llvm dev - Mar 2018 - how to simplify FP ops with an undef operand?

On 2/28/2018 5:46 PM, Chris Lattner wrote:
> On Feb 28, 2018, at 3:29 PM, Kaylor, Andrew via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>> For the first part of Sanjay’s question, I think the answer is, “Yes, 
>> we can fold all of these to NaN in the general case.”
>
> Agreed.  Those IR instructions are undefined on SNAN, and that undef 
> could take on an SNAN value.  Folding these instructions to undef 
> seems reasonable, and it is arguable that you could even fold it to an 
> ‘unreachable'.
fdiv snan, snan is undefined?  As opposed to producing a qnan, as 
specified by IEEE-754?

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

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> On Feb 28, 2018, at 6:33 PM, Friedman, Eli <efriedma at
codeaurora.org> wrote:
> 
> On 2/28/2018 5:46 PM, Chris Lattner wrote:
>> On Feb 28, 2018, at 3:29 PM, Kaylor, Andrew via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> For the first part of Sanjay’s question, I think the answer is,
“Yes, we can fold all of these to NaN in the general case.”
>> 
>> Agreed.  Those IR instructions are undefined on SNAN, and that undef
could take on an SNAN value.  Folding these instructions to undef seems
reasonable, and it is arguable that you could even fold it to an
‘unreachable'.
> 
> fdiv snan, snan is undefined?  As opposed to producing a qnan, as specified
by IEEE-754?

You’re talking about IEEE, I’m talking about LLVM IR.  LLVM IR is undefined on
SNaNs.  It looks like LangRef isn’t clear about this, the only mention of SNaNs
is in this statement:

	"fdiv is not (currently) defined on SNaN’s.”

However, fdiv/fmul/etc are pervasively treated as not having side effects.  The
intention, and the only sensible definition for them, is that they are undefined
on SNaNs.

-Chris


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So you don’t think sNaNs can just be treated as if they were qNaNs? I understand
why we would want to ignore the signaling part of things, but the rules for
operating on NaNs are pretty clear and reasonable to implement. The signaling
aspect can, I think, be safely ignored when we are in the mode of assuming the
default FP environment.

As for the distinction between IEEE and LLVM IR, I would think we would want to
define LLVM IR in such a way that it is possible to create and IEEE-compliant
compiler. I know we’re not there yet, but we’re working toward it.

From: Chris Lattner [mailto:clattner at nondot.org]
Sent: Wednesday, February 28, 2018 8:42 PM
To: Friedman, Eli <efriedma at codeaurora.org>
Cc: Kaylor, Andrew <andrew.kaylor at intel.com>; Sanjay Patel <spatel
at rotateright.com>; Matt Arsenault <arsenm2 at gmail.com>; llvm-dev
<llvm-dev at lists.llvm.org>; John Regehr <regehr at cs.utah.edu>
Subject: Re: [llvm-dev] how to simplify FP ops with an undef operand?




On Feb 28, 2018, at 6:33 PM, Friedman, Eli <efriedma at
codeaurora.org<mailto:efriedma at codeaurora.org>> wrote:

On 2/28/2018 5:46 PM, Chris Lattner wrote:
On Feb 28, 2018, at 3:29 PM, Kaylor, Andrew via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
For the first part of Sanjay’s question, I think the answer is, “Yes, we can
fold all of these to NaN in the general case.”

Agreed.  Those IR instructions are undefined on SNAN, and that undef could take
on an SNAN value.  Folding these instructions to undef seems reasonable, and it
is arguable that you could even fold it to an ‘unreachable'.

fdiv snan, snan is undefined?  As opposed to producing a qnan, as specified by
IEEE-754?

You’re talking about IEEE, I’m talking about LLVM IR.  LLVM IR is undefined on
SNaNs.  It looks like LangRef isn’t clear about this, the only mention of SNaNs
is in this statement:

            "fdiv is not (currently) defined on SNaN’s.”

However, fdiv/fmul/etc are pervasively treated as not having side effects.  The
intention, and the only sensible definition for them, is that they are undefined
on SNaNs.

-Chris


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Wait, back up, what? The invalid flag raised by operations on sNaN is no
different from any other flag in fenv. There’s nothing sensible about saying
that every operation that raises underflow/overflow/inexact is undefined, why
are operations on sNaN any different?

– Steve
> On Feb 28, 2018, at 11:41 PM, Chris Lattner via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> However, fdiv/fmul/etc are pervasively treated as not having side effects. 
The intention, and the only sensible definition for them, is that they are
undefined on SNaNs.
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