Re: RFA: New pass to delete unexecutable paths in the CFG

[Kai Tietz <ktietz70 at googlemail dot com>]

2011/11/7 Richard Guenther <richard.guenther@gmail.com>:
> On Mon, Nov 7, 2011 at 5:07 PM, Jeff Law <law@redhat.com> wrote:
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>>
>>>
>>> I also wonder if instead of a new pass control-dependent DCE can be
>>> taught of this?
>> It could. ? I'm not sure it really buys us anything except maybe being
>> able to reuse the pdom & control-dependency analysis.
>>
>> It actually more naturally belongs in cprop because it's cprop that
>> exposes NULL pointers in useful ways (in memory dereferences and as
>> PHI args).
>
> Heh, indeed, that as well. ?I suppose every pass that handles
> unexecutable edges in some way might benefit from this. ?Like for
>
> ?if (i)
> ? j_1 = 1;
> ?else
> ? {
> ? ?j_2 = 2;
> ? ?*0 = ...;
> ? }
> ?j_3 = PHI <j_1, j_2>
>
> we could CCP j_1 = 1 to j_3 ...
>
> Even without actually removing the edge and the trapping stmt.
>
> For DCE the most interesting part would be to end the block at
> the point of ?the trap - thus - replace the store with a __builtin_trap ()
> which then trivially makes all code w/o side-effects on the path
> dead.
>
> Richard.

Well, easiest sample to see effect of if-joining is on patterns like
this.  As here neither PHI-opt nor tail-merge helps pretty much, as
condition chain doesn't get reduced to simple form.  The sample above
seems to be too less explicit to get the point.

Eg:

int doo (void);

int foo (unsigned int a, unsigned int b)
{
  if (a == 0) rerturn doo ();
  if (b == 0) return doo ();
  if (a == 1) return doo ();
  if (b == 1) return doo ();
  if (a == 2) return doo ();
  if (b == 2) return doo ();
  return -2;
}

This sample would be the same (in associative form) as

int doo (void);

int foo (unsigned int a, unsigned int b)
{
  if (a == 0 || b == 0 || a == 1 || b == 1 || a == 2 || b == 2) rerturn doo ();
  return -2;
}

(btw both samples won't be folded for IA due current BC).

With the first patch doing the transformation to associative
truth-bitwise for all simple operands, the second example will be
folded by reassoc-pass to
...
if (a <= 2 || b <=2) return doo ();
return -2;
...

With the if-joining also the first example gets optimized to this simple form.

Regards,
Kai