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tcg: Add reachable_code_pass

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Delete trivially dead code that follows unconditional branches and
noreturn helpers.  These can occur either via optimization or via
the structure of a target's translator following an exception.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2018-11-26 14:28:28 -08:00
parent d88a117eaa
commit b4fc67c7af
1 changed files with 76 additions and 0 deletions
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76
tcg/tcg.c
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@ -2239,6 +2239,81 @@ TCGOp *tcg_op_insert_after(TCGContext *s, TCGOp *old_op, TCGOpcode opc)
return new_op;
}
/* Reachable analysis : remove unreachable code. */
static void reachable_code_pass(TCGContext *s)
{
TCGOp *op, *op_next;
bool dead = false;
QTAILQ_FOREACH_SAFE(op, &s->ops, link, op_next) {
bool remove = dead;
TCGLabel *label;
int call_flags;
switch (op->opc) {
case INDEX_op_set_label:
label = arg_label(op->args[0]);
if (label->refs == 0) {
/*
* While there is an occasional backward branch, virtually
* all branches generated by the translators are forward.
* Which means that generally we will have already removed
* all references to the label that will be, and there is
* little to be gained by iterating.
*/
remove = true;
} else {
/* Once we see a label, insns become live again. */
dead = false;
remove = false;
/*
* Optimization can fold conditional branches to unconditional.
* If we find a label with one reference which is preceded by
* an unconditional branch to it, remove both. This needed to
* wait until the dead code in between them was removed.
*/
if (label->refs == 1) {
TCGOp *op_prev = QTAILQ_PREV(op, TCGOpHead, link);
if (op_prev->opc == INDEX_op_br &&
label == arg_label(op_prev->args[0])) {
tcg_op_remove(s, op_prev);
remove = true;
}
}
}
break;
case INDEX_op_br:
case INDEX_op_exit_tb:
case INDEX_op_goto_ptr:
/* Unconditional branches; everything following is dead. */
dead = true;
break;
case INDEX_op_call:
/* Notice noreturn helper calls, raising exceptions. */
call_flags = op->args[TCGOP_CALLO(op) + TCGOP_CALLI(op) + 1];
if (call_flags & TCG_CALL_NO_RETURN) {
dead = true;
}
break;
case INDEX_op_insn_start:
/* Never remove -- we need to keep these for unwind. */
remove = false;
break;
default:
break;
}
if (remove) {
tcg_op_remove(s, op);
}
}
}
#define TS_DEAD 1
#define TS_MEM 2
@ -3515,6 +3590,7 @@ int tcg_gen_code(TCGContext *s, TranslationBlock *tb)
atomic_set(&prof->la_time, prof->la_time - profile_getclock());
#endif
reachable_code_pass(s);
liveness_pass_1(s);
if (s->nb_indirects > 0) {