Re: [PATCH] A jump threading opportunity for condition branch

[Jiufu Guo <guojiufu@linux.ibm.com>]

Richard Biener <rguenther@suse.de> writes:

> On May 29, 2019 10:21:46 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>>On 5/24/19 6:45 AM, Richard Biener wrote:
>>[ Aggressive snipping ]
>>
>>> As said in my first review I'd just check whether for the
>>> edge we want to thread through the definition comes from a CMP.
>>> Suppose you have
>>> 
>>>  # val_1 = PHI <a_2, b_3, c_4>
>>>  if (val_1 != 0)
>>> 
>>> and only one edge has a b_3 = d_5 != 0 condition it's still
>>> worth tail-duplicating the if block.
>>Agreed.  The cost of tail duplicating here is so small we should be
>>doing it highly aggressively.  About the only case where we might not
>>want to would be if we're optimizing for size rather than speed.  That
>>case isn't clearly a win either way.
>
> Even there the PHI likely causes edge copies to be inserted. So I
> wouldn't care for the moment. The proper check would be !
> Optimize_edge_for_size_p (e).
For most of this kind of case where the bb contains just one conditional
jump stmt, it may not increase the size especially for there are
combinings in follow passes -- it may save size ;)

>
> Richard. 
>
>>jeff