[llvm-dev] [RFC] A nofree (and nosynch) function attribute: Mixing dereferenceable and delete

[Hal Finkel via llvm-dev]

[+Richard]


On 07/11/2018 08:29 AM, Sanjoy Das wrote:
> I'm not sure if nosynch is sufficient.  What if we had:
>
> void f(int& x) {
>   if (false) {
>     int r0 = x;
>   }
> }
>
> // other thread
> free(<pointer to x>);
>
> The source program is race free, but LLVM may speculate the read from
> x (seeing that it is dereferenceable) creating a race.

Interestingly, I'm not sure. I trust that Richard can answer this
question. :-)

So, if we had:

int y = ...;
...
f(y);

then I think that Clang's use of dereferenceable is almost certainly
okay (because the standard explicitly says, 9.2.3.2p5, "A reference
shall be initialized to refer to a valid object or
function."). Because the reference must have been valid when f(y) began
executing, unless it synchronizes somehow with the other thread, any
asynchronous deletion of y must be a race.

On the other hand, if we have:

int &y = ...;
...
f(y);

do we know that, when f(y) begins executing, the reference points to a
valid object? My reading of 9.3.3p2, which says, "Argument passing
(7.6.1.2) and
function value return (8.6.3) are initializations.", combined with the
statement above, implies that, perhaps surprisingly, the same holds
here. When the argument to f is initialized, it must refer to a valid
object (even if the initializer is another reference).

Richard, what do you think?

Thanks again,
Hal

P.S. If I'm right, then I might be happy, but it's also somewhat scary
(although we've been doing this optimization for multiple releases and I
don't think we have a bug along these lines), and I'd at least smell the
need for a sanitizer.

>
> -- Sanjoy
> On Tue, Jul 10, 2018 at 7:01 PM Hal Finkel via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Hi, everyone,
>>
>> I'd like to propose adding a nofree function attribute to indicate that
>> a function does not, directly or indirectly, call a memory-deallocation
>> function (e.g., free, C++'s operator delete). Clang/LLVM can currently
>> misoptimize functions that:
>>
>>  1. Have a reference argument.
>>
>>  2. Free the memory backing the object to which the reference is bound
>> during the function's execution.
>>
>> Because we tag, in Clang, all reference arguments using the
>> dereferenceable attribute, LLVM assumes that the pointer is
>> unconditionally dereferenceable throughout the course of the entire
>> function. This isn't true, however, if the memory is freed during the
>> execution of the function. For more information, please see the
>> discussion in https://reviews.llvm.org/D48239.
>>
>> To solve this problem, we need to give LLVM more information in order to
>> help it determine when a pointer, which is dereferenceable when the
>> functions begins to execute, will still be dereferenceable later on in
>> the function's execution. This nofree attribute can be part of that
>> solution. If we know that free (and friends) are not called by the
>> function (nor by any function called by the function, and so on), then
>> we know that pointers that started out dereferenceable will stay that
>> way (except as explained below).
>>
>> I'm initially proposing this to be only a function attribute, although
>> one could easily imagine a parameter attribute as well (that indicates
>> that a particular pointer argument is not freed by the function). This
>> might be useful, but for the use case of helping dereferenceable, it
>> would be subtle to use, unless the parameter was also marked as noalias,
>> because you'd need to know that the parameter was not also aliased with
>> another argument (or had not been captured). Another analysis would need
>> to provide this kind of information.
>>
>> Also, just because a function does not, directly or indirectly, call
>> free does not mean that it cannot cause memory to be deallocated. The
>> function might communicate (synchronize) with another thread causing
>> that other thread to delete the memory. For this reason, to use
>> dereferenceable as we currently do, we also need to know that the
>> function does not synchronize with any other threads. To solve this
>> problem, like nofree, I propose to add a nosynch attribute (to indicate
>> that a function does not use (non-relaxed) atomics or otherwise
>> synchronize with any other threads (e.g., perform I/O or, as a practical
>> matter, use volatile accesses).
>>
>> I've posted a patch for the nofree attribute
>> (https://reviews.llvm.org/D49165). nosynch's implementation would be
>> very similar (except instead of looking for calls to free, it would look
>> for uses of non-relaxed atomics, volatile ops, and known functions that
>> are not I/O functions).
>>
>> With both of these attributes (nofree and nosynch), a function argument
>> with the dereferenceable attribute will be known to be dereferenceable
>> throughout the execution of the attributed function. We can update
>> isDereferenceableAndAlignedPointer to include these additional checks on
>> the current function.
>>
>> One more choice we have: We can, as I proposed above, essentially weaken
>> the current semantics of dereferenceable to not exclude
>> mid-function-execution deallocation. We can also add a second attribute
>> with the current, stronger, semantics. We can keep the current attribute
>> as-is, and add a second attribute with the weaker semantics (and switch
>> Clang to use that).
>>
>> Please let me know what you think.
>>
>> Thanks again,
>>
>> Hal
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
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>> llvm-dev at lists.llvm.org
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory