[llvm-dev] The semantics of nonnull attribute

Wed Feb 19 17:52:06 PST 2020

Two thoughts:

 1. I think that we should aim for regularity, to the extent possible, and so we should treat nonnull, align, etc. similarly w.r.t. to whether they produce poison or UB.

 2. I was thinking about the following last night, and it clarified for me why having an not_poison attribute makes sense and seems useful, and how poison/UB might affect things on a function-call boundary itself. Imagine that we had a fastcc lowering strategy that took a pointer argument with an alignment attribute, followed by a suitably-small integer argument, and implemented a calling convention that passed both in the same register. If the pointer value might be poison, and thus violate the alignment attribute (or might violate the alignment attribute otherwise and produce poison), then we can't implement this just by anding together the two values (to pass them in the one register). We need to mask off the low bits first. If the value can't be or generate poison, and violating the alignment constraint produces UB, then the masking is not needed and we can just and together the two values (confident that the low bits will always be zero).

 -Hal

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Finkel, Hal J. via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Wednesday, February 19, 2020 2:29 AM
To: Doerfert, Johannes <johannesdoerfert at gmail.com>; Juneyoung Lee <juneyoung.lee at sf.snu.ac.kr>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Nuno Lopes <nuno.lopes at ist.utl.pt>; John Regehr <regehr at cs.utah.edu>
Subject: Re: [llvm-dev] The semantics of nonnull attribute

On 2/19/20 1:16 AM, Doerfert, Johannes via llvm-dev wrote:

On 02/19, Juneyoung Lee via llvm-dev wrote:

Hello,

 Would it be correct to resolve this by saying that dereferenceable(N)
*implies* not_poison? This would be helpful as a clarification of how
it all fits together.

Yes, I think it makes sense.

I don't we should do that.

Take the `gep inbounds` example:

char* foo(char *arg) {
  return `gep inbounds %arg, -100`
}

Here it depends if we want to deduce the output is dereferenceable(100)
or not. If we do, we need dereferenceable to mean poison if violated, as
with nonnull, because it is derived from poison. Only if we don't derive
dereferenceable for the return value we can go for dereferenceable
violations are UB.

Can you please clarify what it means for the output of dereferenceable to be poison? If we tag a memory address as dereferenceable, is the optimizer free to insert a load of the address immediately following that? Or we need to see some other access (prior to any thread synchronization?) to say that's valid?

Thanks again,

Hal

In the end, I think, it boils down to the question if there are
situations where violation of some attributes should be poison and
violation of others should be UB. If such situations exists it is
unclear to me what makes the UB/poison ones special.

On Wed, Feb 19, 2020 at 12:14 PM Nicolai Hähnle <nhaehnle at gmail.com><mailto:nhaehnle at gmail.com> wrote:

On Wed, Feb 19, 2020 at 3:51 AM Juneyoung Lee via llvm-dev
<llvm-dev at lists.llvm.org><mailto:llvm-dev at lists.llvm.org> wrote:

I think not_poison (Johannes's used keyword) makes sense. We can

simulate the original UB semantics by simply attaching it, as explained.

For the attributes other than nonnull, we may need more discussion;

align attribute seems to be okay with defining it as poison,
dereferenceable may need UB even without nonnull (because it needs to be
non-poison as shown Nuno's hoisting example).

For reference, the hoisting example was:

f(dereferenceable(4) %p) {
  loop() {
    %v = load %p
    use(%v)
  }
}
=>
f(dereferenceable(4) %p) {
  %v = load %p
  loop() {
    use(%v)
  }
}

Would it be correct to resolve this by saying that dereferenceable(N)
*implies* not_poison? This would be helpful as a clarification of how
it all fits together.

Cheers,
Nicolai

--

Juneyoung Lee
Software Foundation Lab, Seoul National University

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