[llvm-dev] how to simplify FP ops with an undef operand?

Thu Mar 1 10:26:23 PST 2018

On Thu, Mar 1, 2018 at 2:08 AM, Nuno Lopes <nunoplopes at sapo.pt> wrote:

> We can do "add %x, undef" => "undef" because for any value of %x, we can
> always find a value that when added to %x produces any value in the domain
> of integers.
>
> This is not the case with floats since with some inputs, e.g., NaNs, we
> are not able to produce some values in the domain (e.g., there's no value
> of %x that makes "fadd NaN, %x" return 42.0).
>
> In summary, since there's no NaN constant in the IR

Isn't "float 0x7FF8000000000000" a NaN constant?

> , all we can do is to canonicalize to some instruction that yields NaN
> (and treat that as a NaN constant throughout the pipeline, I guess).
>
> Nuno
>
> P.S.: With poison you can always fold operations with a poison input to
> poison (e.g., "fadd %x, poison" => poison).
>
>
> -----Original Message----- From: Kaylor, Andrew
> Sent: Wednesday, February 28, 2018 11:29 PM
> Subject: RE: how to simplify FP ops with an undef operand?
>
>
>
>
> 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.” For the second part,
> which the nnan FMF is present, I’m not sure. The particulars of the
> semantics of nnan are unclear to me.
>
>
>
> But let me explore what Eli is saying. It sounds reasonable, but I have a
> question about it.
>
>
>
> Suppose we have the nnan FMF set, and we encounter this:
>
>
>
> %y = fdiv float %x, undef
>
>
>
> If I’ve understood Eli’s interpretation correctly, any of the following
> transformations would be legal and safe:
>
>
>
> %y = 0.0
>
> %y = -1.0
>
> %y = inf
>
> %y = NaN
>
>
>
> And so on, covering all possible concrete values, right? Now suppose I
> don’t change it at all. Now I might have IR that looks like this.
>
>
>
> %y = fdiv float %x, undef
>
> %z = fmul float %q, %y
>
>
>
> At this point, while working on %z, I could reconsider and say “If I had
> transformed the fdiv into ‘%y = 0.0’ I could optimize this fmul away too.”
> So at that point I can choose to do that, right? And in general I can
> “retroactively” choose any concrete value that would be convenient for the
> next transformation. You see where I’m going with this?
>
>
>
> How is that different from just folding the fdiv into undef to begin with?
> Is it because I can’t choose different values on different code paths?
>
>
>
> -Andy
>
>
>
>
>
>
>
> From: Friedman, Eli [mailto:efriedma at codeaurora.org]
> Sent: Wednesday, February 28, 2018 3:07 PM
> To: Sanjay Patel <spatel at rotateright.com>; 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>; Matt Arsenault <arsenm2 at gmail.com>;
> Kreitzer, David L <david.l.kreitzer at intel.com>
> Subject: Re: how to simplify FP ops with an undef operand?
>
>
>
>
>
>
>
> I'm pretty sure that isn't what nnan is supposed to mean. If the result of
> nnan math were undefined in the sense of "undef", programs using nnan could
> have undefined behavior if the result is used in certain ways which would
> not be undefined for any actual float value (e.g. converting the result to
> a string), which seems like a surprising result.  And I don't think we gain
> any useful optimization power from saying we can fold to undef instead of
> something else.
>
> So I think it's supposed to say "the result is not specified" or something
> (so an nnan operation which would produce a nan can instead produce any
> value that isn't undef/poison).
>
> -Eli
>
> On 2/28/2018 2:45 PM, Sanjay Patel wrote:
>
>
>
> Ah, thanks for explaining. So given that any of these ops will return NaN
> with a NaN operand, let's choose the undef operand value to be NaN. That
> means we can fold all of these to a NaN constant in the general case.
>
> But if we have 'nnan' FMF, then we can fold harder to undef?
> nnan - Allow optimizations to assume the arguments and result are not NaN.
> Such optimizations are required to retain defined behavior over NaNs, but
> the value of the result is undefined.
>
>
>
>
>
>
>
> On Wed, Feb 28, 2018 at 3:25 PM, Kaylor, Andrew <andrew.kaylor at intel.com>
> wrote:
>
>
>
> What I’m saying is that if we have one operand that is not an undef value
> then that operand might be NaN and if it is then the result must be NaN. So
> while it may be true that we don’t have a NaN, it is not true that we
> definitely do not have a NaN in the example. This is analogous to the
> example in the language reference where it says “%A = or %X, undef” -> “%A
> = undef” is unsafe because any bits that are set in %A must be set in the
> result. If any floating point operand is NaN dynamically, then the result
> must be NaN.
>
>
>
> I don’t believe it’s accurate to say that NaN is “morally equivalent” to
> undef. There are some similarities, but the important difference is that
> NaN always has well defined behavior with a specific correct result. There
> is, perhaps, a sense in which it is analogous to a poison value, but for
> the purposes of reasoning about the correctness of floating point
> operations I think it’s best to be pedantic about treating it as the
> specific value that it is.
>
>
>
> Finally, I was pretty sure you knew that fdiv by zero wasn’t undefined. I
> just wanted to clarify that the “?” in your comment was indicating that the
> assertion in the language reference was questionable as opposed to this
> point being in any way actually uncertain.
>
>
>
> From: Sanjay Patel [mailto:spatel at rotateright.com]
> Sent: Wednesday, February 28, 2018 1:05 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>; Kreitzer, David L <
> david.l.kreitzer at intel.com>
>
>
>
>
> Subject: Re: how to simplify FP ops with an undef operand?
>
>
>
>
>
>
>
> 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.
>
>
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> 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?
>
>
>
> -- 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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