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.

tccgen.c

@@ -5367,7 +5367,7 @@ static int gcase(struct case_t **base, int len, int case_reg, int *bsym)
 
 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 */
@@ -5386,22 +5386,31 @@ static void block(int *bsym, int *csym, int is_expr)
 
     if (tok == TOK_IF) {
         /* if test */
+	int saved_nocode_wanted = nocode_wanted;
         next();
         skip('(');
         gexpr();
         skip(')');
+	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();
@@ -5564,6 +5573,7 @@ static void block(int *bsym, int *csym, int is_expr)
         skip(';');
     } else if (tok == TOK_FOR) {
         int e;
+	nocode_wanted &= ~2;
         next();
         skip('(');
         s = local_stack;
@@ -5607,6 +5617,7 @@ static void block(int *bsym, int *csym, int is_expr)
 
     } else 
     if (tok == TOK_DO) {
+	nocode_wanted &= ~2;
         next();
         a = 0;
         b = 0;
@@ -5658,6 +5669,7 @@ static void block(int *bsym, int *csym, int is_expr)
         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) {
@@ -5735,6 +5747,7 @@ static void block(int *bsym, int *csym, int is_expr)
             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

@@ -88,6 +88,7 @@ void struct_test();
 void array_test();
 void expr_ptr_test();
 void bool_test();
+void optimize_out();
 void expr2_test();
 void constant_expr_test();
 void expr_cmp_test();
@@ -695,6 +696,7 @@ int main(int argc, char **argv)
     array_test();
     expr_ptr_test();
     bool_test();
+    optimize_out();
     expr2_test();
     constant_expr_test();
     expr_cmp_test();
@@ -1165,6 +1167,31 @@ void bool_test()
     printf ("bits = 0x%x\n", calc_vm_flags (0x1));
 }
 
+extern int undefined_function(void);
+extern int defined_function(void);
+
+void optimize_out(void)
+{
+  int i = 0 ? undefined_function() : defined_function();
+  printf ("oo:%d\n", i);
+  int j = 1 ? defined_function() : undefined_function();
+  printf ("oo:%d\n", j);
+  if (0)
+    printf("oo:%d\n", undefined_function());
+  else
+    printf("oo:%d\n", defined_function());
+  if (1)
+    printf("oo:%d\n", defined_function());
+  else
+    printf("oo:%d\n", undefined_function());
+}
+
+int defined_function(void)
+{
+  static int i = 40;
+  return i++;
+}
+
 /* GCC accepts that */
 static int tab_reinit[];
 static int tab_reinit[10];

tests/tests2/87_dead_code.c

@@ -0,0 +1,122 @@
+/* This checks various ways of dead code inside if statements
+   where there are non-obvious ways of how the code is actually
+   not dead due to reachable by labels.  */
+extern int printf (const char *, ...);
+static void kb_wait_1(void)
+{
+  unsigned long timeout = 2;
+  do {
+      /* Here the else arm is a statement expression that's supposed
+         to be suppressed.  The label inside the while would unsuppress
+	 code generation again if not handled correctly.  And that
+	 would wreak havok to the cond-expression because there's no
+	 jump-around emitted, the whole statement expression really
+	 needs to not generate code (perhaps except useless forward jumps).  */
+      (1 ? 
+       printf("timeout=%ld\n", timeout) :
+       ({
+	int i = 1;
+	while (1)
+	  while (i--)
+	    some_label:
+	      printf("error\n");
+	goto some_label;
+	})
+      );
+      timeout--;
+  } while (timeout);
+}
+int main (void)
+{
+  int i = 1;
+  kb_wait_1();
+
+  /* Simple test of dead code at first sight which isn't actually dead. */
+  if (0) {
+yeah:
+      printf ("yeah\n");
+  } else {
+      printf ("boo\n");
+  }
+  if (i--)
+    goto yeah;
+
+  /* Some more non-obvious uses where the problems are loops, so that even
+     the first loop statements aren't actually dead.  */
+  i = 1;
+  if (0) {
+      while (i--) {
+	  printf ("once\n");
+enterloop:
+	  printf ("twice\n");
+      }
+  }
+  if (i >= 0)
+    goto enterloop;
+
+  /* The same with statement expressions.  One might be tempted to
+     handle them specially by counting if inside statement exprs and
+     not unsuppressing code at loops at all then.
+     See kb_wait_1 for the other side of the medal where that wouldn't work.  */
+  i = ({
+      int j = 1;
+      if (0) {
+	  while (j--) {
+	      printf ("SEonce\n");
+    enterexprloop:
+	      printf ("SEtwice\n");
+	  }
+      }
+      if (j >= 0)
+	goto enterexprloop;
+      j; });
+
+  /* The other two loop forms: */
+  i = 1;
+  if (0) {
+      for (i = 1; i--;) {
+	  printf ("once2\n");
+enterloop2:
+	  printf ("twice2\n");
+      }
+  }
+  if (i > 0)
+    goto enterloop2;
+
+  i = 1;
+  if (0) {
+      do {
+	  printf ("once3\n");
+enterloop3:
+	  printf ("twice3\n");
+      } while (i--);
+  }
+  if (i > 0)
+    goto enterloop3;
+
+  /* And check that case and default labels have the same effect
+     of disabling code suppression.  */
+  i = 41;
+  switch (i) {
+      if (0) {
+	  printf ("error\n");
+      case 42:
+	  printf ("error2\n");
+      case 41:
+	  printf ("caseok\n");
+      }
+  }
+
+  i = 41;
+  switch (i) {
+      if (0) {
+	  printf ("error3\n");
+      default:
+	  printf ("caseok2\n");
+	  break;
+      case 42:
+	  printf ("error4\n");
+      }
+  }
+  return 0;
+}

tests/tests2/87_dead_code.expect

@@ -0,0 +1,18 @@
+timeout=2
+timeout=1
+boo
+yeah
+twice
+once
+twice
+SEtwice
+SEonce
+SEtwice
+twice2
+once2
+twice2
+twice3
+once3
+twice3
+caseok
+caseok2