opt: Start optimizing dead code a bit

If a condition is always zero/non-zero we can omit the then or else code. This is complicated a bit by having to deal with labels that might make such code reachable without us yet knowing during parsing.

Michael Matz committed 10 years ago

Commit 31c7ea01 parent b303a004 files · +182 −2Browse files

Unified · Split

4 files

tccgen.c +15 −2
tests/tcctest.c +27 −0
tests/tests2/87_dead_code.c +122 −0
tests/tests2/87_dead_code.expect +18 −0

tccgen.c+15−2

 static void block(int *bsym, int *csym, int is_expr)
 {
-    int a, b, c, d;
+    int a, b, c, d, cond;
     Sym *s;
     /* generate line number info */
     if (tok == TOK_IF) {
         /* if test */
+	int saved_nocode_wanted = nocode_wanted;
         next();
         skip('(');
         gexpr();
         skip(')');
-        a = gvtst(1, 0);
+	cond = condition_3way();
+	if (cond == 0)
+	    nocode_wanted |= 2;
+	a = gvtst(1, 0);
         block(bsym, csym, 0);
+	nocode_wanted = saved_nocode_wanted;
         c = tok;
         if (c == TOK_ELSE) {
             next();
+	    if (cond == 1)
+		nocode_wanted |= 2;
             d = gjmp(0);
             gsym(a);
             block(bsym, csym, 0);
             gsym(d); /* patch else jmp */
+	    nocode_wanted = saved_nocode_wanted;
         } else
             gsym(a);
     } else if (tok == TOK_WHILE) {
+	nocode_wanted &= ~2;
         next();
         d = ind;
         vla_sp_restore();
         skip(';');
     } else if (tok == TOK_FOR) {
         int e;
+	nocode_wanted &= ~2;
         next();
         skip('(');
         s = local_stack;
     } else
     if (tok == TOK_DO) {
+	nocode_wanted &= ~2;
         next();
         a = 0;
         b = 0;
         struct case_t *cr = tcc_malloc(sizeof(struct case_t));
         if (!cur_switch)
             expect("switch");
+	nocode_wanted &= ~2;
         next();
         cr->v1 = cr->v2 = expr_const();
         if (gnu_ext && tok == TOK_DOTS) {
             vla_sp_restore();
             /* we accept this, but it is a mistake */
         block_after_label:
+	    nocode_wanted &= ~2;
             if (tok == '}') {
                 tcc_warning("deprecated use of label at end of compound statement");
             } else {

tests/tcctest.c+27−0

@@ -88,6 +88,7 @@ void struct_test(); · 87 unmodified lines
88	void array_test();	88	void array_test();
89	void expr_ptr_test();	89	void expr_ptr_test();
90	void bool_test();	90	void bool_test();
		91	void optimize_out();
91	void expr2_test();	92	void expr2_test();
92	void constant_expr_test();	93	void constant_expr_test();
93	void expr_cmp_test();	94	void expr_cmp_test();
@@ -695,6 +696,7 @@ int main(int argc, char **argv) · 601 unmodified lines
695	array_test();	696	array_test();
696	expr_ptr_test();	697	expr_ptr_test();
697	bool_test();	698	bool_test();
		699	optimize_out();
698	expr2_test();	700	expr2_test();
699	constant_expr_test();	701	constant_expr_test();
700	expr_cmp_test();	702	expr_cmp_test();
@@ -1165,6 +1167,31 @@ void bool_test() · 464 unmodified lines
1165	printf ("bits = 0x%x\n", calc_vm_flags (0x1));	1167	printf ("bits = 0x%x\n", calc_vm_flags (0x1));
1166	}	1168	}
1167		1169
		1170	extern int undefined_function(void);
		1171	extern int defined_function(void);
		1172
		1173	void optimize_out(void)
		1174	{
		1175	int i = 0 ? undefined_function() : defined_function();
		1176	printf ("oo:%d\n", i);
		1177	int j = 1 ? defined_function() : undefined_function();
		1178	printf ("oo:%d\n", j);
		1179	if (0)
		1180	printf("oo:%d\n", undefined_function());
		1181	else
		1182	printf("oo:%d\n", defined_function());
		1183	if (1)
		1184	printf("oo:%d\n", defined_function());
		1185	else
		1186	printf("oo:%d\n", undefined_function());
		1187	}
		1188
		1189	int defined_function(void)
		1190	{
		1191	static int i = 40;
		1192	return i++;
		1193	}
		1194
1168	/* GCC accepts that */	1195	/* GCC accepts that */
1169	static int tab_reinit[];	1196	static int tab_reinit[];
1170	static int tab_reinit[10];	1197	static int tab_reinit[10];

tests/tests2/87_dead_code.c+122−0

@@ -0,0 +1,122 @@
	1	/* This checks various ways of dead code inside if statements
	2	where there are non-obvious ways of how the code is actually
	3	not dead due to reachable by labels. */
	4	extern int printf (const char *, ...);
	5	static void kb_wait_1(void)
	6	{
	7	unsigned long timeout = 2;
	8	do {
	9	/* Here the else arm is a statement expression that's supposed
	10	to be suppressed. The label inside the while would unsuppress
	11	code generation again if not handled correctly. And that
	12	would wreak havok to the cond-expression because there's no
	13	jump-around emitted, the whole statement expression really
	14	needs to not generate code (perhaps except useless forward jumps). */
	15	(1 ?
	16	printf("timeout=%ld\n", timeout) :
	17	({
	18	int i = 1;
	19	while (1)
	20	while (i--)
	21	some_label:
	22	printf("error\n");
	23	goto some_label;
	24	})
	25	);
	26	timeout--;
	27	} while (timeout);
	28	}
	29	int main (void)
	30	{
	31	int i = 1;
	32	kb_wait_1();
	33
	34	/* Simple test of dead code at first sight which isn't actually dead. */
	35	if (0) {
	36	yeah:
	37	printf ("yeah\n");
	38	} else {
	39	printf ("boo\n");
	40	}
	41	if (i--)
	42	goto yeah;
	43
	44	/* Some more non-obvious uses where the problems are loops, so that even
	45	the first loop statements aren't actually dead. */
	46	i = 1;
	47	if (0) {
	48	while (i--) {
	49	printf ("once\n");
	50	enterloop:
	51	printf ("twice\n");
	52	}
	53	}
	54	if (i >= 0)
	55	goto enterloop;
	56
	57	/* The same with statement expressions. One might be tempted to
	58	handle them specially by counting if inside statement exprs and
	59	not unsuppressing code at loops at all then.
	60	See kb_wait_1 for the other side of the medal where that wouldn't work. */
	61	i = ({
	62	int j = 1;
	63	if (0) {
	64	while (j--) {
	65	printf ("SEonce\n");
	66	enterexprloop:
	67	printf ("SEtwice\n");
	68	}
	69	}
	70	if (j >= 0)
	71	goto enterexprloop;
	72	j; });
	73
	74	/* The other two loop forms: */
	75	i = 1;
	76	if (0) {
	77	for (i = 1; i--;) {
	78	printf ("once2\n");
	79	enterloop2:
	80	printf ("twice2\n");
	81	}
	82	}
	83	if (i > 0)
	84	goto enterloop2;
	85
	86	i = 1;
	87	if (0) {
	88	do {
	89	printf ("once3\n");
	90	enterloop3:
	91	printf ("twice3\n");
	92	} while (i--);
	93	}
	94	if (i > 0)
	95	goto enterloop3;
	96
	97	/* And check that case and default labels have the same effect
	98	of disabling code suppression. */
	99	i = 41;
	100	switch (i) {
	101	if (0) {
	102	printf ("error\n");
	103	case 42:
	104	printf ("error2\n");
	105	case 41:
	106	printf ("caseok\n");
	107	}
	108	}
	109
	110	i = 41;
	111	switch (i) {
	112	if (0) {
	113	printf ("error3\n");
	114	default:
	115	printf ("caseok2\n");
	116	break;
	117	case 42:
	118	printf ("error4\n");
	119	}
	120	}
	121	return 0;
	122	}

tests/tests2/87_dead_code.expect+18−0

@@ -0,0 +1,18 @@
	1	timeout=2
	2	timeout=1
	3	boo
	4	yeah
	5	twice
	6	once
	7	twice
	8	SEtwice
	9	SEonce
	10	SEtwice
	11	twice2
	12	once2
	13	twice2
	14	twice3
	15	once3
	16	twice3
	17	caseok
	18	caseok2