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opt: Start optimizing dead code a bit

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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
2016-09-19 18:38:12 +02:00
parent b303a00ce0
commit 31c7ea0165
4 changed files with 182 additions and 2 deletions
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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)
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(')');
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();
@ -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 {