[llvm-dev] The semantics of nonnull attribute (original) (raw)

Nicolai Hähnle via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 19 00:26:31 PST 2020

Hi Johannes,

On Wed, Feb 19, 2020 at 8:17 AM Doerfert, Johannes <johannesdoerfert at gmail.com> wrote:

On 02/19, Juneyoung Lee via llvm-dev wrote: > Hello, > > > Would it be correct to resolve this by saying that dereferenceable(N) > > implies notpoison? 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.

That's a fair point. The same kind of argument actually applies to an analog of the example 4 that started this thread. If the argument is dead, then:

f(dereferenceable(N) %ptr) ==> f(dereferenceable(N) undef)

... would introduce UB and therefore be forbidden. So if that example serves as motivation for making nonnull weaker and introducing not_poison, then it should really serve as motivation for making all function argument attributes weaker.

Besides, there's a certain elegance in treating all attributes on function arguments the same:

That seems like it would be far less error prone than having to remember on a case-by-case basis whether certain attributes cause poison or immediate UB.

Cheers, Nicolai

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> wrote: > > > On Wed, Feb 19, 2020 at 3:51 AM Juneyoung Lee via llvm-dev > > <llvm-dev at lists.llvm.org> wrote: > > > I think notpoison (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 notpoison? This would be helpful as a clarification of how > > it all fits together. > > > > Cheers, > > Nicolai > > > > > -- > > Juneyoung Lee > Software Foundation Lab, Seoul National University _> ________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev -- Johannes Doerfert Researcher Argonne National Laboratory Lemont, IL 60439, USA jdoerfert at anl.gov

-- Lerne, wie die Welt wirklich ist, aber vergiss niemals, wie sie sein sollte.

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