On Thu, Jul 5, 2012 at 8:45 PM, Tom de Vries <tom_devr...@mentor.com> wrote:
> On 05/07/12 15:30, Michael Matz wrote:
>> Hi,
>>
>> On Thu, 5 Jul 2012, Tom de Vries wrote:
>>
>>> The asserts allow the return result to be optimized, but not the cfg
>>> conditions.
>>>
>>> AFAIU, we can insert the asserts earlier. F.i., we can insert
>>> aD.1711_6 = ASSERT_EXPR <aD.1711_1(D), aD.1711_1(D) > 0>
>>> before the GIMPLE_COND in bb2.
>>
>> Nope. That would require some more checks, in particular that the BB
>> containing builtin_unreachable doesn't contain any other side-effects.
>> Given this:
>>
>> if (i < 0)
>> { do_something_interesting();
>> __builtin_unreachable();
>> }
>>
>> moving the assert before the if would remove the if condition, hence
>> the call to do_something_interesting. You need to retain side-effects if
>> there are any.
>>
>
> Michael,
>
> Thanks for pointing that out.
>
> I tried a first stab at your suggestion of implementing the optimization in
> pass_fold_builtins, it works for the test-case.
+static bool
+optimize_unreachable (gimple_stmt_iterator i)
+{
+ gimple stmt;
+ basic_block bb;
+ edge_iterator ei;
+ edge e;
+
+ for (gsi_prev (&i); !gsi_end_p (i); gsi_prev (&i))
+ {
+ stmt = gsi_stmt (i);
+ if (gimple_has_side_effects (stmt))
+ return false;
+ }
I think we should rely on DCE to remove stmts without side-effects before
__builtin_unreachable. Thus I'd make this
basic_block bb = gsi_bb (i);
for (gsi = gsi_start (bb); !gsi_end_p (i); gsi_next (&gsi))
{
if (is_gimple_debug ())
continue;
if (gimple_code () == GIMPLE_LABEL)
/* Verify we do not need to preserve the label. */;
if (gsi_stmt () != gsi_stmt (i))
return false;
}
...
thus simply require the builtin be the first statement in the block.
As for the label I'm concerned about
void foo (int b, int c)
{
void *x = &&lab;
if (b)
{
lab:
__builtin_unreachable ();
}
lab2:
if (c)
x = &&lab2;
goto *x;
}
non-sensical, of course, but "valid".
Richard.
> Thanks,
> - Tom
>
>>
>> Ciao,
>> Michael.
>>
>
>
