[LLVMdev] Float undef value propagation

[Raoux, Thomas F]

Well in IEEE-754 Nan + any value returns Nan, so doing Nan + undef -> undef is wrong.

I agree that undef are usually the results of a bad application behavior I'm just wondering what is the right thing to do.

Thomas

-----Original Message-----
From: Tim Northover [mailto:t.p.northover at gmail.com] 
Sent: Wednesday, December 11, 2013 12:15 PM
To: Raoux, Thomas F
Cc: Philip Reames; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Float undef value propagation

On 11 December 2013 20:08, Raoux, Thomas F <thomas.f.raoux at intel.com> wrote:
> You are right some cases would definitely not be right like undef + 
> Nan -> undef. For 2.0f case I'm not sure either if any bits could be known.

Do you have an example where that's unsafe? You usually have to do something pretty bad to get an undef in the first place. Since
IEEE-754 doesn't have that concept I can't see how it could have anything to say on how LLVM optimises it.

Cheers.

Tim.