[llvm-dev] getelementptr inbounds with offset 0

Tue Mar 26 10:20:09 PDT 2019

Hi Johannes,

>> So, the thinking here is: LLVM cannot exclude the possibility of an
>> object of size 0 existing at any given address.  The pointer returned
>> by "GEPi p 0" then would be one-past-the-end of such a 0-sized object.
>> Thus, "GEPi p 0" is the identitiy function for any p, it will not
>> return poison.
> 
> I don't see the problem. The behavior I hope we want and implement is:
> 
> Either LLVM knows that %p points to an invalid address (=non-object) or
> it doesn't. If it does, %p and all GEPs on it yield poison. If it
> doesn't, it has to assume %p points to a valid address and offset 0, 1,
> 2, ... might all yield valid pointers. The special case is when we know
> %p is valid and has extend of (at most) S, then all offsets <= S,
> including 0, are potentially valid (negative extends are similar).

So you are basically saying whether the offset is 0 or not does not matter, but
whether the base is an object LLVM can now about or not does?  I see.  That
makes sense.

The reason I restricted myself to offset 0 is that we'd like to do this without
actually having any accessible objects anywhere, which works out if the objects
have size 0.

FWIW, in <https://people.mpi-sws.org/~jung/twinsem/twinsem.pdf> we anyway had to
make "getelementptr inbounds" on integer pointers (pointers obtained by casting
an integer to a pointer) never yield poison directly and instead defer the
in-bound check to the time when the actual access happens.  That nicely
accommodates all uses of getelementptr that just compute addresses without ever
using them for a memory access (using them only, e.g. to compute offsets or
compare pointers).  But this is not how the LLVM LangRef is written, unfortunately.

>> # example1
>>
>> %P1 = int2ptr 4
>> %G1 = gep inbounds %P1 0
>>
>> # example2
>>
>> %P2 = call noalias i8* @malloc(i64 12)
>> call void @free(i8* %P2)
>> %G2 = gep inbounds %P2 0
>>
>> The first happens in Rust all the time, and we rely on not getting
>> poison.  The second doesn't occur in Rust (to my knowledge), but it
>> seems somewhat inconsistent to return poison in one case and not the
>> other.
> 
> Let's start with example2, note that I renamed the values above.
> 
> %P2 is dangling (and we know it) after the free. %P2 is therefore
> poison* and so is %G2.
> 
> * or undef I'm always confused which might be bad in this conversation.

Wait, I know that C has a rule that dangling pointers are "indeterminate" but
this is the first time I hear that LLVM has it as well.  Is that written down
anywhere?  Rust relies heavily in dangling pointers being well-behaved when used
only on comparisons and casts (no accesses), so this would be a big deal.
(Also, this rule in C is pretty much impossible to formalize and serves no
purpose that I know of, but that is a separate discussion.)

> In example1, without further information, I'd say that there is no
> poison (statically). Address 4 could be an allocated object until proven
> otherwise.
> 
> 
> I am still a little confused about the problem you see. If what I wrote
> about the implemented behavior holds true (which I am not totally sure
> of), you should not have a problem with poison even if you would
> sprinkle GEP (inbounds) %p 0 all over the place. Either %p was known to
> be invalid and so is the GEP, or %p was not known to be invalid and
> neither is the GEP. Am I missing something here?

The thing is, I am not asking about the behavior implemented today but about the
behavior of the "abstract LLVM machine" that is described by the LangRef and
that the optimizer has to justify its transformations against.  Analyses become
smarter every day, so looking at what LLVM deduces from certain instructions is
but a snapshot.

But also, your response assumes "dangling pointers are undef/posion", which is
new to me.  I'd be rather shocked if this is something LLVM actually relies on
anywhere.

Kind regards,
Ralf

> 
> Cheers,
>   Johannes
> 
>>> A side-effect based on the GEP will however __locally__ introduce an
>>> dereferencability assumption (in my opinion at least). Let's say the
>>> code looks like this:
>>>
>>>
>>>   %G = gep inbounds (int2ptr 4) 0 ; We don't know anything about the
>>>   dereferencability of ; the memory at address 4 here.  br %cnd,
>>>   %BB0, %BB1
>>>
>>> BB0: ; We don't know anything about the dereferencability of ; the
>>> memory at address 4 here.  load %G ; We know the memory at address 4
>>> is dereferenceable here.  ; Though, that is due to the load and not
>>> the inbounds.  ...  br %BB1
>>>
>>> BB1: ; We don't know anything about the dereferencability of ; the
>>> memory at address 4 here.
>>>
>>>
>>> It is a different story if you start to use the GEP in other
>>> operations, e.g., to alter control flow. Then the (potential)
>>> undefined value can propagate.
>>>
>>>
>>> Any thought on this? Did I at least get your problem description
>>> right?
>>>
>>> Cheers, Johannes
>>>
>>>
>>>
>>> P.S. Sorry if this breaks the thread and apologies that I had to
>>> remove Bruce from the CC. It turns out replying to an email you did
>>> not receive is complicated and getting on the LLVM-Dev list is
>>> nowadays as well...
>>>
>>>
>>> On 02/25, Ralf Jung via llvm-dev wrote:
>>>> Hi Bruce,
>>>>
>>>> On 25.02.19 13:10, Bruce Hoult wrote:
>>>>> LLVM has no idea whether the address computed by GEP is actually
>>>>> within a legal object. The "inbounds" keyword is just you, the
>>>>> programmer, promising LLVM that you know it's ok and that you
>>>>> don't care what happens if it is actually out of bounds.
>>>>>
>>>>> https://llvm.org/docs/GetElementPtr.html#what-happens-if-an-array-index-is-out-of-bounds
>>>>
>>>> The LangRef says I get a poison value when I am violating the
>>>> bounds. What I am asking is what exactly this means when the offset
>>>> is 0 -- what *are* the conditions under which an offset-by-0 is
>>>> "out of bounds" and hence yields poison?  Of course LLVM cannot
>>>> always statically determine this, but it relies on (dynamically, on
>>>> the "LLVM abstract machine") such things not happening, and I am
>>>> asking what exactly these dynamic conditions are.
>>>>
>>>> Kind regards, Ralf
>>>>
>>>>>
>>>>> On Sun, Feb 24, 2019 at 9:05 AM Ralf Jung via llvm-dev
>>>>> <llvm... at lists.llvm.org> wrote:
>>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> What exactly are the rules for `getelementptr inbounds` with
>>>>>> offset 0?
>>>>>>
>>>>>> In Rust, we are relying on the fact that if we use, for example,
>>>>>> `inttoptr` to turn `4` into a pointer, we can then do
>>>>>> `getelementptr inbounds` with offset 0 on that without LLVM
>>>>>> deducing that there actually is any dereferencable memory at
>>>>>> location 4.  The argument is that we can think of there being a
>>>>>> zero-sized allocation. Is that a reasonable assumption?  Can
>>>>>> something like this be documented in the LangRef?
>>>>>>
>>>>>> Relatedly, how does the situation change if the pointer is not
>>>>>> created "out of thin air" from a fixed integer, but is actually a
>>>>>> dangling pointer obtained previously from `malloc` (or `alloca`
>>>>>> or whatever)?  Is getelementptr inbounds` with offset 0 on such a
>>>>>> pointer a NOP, or does it result in `poison`?  And if that makes
>>>>>> a difference, how does that square with the fact that, e.g., the
>>>>>> integer `0x4000` could well be inside such an allocation, but
>>>>>> doing `getelementptr inbounds` with offset 0 on that would fall
>>>>>> under the first question above?
>>>>>>
>>>>>> Kind regards, Ralf
>>>>>> _______________________________________________ LLVM Developers
>>>>>> mailing list llvm... at lists.llvm.org
>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>> _______________________________________________ LLVM Developers
>>>> mailing list llvm... at lists.llvm.org
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>>>
> 