[llvm-dev] PHI nodes and connected ICMp

[Anastasiya Ruzhanskaya via llvm-dev]

Maybe you, as a person connected with this theme, have some
recommendations? Analyzing SCEV didn't help me, as I am not able to extract
information, that is crucial for me.

2017-08-10 9:55 GMT+02:00 Anastasiya Ruzhanskaya <
anastasiya.ruzhanskaya at frtk.ru>:

> Hi!
> By only two cases I mean , that in exiting block when computing the
> condition related to PHI node I can expect only icmp on one of incoming
> values or on phi node itself... I tried to come up with some more complex
> examples but I always receive only these two cases, that is why I am asking.
>
> This problem still relates to the problem of all induction, cumulative and
> so on variables in loop. SCEV didn't help me, as it doesn't provide me with
> the value against which I am comparing when exiting, and when it cannot
> detect loop trip count directly I need this "exiting value". That is why I
> am searching for all compare instructions in exiting blocks that are using
> either "phi" itself or "incoming value".
>
> 2017-08-10 9:47 GMT+02:00 Sanjoy Das <sanjoy at google.com>:
>
>> It may be helpful to see the incoming value as the incoming value of
>> the "next" PHI (i.e. the instance of the PHI node on the next
>> iteration).  With that interpretation in mind, you can see that
>> comparing the PHI node itself is equivalent to:
>>
>> do {
>>   ...
>>   // I is the value that becomes the PHI node
>> } while (I++ < N);
>>
>> and comparing with the incoming value of the PHI is equivalent to:
>>
>> do {
>>   ...
>> } while (++I < N);
>>
>>
>> I can't think of any fundamental reason why one would be preferred for
>> increasing induction variables and the other would be preferred for
>> decreasing induction variables.  I suspect what you're seeing is just
>> an artifact of how clang lowers these loops.
>>
>> I'm also not sure what you mean by "I can have only two cases".   Loop
>> backedge conditions can be arbitrarily complex, if that's what you're
>> asking.
>>
>> -- Sanjoy
>>
>>
>> On Thu, Aug 10, 2017 at 12:34 AM, Anastasiya Ruzhanskaya via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > Hello,
>> > I have one more question about how phi nodes and their corresponding
>> ICmp
>> > instructions are associated. maybe it is simple, but at first I thought
>> that
>> > we always compare against one of incoming value. Is it true that  I can
>> have
>> > only  two cases:
>> > %indvars.iv = phi i64 [ %indvars.iv.next, %1 ], [ 0, %0 ]
>> > ...
>> > %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>> > %exitcond = icmp eq i64 %indvars.iv.next, 32
>> >
>> >
>> > and
>> >
>> > %i.11 = phi i32 [ %i.11.be, %.backedge ], [ 32, %1 ]
>> > ...
>> > %13 = icmp sgt i32 %i.11, 3
>> > ?
>> > In the first one we always have icmp on the incoming value after
>> addition,
>> > multiplication and so on.
>> > In the second - we compare at first against our phi variable and then
>> > perform operations. I have noticed, that the first case correspond to
>> "up"
>> > operations - +=, *= ans do on, the second - to "down" : -=, /= and so
>> on.
>> > But maybe it depends on logic of the cycle too... So, are their two
>> cases :
>> > comparing in exiting block against PHI variable or against one of its'
>> > incoming value?
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>>
>
>
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