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

Yes, if %x is a NaN, we should expect that NaN is propagated.

I'm still not sure what to do here. We can take comfort in knowing that
whatever we do is likely an improvement over the current situation though.
:)
That's because the code in InstSimplify is inconsistent with the LangRef:
http://llvm.org/docs/LangRef.html#undefined-values (UB for fdiv by 0?)
...and both of those are inconsistent with undef handling in SDAG.

Let me propose an alternate interpretation:

1. The meaning of snan as written in IEEE754-2008 is: "Signaling NaNs
afford representations for uninitialized variables..."
2. That matches our intent with 'undef' here in IR as written in the
LangRef: "unspecified bit-pattern".
3. The current fdiv transform is actually correct (any SNaN UB/trapping
commentary is irrelevant because we assume exceptions are off by default).

The undef operand represents an uninitialized variable, and the result of
any FP op with that uninitialized variable is well-defined: it's another
NaN which is just 'undef' in IR.





On Wed, Feb 28, 2018 at 11:43 AM, Kaylor, Andrew <andrew.kaylor at
intel.com>
wrote:
> I’m not sure the transformation happening with fdiv is correct. If we have
> “%y = fdiv float %x, undef” and %x is a NaN then the result will be NaN for
> any value of the undef, right? So if I understand the undef rules correctly
> (never a certainty) then we can’t safely replace the expression with undef.
> We could, I think, replace it with “%y = %x” though. I think the same is
> true for fadd, fsub, fmul, and frem.
>
>
>
> -Andy
>
>
>
>
> %y = fadd float %x, undef
>
>
>
> Can we simplify this?
>
> Currently in IR, we do nothing for fadd/fsub/fmul. For fdiv/frem, we
> propagate undef. The code comment for fdiv/frem says:
> "the undef could be a snan"
>
>
>
> If that's correct, then shouldn't it be the same for
fadd/fsub/fmul? But
> this can't be correct because we support targets that don't raise
> exceptions...and even targets that raise exceptions do not trap by default
> on snan?
>
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Why is NaN “just ‘undef’ in IR”? NaN is a specific value with well-defined
behavior. I would think that unless the no-NaNs flag is used we need to preserve
the behavior of NaNs.

From: Sanjay Patel [mailto:spatel at rotateright.com]
Sent: Wednesday, February 28, 2018 12:08 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Nuno Lopes <nunoplopes at
sapo.pt>; Stephen Canon <scanon at apple.com>; David Majnemer
<david.majnemer at gmail.com>; John Regehr <regehr at cs.utah.edu>;
Sanjoy Das <sanjoy at playingwithpointers.com>; Friedman, Eli <efriedma
at codeaurora.org>; Matt Arsenault <arsenm2 at gmail.com>
Subject: Re: how to simplify FP ops with an undef operand?

Yes, if %x is a NaN, we should expect that NaN is propagated.

I'm still not sure what to do here. We can take comfort in knowing that
whatever we do is likely an improvement over the current situation though. :)
That's because the code in InstSimplify is inconsistent with the LangRef:
http://llvm.org/docs/LangRef.html#undefined-values (UB for fdiv by 0?)
...and both of those are inconsistent with undef handling in SDAG.
Let me propose an alternate interpretation:

1. The meaning of snan as written in IEEE754-2008 is: "Signaling NaNs
afford representations for uninitialized variables..."
2. That matches our intent with 'undef' here in IR as written in the
LangRef: "unspecified bit-pattern".
3. The current fdiv transform is actually correct (any SNaN UB/trapping
commentary is irrelevant because we assume exceptions are off by default).

The undef operand represents an uninitialized variable, and the result of any FP
op with that uninitialized variable is well-defined: it's another NaN which
is just 'undef' in IR.





On Wed, Feb 28, 2018 at 11:43 AM, Kaylor, Andrew <andrew.kaylor at
intel.com<mailto:andrew.kaylor at intel.com>> wrote:
I’m not sure the transformation happening with fdiv is correct. If we have “%y =
fdiv float %x, undef” and %x is a NaN then the result will be NaN for any value
of the undef, right? So if I understand the undef rules correctly (never a
certainty) then we can’t safely replace the expression with undef. We could, I
think, replace it with “%y = %x” though. I think the same is true for fadd,
fsub, fmul, and frem.

-Andy


%y = fadd float %x, undef

Can we simplify this?

Currently in IR, we do nothing for fadd/fsub/fmul. For fdiv/frem, we propagate
undef. The code comment for fdiv/frem says:
"the undef could be a snan"

If that's correct, then shouldn't it be the same for fadd/fsub/fmul? But
this can't be correct because we support targets that don't raise
exceptions...and even targets that raise exceptions do not trap by default on
snan?

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Correct - NaN is not undef in IR. But we don't have a NaN in this example.
We have its moral equivalent in LLVM - an uninitialized value, undef.

So we're not introducing any extra uncertainty by propagating the undef.
The backend can choose whatever encoding of undef makes sense when lowering?

And yes, I don't know why FP-div-by-zero would ever be UB. I think that
text in the LangRef should be removed regardless of any other outcome here.

On Wed, Feb 28, 2018 at 1:18 PM, Kaylor, Andrew <andrew.kaylor at
intel.com>
wrote:
> Why is NaN “just ‘undef’ in IR”? NaN is a specific value with well-defined
> behavior. I would think that unless the no-NaNs flag is used we need to
> preserve the behavior of NaNs.
>
>
>
> *From:* Sanjay Patel [mailto:spatel at rotateright.com]
> *Sent:* Wednesday, February 28, 2018 12:08 PM
> *To:* Kaylor, Andrew <andrew.kaylor at intel.com>
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>; Nuno Lopes
<nunoplopes at sapo.pt>;
> Stephen Canon <scanon at apple.com>; David Majnemer
<david.majnemer at gmail.com>;
> John Regehr <regehr at cs.utah.edu>; Sanjoy Das <sanjoy at
playingwithpointers.
> com>; Friedman, Eli <efriedma at codeaurora.org>; Matt Arsenault
<
> arsenm2 at gmail.com>
> *Subject:* Re: how to simplify FP ops with an undef operand?
>
>
>
> Yes, if %x is a NaN, we should expect that NaN is propagated.
>
> I'm still not sure what to do here. We can take comfort in knowing that
> whatever we do is likely an improvement over the current situation though.
> :)
>
> That's because the code in InstSimplify is inconsistent with the
LangRef:
> http://llvm.org/docs/LangRef.html#undefined-values (UB for fdiv by 0?)
>
> ...and both of those are inconsistent with undef handling in SDAG.
>
> Let me propose an alternate interpretation:
>
> 1. The meaning of snan as written in IEEE754-2008 is: "Signaling NaNs
> afford representations for uninitialized variables..."
> 2. That matches our intent with 'undef' here in IR as written in
the
> LangRef: "unspecified bit-pattern".
> 3. The current fdiv transform is actually correct (any SNaN UB/trapping
> commentary is irrelevant because we assume exceptions are off by default).
>
> The undef operand represents an uninitialized variable, and the result of
> any FP op with that uninitialized variable is well-defined: it's
another
> NaN which is just 'undef' in IR.
>
>
>
>
>
>
>
>
>
>
>
> On Wed, Feb 28, 2018 at 11:43 AM, Kaylor, Andrew <andrew.kaylor at
intel.com>
> wrote:
>
> I’m not sure the transformation happening with fdiv is correct. If we have
> “%y = fdiv float %x, undef” and %x is a NaN then the result will be NaN for
> any value of the undef, right? So if I understand the undef rules correctly
> (never a certainty) then we can’t safely replace the expression with undef.
> We could, I think, replace it with “%y = %x” though. I think the same is
> true for fadd, fsub, fmul, and frem.
>
>
>
> -Andy
>
>
>
>
>
> %y = fadd float %x, undef
>
>
>
> Can we simplify this?
>
> Currently in IR, we do nothing for fadd/fsub/fmul. For fdiv/frem, we
> propagate undef. The code comment for fdiv/frem says:
> "the undef could be a snan"
>
>
>
> If that's correct, then shouldn't it be the same for
fadd/fsub/fmul? But
> this can't be correct because we support targets that don't raise
> exceptions...and even targets that raise exceptions do not trap by default
> on snan?
>
>
>
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