[LLVMdev] Float undef value propagation

[Kuperstein, Michael M]

For every non-NaN, non-Inf constant C, C + (-C) = 0, right? 
So I guess you could also take undef = -C, and get 0.

I'm not sure whether you can do the same for multiplication (taking undef = 1/C and getting 1), though.

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Philip Reames
Sent: Monday, December 16, 2013 03:12
To: Dan Gohman; Owen Anderson
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Float undef value propagation

On 12/14/2013 05:18 PM, Dan Gohman wrote:
> On Thu, Dec 12, 2013 at 5:43 PM, Owen Anderson <resistor at mac.com 
> <mailto:resistor at mac.com>> wrote:
>
>
>     On Dec 12, 2013, at 4:57 PM, Philip Reames
>     <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>
>>     undef + any == NaN (since undef can be NaN) or undef + any (since
>>     undef could be zero)
>
>     undef + non-NaN is still undef.  The compiler is free to choose any
>     value of the type it wishes when simplifying an undef expression.
>       The important point is that it still has to be a value /of that
>     type. /Hence, predicates that are true for /any/ choice of value
>     must still be respected.  This is the case for NaN + undef == NaN:
>     while the compiler is free to choose any value for the undef, there
>     is no such value for which the result is not NaN.
>
>
> undef + non-NaN is not undef. This is because while you can pick any 
> value for the original undef, if you leave an undef behind, you can't 
> control what someone else might pick for it. There are floating-point 
> values which cannot be the result of undef + non-NaN for many values 
> of non-NaN, so the result of undef + non-Nan is not an unconstrained undef.
This is a very good point.  Does this mean that the only valid transformations of "undef + Constant" are "Constant" (by undef == 0) or "NaN" (by undef == NaN)?  Or are there known easily describible subsets of Constants which allow a more general transform?

As an example, when Constant == 0, it would appear that propagating the undef is fine.  (I think.)  Are there other cases like this?  Or am I wrong about the legality of this example?

Philip
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