From: Jeff Law <law@redhat.com>
To: Richard Biener <rguenther@suse.de>
Cc: Jiufu Guo <guojiufu@linux.ibm.com>,
gcc-patches@gcc.gnu.org, Jakub Jelinek <jakub@redhat.com>,
Daniel Berlin <dberlin@dberlin.org>,
segher@kernel.crashing.org, wschmidt@linux.ibm.com
Subject: Re: [PATCH] A jump threading opportunity for condition branch
Date: Fri, 31 May 2019 15:28:00 -0000 [thread overview]
Message-ID: <47b8d7f9-0039-593e-0813-7dc5e58acfe9@redhat.com> (raw)
In-Reply-To: <alpine.LSU.2.20.1905310923170.10704@zhemvz.fhfr.qr>
On 5/31/19 1:24 AM, Richard Biener wrote:
> On Thu, 30 May 2019, Jeff Law wrote:
>
>> On 5/30/19 12:41 AM, Richard Biener wrote:
>>> On May 29, 2019 10:18:01 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>>>> On 5/23/19 6:11 AM, Richard Biener wrote:
>>>>> On Thu, 23 May 2019, Jiufu Guo wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> Richard Biener <rguenther@suse.de> writes:
>>>>>>
>>>>>>> On Tue, 21 May 2019, Jiufu Guo wrote:
>>>>
>>>>>>>> + }
>>>>>>>> +
>>>>>>>> + if (TREE_CODE_CLASS (gimple_assign_rhs_code (def)) !=
>>>> tcc_comparison)
>>>>>>>> + return false;
>>>>>>>> +
>>>>>>>> + /* Check if phi's incoming value is defined in the incoming
>>>> basic_block. */
>>>>>>>> + edge e = gimple_phi_arg_edge (phi, index);
>>>>>>>> + if (def->bb != e->src)
>>>>>>>> + return false;
>>>>>>> why does this matter?
>>>>>>>
>>>>>> Through preparing pathes and duplicating block, this transform can
>>>> also
>>>>>> help to combine a cmp in previous block and a gcond in current
>>>> block.
>>>>>> "if (def->bb != e->src)" make sure the cmp is define in the incoming
>>>>>> block of the current; and then combining "cmp with gcond" is safe.
>>>> If
>>>>>> the cmp is defined far from the incoming block, it would be hard to
>>>>>> achieve the combining, and the transform may not needed.
>>>>> We're in SSA form so the "combining" doesn't really care where the
>>>>> definition comes from.
>>>> Combining doesn't care, but we need to make sure the copy of the
>>>> conditional ends up in the right block since it wouldn't necessarily be
>>>> associated with def->bb anymore. But I'd expect the sinking pass to
>>>> make this a non-issue in practice anyway.
>>>>
>>>>>
>>>>>>>> +
>>>>>>>> + if (!single_succ_p (def->bb))
>>>>>>>> + return false;
>>>>>>> Or this? The actual threading will ensure this will hold true.
>>>>>>>
>>>>>> Yes, other thread code check this and ensure it to be true, like
>>>>>> function thread_through_normal_block. Since this new function is
>>>> invoked
>>>>>> outside thread_through_normal_block, so, checking single_succ_p is
>>>> also
>>>>>> needed for this case.
>>>>> I mean threading will isolate the path making this trivially true.
>>>>> It's also no requirement for combining, in fact due to the single-use
>>>>> check the definition can be sinked across the edge already (if
>>>>> the edges dest didn't have multiple predecessors which this threading
>>>>> will fix as well).
>>>> I don't think so. The CMP source block could end with a call and have
>>>> an abnormal edge (for example). We can't put the copied conditional
>>>> before the call and putting it after the call essentially means
>>>> creating
>>>> a new block.
>>>>
>>>> The CMP source block could also end with a conditional. Where do we
>>>> put
>>>> the one we want to copy into the CMP source block in that case? :-)
>>>>
>>>> This is something else we'd want to check if we ever allowed the the
>>>> CMP
>>>> defining block to not be the immediate predecessor of the conditional
>>>> jump block. If we did that we'd need to validate that the block where
>>>> we're going to insert the copy of the jump has a single successor.
>>>
>>> But were just isolating a path here. The actual combine job is left to followup cleanups.
>> Absolutely agreed. My point was that there's some additional stuff we'd
>> have to verify does the right thing if we wanted to allow the CMP to be
>> somewhere other than in the immediate predecessor of the conditional
>> jump block.
>
> For correctness? No. For the CMP to be forwarded? No. For optimality
> maybe - forwarding a binary operation always incurs register pressure
> increase.
For correctness of the patch. Conceptually I have _no_ issues with
having the CMP in a different block than an immediate predecessor of the
conditional jump block. But the patch does certain code which would
need to be audited with that change in mind.
>
> Btw, as you already said sinking should have sinked the CMP to the
> predecessor (since we have a single use in the PHI).
>
> So I hardly see the point of making this difference.
:-)
jeff
next prev parent reply other threads:[~2019-05-31 15:03 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-21 13:45 Jiufu Guo
2019-05-22 12:38 ` Richard Biener
2019-05-23 12:06 ` Jiufu Guo
2019-05-23 12:11 ` Richard Biener
2019-05-23 14:40 ` Jiufu Guo
2019-05-24 12:45 ` Richard Biener
2019-05-24 14:52 ` Jiufu Guo
2019-05-28 14:07 ` [PATCH V2] " Jiufu Guo
2019-05-29 1:51 ` Jiufu Guo
2019-05-29 12:40 ` Richard Biener
2019-05-29 19:47 ` Jeff Law
2019-05-30 15:09 ` Jiufu Guo
2019-05-30 23:55 ` Jeff Law
2019-05-31 7:34 ` Richard Biener
2019-06-04 3:03 ` Jiufu Guo
2019-05-30 15:34 ` Jeff Law
2019-06-03 2:18 ` [PATCH V3] " Jiufu Guo
2019-06-04 5:30 ` [PATCH V4] " Jiufu Guo
2019-06-13 18:56 ` Jeff Law
2019-06-14 12:51 ` Jiufu Guo
2019-06-14 16:34 ` Jeff Law
2019-05-29 20:26 ` [PATCH] " Jeff Law
2019-05-30 6:57 ` Richard Biener
2019-05-30 6:58 ` Jiufu Guo
2019-05-30 14:59 ` Jeff Law
2019-05-30 15:03 ` Jeff Law
2019-05-29 20:22 ` Jeff Law
2019-05-30 6:40 ` Jiufu Guo
2019-05-30 6:44 ` Richard Biener
2019-05-30 20:17 ` Jeff Law
2019-05-31 7:30 ` Richard Biener
2019-05-31 15:28 ` Jeff Law [this message]
2019-06-04 5:19 ` Jiufu Guo
2019-06-04 7:07 ` Richard Biener
2019-06-07 0:05 ` Jeff Law
2019-05-29 20:18 ` Jeff Law
2019-05-30 6:41 ` Richard Biener
2019-05-29 20:12 ` Jeff Law
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