On Wed, Mar 9, 2011 at 8:29 AM, Richard Guenther
<richard.guent...@gmail.com> wrote:
> On Wed, Mar 9, 2011 at 5:24 PM, Xinliang David Li <davi...@google.com> wrote:
>> On Wed, Mar 9, 2011 at 6:03 AM, Jeff Law <l...@redhat.com> wrote:
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>>> On 03/09/11 02:45, Richard Guenther wrote:
>>>> On Tue, Mar 8, 2011 at 11:04 PM, Jeff Law <l...@redhat.com> wrote:
>>>
>>>> True. I've been repeatedly thinking of building some on-the-side CFG
>>>> with value-numbered predicates to also catch the CFG vs. scalar-code
>>>> predicate combinations we have. On such on-the-side data structure
>>>> we could do very aggressive jump-threading just for analysis purposes
>>>> (experiments when working on separating conditions shows that
>>>> a PRE-like algorithm could drive this).
>>> I'm pondering the same kind of thing. PRE on the predicates with a more
>>> structured approach to block copying to isolate the paths seems to be
>>> the way to go.
>>
>>
>> Any simple examples to show how your idea would work?
>
> You basically would copy all paths with redundant predicate computations,
> removing them and leaving no redundant checks around. Of course you
> wouldn't really do this code-transformation but just do it to help analyses.
You will also need to compute SSA on the alternate CFG as well, right?
How will partially redundant predicates handled? Split the predicates?
if (p1 OR p2)
foo();
bar();
if (p1)
baz();
==>
if (p1)
{
foo(); bar(); baz();
}
else if (p2)
{
foo(); bar();
}
else
bar();
Things can be complicated when dealing with slightly complex
predicates. For instance
p1 = x | y; // bitwise OR
p2_1 = (x!=0);
p2_2 = (y!=0);
p2 = (p2_1 & p2_2);
p1 can be split into (x AND !y), p2 , and (!x AND y).
How about short circuite &&, || expressions lowered into if-then-else?
Will the side CFG collapse the expanded branches and expose the
'original' predicates that are easier to analyze?
Thanks,
David
>
> Like in
>
> if (p)
> foo();
> bar();
> if (p)
> baz();
>
> you'd generate
>
> if(p)
> {
> foo();
> bar();
> baz();
> }
> else
> bar();
>
> for all, even partially redundant predicates. See the patch I linked to, it
> causes PRE to identify them (but do nothing too useful yet)
>
> Richard.
>
>> Thanks,
>>
>> David
>>>
>>> jeff
>>>
>>>
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>>>
>>
>
