[LLVMdev] SCEV bottom value

[Preston Briggs]

In my case, it indicates an unknown value. It's certainly a number of
some sort, but I can't prove anything about it (positive, negative,
size, etc). So I use this special indication to indicate I must make
worst-case assumptions. But, yes, I'd like it to work almost like a
floating-point NaN. The exception is that 0*magic should yield 0.

Preston


On Sun, Oct 7, 2012 at 10:51 PM, Sahasrabuddhe, Sameer
<Sameer.Sahasrabuddhe at amd.com> wrote:
> Hi Preston,
>
> I was wondering ... "Bottom" is a bit overloaded as far as terms go. Would SCEVNaN be a better name for this beast?
>
> Sameer.
>
>> -----Original Message-----
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
>> Behalf Of Sameer Sahasrabuddhe
>> Sent: Monday, October 08, 2012 9:16 AM
>> To: preston.briggs at gmail.com
>> Cc: LLVM Developers Mailing List
>> Subject: Re: [LLVMdev] SCEV bottom value
>>
>> On Sun, 7 Oct 2012 18:53:59 -0700
>> Preston Briggs <preston.briggs at gmail.com> wrote:
>>
>> > I'd like a value, call it Bottom, such that
>> >
>> > SE->getAddExpr(Bottom, X) => Bottom
>> > SE->getMulExpr(Bottom, X,) => Bottom
>> > isKnownPredicate(any, Bottom, X) => false
>> > etc.
>> >
>> >
>> > I can write code to make NULL work like I want, but it would be
>> > simpler if something was already defined. I'm wondering about
>> > SCEV::Unknown.  The documentation suggests I could perhaps use it for
>> > a "bottom" value.
>> >
>> > Think it would work?
>>
>> The documentation definitely says that SCEVUnknown is the "bottom"
>> value. But the semantics you have listed here are different from how
>> SCEVUnknown works. For example, B = A + Unknown is not Unknown, but it
>> is in fact an add expression. If B is an operand for another add
>> expression C, then it can be flattened so that A participates in any
>> folding happening for C. But instead, if B was the expression A +
>> Bottom, then B would become bottom, and so would C, and no folding will
>> ever be allowed.
>>
>> In some sense, Bottom is the anti-thesis to Unknown. Unknown provides
>> a boundary that encapsulates unknown semantics, while Bottom
>> invalidates anything it touches!
>>
>> Also, how would one uniquify Bottom expressions? Should they all be
>> considered identical?
>>
>> Sameer.
>>
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>
>