100644
--- a/expr.c
+++ b/expr.c
@@ -1007,49 +1007,53 @@ binaryexpr(struct scope *s, struct expr *l, int i)
 static struct expr *
 condexpr(struct scope *s)
 {
-	struct expr *r, *e;
-	struct type *t, *f;
+	struct expr *e, *l, *r;
+	struct type *t, *lt, *rt;
 	enum typequal tq;
 
-	r = binaryexpr(s, NULL, 0);
+	e = binaryexpr(s, NULL, 0);
 	if (!consume(TQUESTION))
-		return r;
-	e = mkexpr(EXPRCOND, NULL, r);
-	e->cond.t = expr(s);
+		return e;
+	l = expr(s);
 	expect(TCOLON, "in conditional expression");
-	e->cond.f = condexpr(s);
-	t = e->cond.t->type;
-	f = e->cond.f->type;
-	if (t == f) {
-		e->type = t;
-	} else if (t->prop & PROPARITH && f->prop & PROPARITH) {
-		e->type = commonreal(&e->cond.t, &e->cond.f);
-	} else if (t == &typevoid && f == &typevoid) {
-		e->type = &typevoid;
+	r = condexpr(s);
+
+	lt = l->type;
+	rt = r->type;
+	if (lt == rt) {
+		t = lt;
+	} else if (lt->prop & PROPARITH && rt->prop & PROPARITH) {
+		t = commonreal(&l, &r);
+	} else if (lt == &typevoid && rt == &typevoid) {
+		t = &typevoid;
 	} else {
-		e->cond.t = eval(e->cond.t, EVALARITH);
-		e->cond.f = eval(e->cond.f, EVALARITH);
-		if (nullpointer(e->cond.t) && f->kind == TYPEPOINTER) {
-			e->type = f;
-		} else if (nullpointer(e->cond.f) && t->kind == TYPEPOINTER) {
-			e->type = t;
-		} else if (t->kind == TYPEPOINTER && f->kind == TYPEPOINTER) {
-			tq = t->qual | f->qual;
-			t = t->base;
-			f = f->base;
-			if (t == &typevoid || f == &typevoid) {
-				e->type = &typevoid;
-			} else {
-				if (!typecompatible(t, f))
-					error(&tok.loc, "operands of conditional operator must have compatible types");
-				e->type = typecomposite(t, f);
-			}
-			e->type = mkpointertype(e->type, tq);
+		l = eval(l, EVALARITH);
+		r = eval(r, EVALARITH);
+		if (nullpointer(l) && rt->kind == TYPEPOINTER) {
+			t = rt;
+		} else if (nullpointer(r) && lt->kind == TYPEPOINTER) {
+			t = lt;
+		} else if (lt->kind == TYPEPOINTER && rt->kind == TYPEPOINTER) {
+			tq = lt->qual | rt->qual;
+			lt = lt->base;
+			rt = rt->base;
+			if (lt == &typevoid || rt == &typevoid)
+				t = &typevoid;
+			else if (typecompatible(lt, rt))
+				t = typecomposite(lt, rt);
+			else
+				error(&tok.loc, "operands of conditional operator must have compatible types");
+			t = mkpointertype(t, tq);
 		} else {
 			error(&tok.loc, "invalid operands to conditional operator");
 		}
 	}
-
+	e = eval(e, EVALARITH);
+	if (e->kind == EXPRCONST && e->type->prop & PROPINT)
+		return exprconvert(e->constant.i ? l : r, t);
+	e = mkexpr(EXPRCOND, t, e);
+	e->cond.t = l;
+	e->cond.f = r;
 	return e;
 }
 
diff --git a/test/conditional-compound-literal.c b/test/conditional-compound-literal.c
index 33a5416..40b8ad6 100644
--- a/test/conditional-compound-literal.c
+++ b/test/conditional-compound-literal.c
@@ -1,4 +1,4 @@
 int main(void) {
-	int x = 0, *p = 0 ? 0 : &(int){x};
+	int x = 0, *p = x ? 0 : &(int){x};
 	return *p;
 }
diff --git a/test/conditional-compound-literal.qbe b/test/conditional-compound-literal.qbe
index 739eea4..d2265c6 100644
--- a/test/conditional-compound-literal.qbe
+++ b/test/conditional-compound-literal.qbe
@@ -3,19 +3,20 @@ function w $main() {
 @start.1
 	%.1 =l alloc4 4
 	%.2 =l alloc8 8
-	%.3 =l alloc4 4
+	%.4 =l alloc4 4
 @body.2
 	storew 0, %.1
-	jnz 0, @cond_true.3, @cond_false.4
+	%.3 =w loadw %.1
+	jnz %.3, @cond_true.3, @cond_false.4
 @cond_true.3
 	jmp @cond_join.5
 @cond_false.4
-	%.4 =w loadw %.1
-	storew %.4, %.3
+	%.5 =w loadw %.1
+	storew %.5, %.4
 @cond_join.5
-	%.5 =l phi @cond_true.3 0, @cond_false.4 %.3
-	storel %.5, %.2
-	%.6 =l loadl %.2
-	%.7 =w loadw %.6
-	ret %.7
+	%.6 =l phi @cond_true.3 0, @cond_false.4 %.4
+	storel %.6, %.2
+	%.7 =l loadl %.2
+	%.8 =w loadw %.7
+	ret %.8
 }
diff --git a/test/conditional-constant.c b/test/conditional-constant.c
new file mode 100644
index 0000000..8ebb2a4
--- /dev/null
+++ b/test/conditional-constant.c
@@ -0,0 +1,6 @@
+int main(void) {
+	if ((0 ? 0 : 123) != 123)
+		return 1;
+	if ((1 ? 456 : 0) != 456)
+		return 1;
+}
diff --git a/test/conditional-constant.qbe b/test/conditional-constant.qbe
new file mode 100644
index 0000000..a14e050
--- /dev/null
+++ b/test/conditional-constant.qbe
@@ -0,0 +1,16 @@
+export
+function w $main() {
+@start.1
+@body.2
+	%.1 =w cnew 123, 123
+	jnz %.1, @if_true.3, @if_false.4
+@if_true.3
+	ret 1
+@if_false.4
+	%.2 =w cnew 456, 456
+	jnz %.2, @if_true.5, @if_false.6
+@if_true.5
+	ret 1
+@if_false.6
+	ret 0
+}