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.
This commit is contained in:
Michael Matz
parent
b303a00ce0
commit
31c7ea0165
4 changed files with 182 additions and 2 deletions
17
tccgen.c
17
tccgen.c
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@ -5367,7 +5367,7 @@ static int gcase(struct case_t **base, int len, int case_reg, int *bsym)
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static void block(int *bsym, int *csym, int is_expr)
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{
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int a, b, c, d;
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int a, b, c, d, cond;
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Sym *s;
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/* generate line number info */
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@ -5386,22 +5386,31 @@ static void block(int *bsym, int *csym, int is_expr)
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if (tok == TOK_IF) {
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/* if test */
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int saved_nocode_wanted = nocode_wanted;
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next();
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skip('(');
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gexpr();
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skip(')');
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a = gvtst(1, 0);
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cond = condition_3way();
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if (cond == 0)
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nocode_wanted |= 2;
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a = gvtst(1, 0);
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block(bsym, csym, 0);
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nocode_wanted = saved_nocode_wanted;
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c = tok;
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if (c == TOK_ELSE) {
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next();
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if (cond == 1)
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nocode_wanted |= 2;
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d = gjmp(0);
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gsym(a);
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block(bsym, csym, 0);
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gsym(d); /* patch else jmp */
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nocode_wanted = saved_nocode_wanted;
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} else
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gsym(a);
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} else if (tok == TOK_WHILE) {
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nocode_wanted &= ~2;
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next();
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d = ind;
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vla_sp_restore();
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@ -5564,6 +5573,7 @@ static void block(int *bsym, int *csym, int is_expr)
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skip(';');
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} else if (tok == TOK_FOR) {
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int e;
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nocode_wanted &= ~2;
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next();
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skip('(');
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s = local_stack;
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@ -5607,6 +5617,7 @@ static void block(int *bsym, int *csym, int is_expr)
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} else
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if (tok == TOK_DO) {
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nocode_wanted &= ~2;
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next();
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a = 0;
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b = 0;
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@ -5658,6 +5669,7 @@ static void block(int *bsym, int *csym, int is_expr)
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struct case_t *cr = tcc_malloc(sizeof(struct case_t));
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if (!cur_switch)
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expect("switch");
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nocode_wanted &= ~2;
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next();
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cr->v1 = cr->v2 = expr_const();
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if (gnu_ext && tok == TOK_DOTS) {
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@ -5735,6 +5747,7 @@ static void block(int *bsym, int *csym, int is_expr)
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vla_sp_restore();
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/* we accept this, but it is a mistake */
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block_after_label:
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nocode_wanted &= ~2;
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if (tok == '}') {
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tcc_warning("deprecated use of label at end of compound statement");
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} else {
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@ -88,6 +88,7 @@ void struct_test();
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void array_test();
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void expr_ptr_test();
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void bool_test();
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void optimize_out();
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void expr2_test();
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void constant_expr_test();
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void expr_cmp_test();
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@ -695,6 +696,7 @@ int main(int argc, char **argv)
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array_test();
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expr_ptr_test();
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bool_test();
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optimize_out();
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expr2_test();
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constant_expr_test();
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expr_cmp_test();
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@ -1165,6 +1167,31 @@ void bool_test()
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printf ("bits = 0x%x\n", calc_vm_flags (0x1));
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}
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extern int undefined_function(void);
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extern int defined_function(void);
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void optimize_out(void)
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{
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int i = 0 ? undefined_function() : defined_function();
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printf ("oo:%d\n", i);
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int j = 1 ? defined_function() : undefined_function();
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printf ("oo:%d\n", j);
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if (0)
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printf("oo:%d\n", undefined_function());
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else
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printf("oo:%d\n", defined_function());
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if (1)
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printf("oo:%d\n", defined_function());
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else
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printf("oo:%d\n", undefined_function());
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}
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int defined_function(void)
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{
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static int i = 40;
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return i++;
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}
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/* GCC accepts that */
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static int tab_reinit[];
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static int tab_reinit[10];
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122
tests/tests2/87_dead_code.c
Normal file
122
tests/tests2/87_dead_code.c
Normal file
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@ -0,0 +1,122 @@
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/* This checks various ways of dead code inside if statements
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where there are non-obvious ways of how the code is actually
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not dead due to reachable by labels. */
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extern int printf (const char *, ...);
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static void kb_wait_1(void)
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{
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unsigned long timeout = 2;
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do {
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/* Here the else arm is a statement expression that's supposed
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to be suppressed. The label inside the while would unsuppress
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code generation again if not handled correctly. And that
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would wreak havok to the cond-expression because there's no
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jump-around emitted, the whole statement expression really
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needs to not generate code (perhaps except useless forward jumps). */
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(1 ?
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printf("timeout=%ld\n", timeout) :
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({
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int i = 1;
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while (1)
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while (i--)
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some_label:
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printf("error\n");
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goto some_label;
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})
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);
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timeout--;
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} while (timeout);
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}
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int main (void)
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{
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int i = 1;
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kb_wait_1();
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/* Simple test of dead code at first sight which isn't actually dead. */
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if (0) {
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yeah:
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printf ("yeah\n");
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} else {
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printf ("boo\n");
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}
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if (i--)
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goto yeah;
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/* Some more non-obvious uses where the problems are loops, so that even
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the first loop statements aren't actually dead. */
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i = 1;
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if (0) {
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while (i--) {
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printf ("once\n");
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enterloop:
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printf ("twice\n");
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}
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}
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if (i >= 0)
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goto enterloop;
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/* The same with statement expressions. One might be tempted to
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handle them specially by counting if inside statement exprs and
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not unsuppressing code at loops at all then.
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See kb_wait_1 for the other side of the medal where that wouldn't work. */
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i = ({
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int j = 1;
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if (0) {
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while (j--) {
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printf ("SEonce\n");
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enterexprloop:
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printf ("SEtwice\n");
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}
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}
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if (j >= 0)
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goto enterexprloop;
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j; });
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/* The other two loop forms: */
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i = 1;
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if (0) {
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for (i = 1; i--;) {
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printf ("once2\n");
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enterloop2:
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printf ("twice2\n");
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}
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}
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if (i > 0)
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goto enterloop2;
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i = 1;
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if (0) {
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do {
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printf ("once3\n");
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enterloop3:
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printf ("twice3\n");
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} while (i--);
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}
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if (i > 0)
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goto enterloop3;
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/* And check that case and default labels have the same effect
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of disabling code suppression. */
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i = 41;
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switch (i) {
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if (0) {
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printf ("error\n");
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case 42:
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printf ("error2\n");
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case 41:
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printf ("caseok\n");
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}
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}
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i = 41;
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switch (i) {
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if (0) {
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printf ("error3\n");
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default:
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printf ("caseok2\n");
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break;
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case 42:
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printf ("error4\n");
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}
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}
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return 0;
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}
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18
tests/tests2/87_dead_code.expect
Normal file
18
tests/tests2/87_dead_code.expect
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@ -0,0 +1,18 @@
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timeout=2
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timeout=1
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boo
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yeah
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twice
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once
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twice
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SEtwice
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SEonce
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SEtwice
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twice2
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once2
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twice2
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twice3
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once3
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twice3
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caseok
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caseok2
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