[cfe-dev] Smart Pointer Lifetime Optimizations

[John McCall via cfe-dev]

On 6 Jun 2020, at 13:47, Zoe Carver wrote:
> John,
>
> Thanks, those are good points. I think we can still remove one of the
> destructors (which could also be done by a more powerful DSE+load
> propagation) but, you're right; one needs to stay.

Can you explain in more detail which destructor you think you can
eliminate?

>> This can only be optimized with a more global, interprocedural
> optimization that shifts responsibility to owner to destroy its 
> argument.
>
> I'll think about implementing something like this, but I suspect any
> possible optimizations will already happen with inlining and analysis.

Yeah.  For the narrow case of `std::unique_ptr`, since its operations
are easily inlined and can be easily optimized after copy propagation,
there’s not much more that can be done at a high level.

Note that `trivial_abi` (if it could be adopted on `std::unique_ptr`)
also changes the ABI to make the callee responsible for destruction.
So as part of getting a more efficient low-level ABI, you also get a
more optimizable high-level one.

One idea I’ve personally been kicking around is some way to mark
declarations as having an “unstable ABI”: basically, a guarantee 
that
all the code that uses them will be compiled with a single toolchain,
and therefore a license for the implementation to alter the ABI however
it likes with any code that uses any of those declarations.

A type would be unstable if it was composed even partially from a
declaration marked unstable.  So `class Unstable` would be unstable,
but so would `const Unstable *` — and, crucially, so would
`std::unique_ptr<Unstable>`.  But for soundness reasons, this would
need to ignore type sugar (so no marking `typedef`s), and it wouldn’t
be able to automatically descend into fields.

There are a few ways that we could use that directly in the compiler.
The big restriction is that you’re not breaking ABI globally and so
you always need an unstable “contaminant” that permits using the
unstable ABI.  For example, we can’t just change function ABIs
for all unstable functions because function pointers have to remain
compatible.  On the other hand, programs aren’t allowed to call
function pointers under the wrong type, so if the function type is
unstable, we can change anything we want about its ABI.

(For functions specifically, there’s another option: we could emit
the functions with an unstable ABI and then introduce thunks that
adapt the calling convention when the address is taken.  But that’s
a non-trivial code-size hit that we might have to do unconditionally.
It also can’t adapt a callee-destroy ABI into a caller-destroy one
without introducing an extra move, which isn’t necessarily 
semantically
allowed.)

Probably more importantly, though, we could allow unstable-ness to
be detected with a type trait,  and that would allow the standard
library to take advantage of it.  So `std::unique_ptr<int>` would
be stuck with the stable ABI, but `std::unique_ptr<Unstable>` could
switch to be `trivial_abi`.

That does leave the problem of actually doing the annotation.
Adding an attribute to every class is probably beyond what people
would accept.  There are several ways to do mass annotation.  Pragmas
are problematic because you don’t want to accidentally leave the
pragma on when you exit a file and then have it cover a system
include.  We do have some pragmas that prevent file changes while
the pragma is active, which is a decent solution for that problem.
An alternative is to mark namespaces.  That probably needs to be
lexical: that is, you wouldn’t be able to mark the entire `clang`
namespace, you would mark a specific  `namespace clang` declaration
in a single header.  But that’s still much more manageable, and
after all, the cost to missing an annotation is just a missed
optimization.

We could also implicitly make all anonymous-namespace declarations
unstable.

John.


>
> Thanks for the response,
> Zoe
>
> On Fri, Jun 5, 2020 at 1:09 PM John McCall <rjmccall at apple.com> wrote:
>
>> On 5 Jun 2020, at 14:45, Zoe Carver via cfe-dev wrote:
>>
>> Hello all,
>>
>>
>> I'm planning to do some work to add lifetime optimization passes for 
>> smart
>> pointers and reference-counted objects. I'll use this email as a sort 
>> of
>> proposal for what I hope to do.
>>
>>
>> *Scope*
>>
>>
>> As I'm developing the pass, I'm trying to keep it general and create
>> utilities that could work across multiple smart pointers. But, right 
>> now,
>> I'm focussing on unique_ptr and applying specific ownership 
>> optimizations
>> to
>> unique_ptr only.
>>
>>
>> *unique_ptr Optimzations*
>>
>>
>> The pass I'm currently developing adds a single, simple, 
>> optimization:
>> constant fold the destructor based on ownership information. 
>> unique_ptr has
>> a lot of ownership information communicated with reference semantics. 
>> When
>> a
>> unique_ptr is moved into another function, that function takes over
>> ownership of the unique_ptr, and subsequent destructors can be 
>> eliminated
>> (because they will be no-ops). Otherwise, branchless functions are 
>> often
>> complicated after inlining unique_ptr's destructor so, this 
>> optimization
>> should be fairly beneficial.
>>
>>
>> unique_ptr's reset and release methods both complicate this 
>> optimization a
>> bit. Because they are also able to transfer and remove ownership, all
>> unknown instructions must be ignored. However, in the future, 
>> knowledge of
>> those methods might be able to make the pass more robust.
>>
>>
>> With unique_ptr, it's difficult to prove liveness. So, it is hard to
>> constant fold the destructor call to always be there. Maybe in the 
>> future,
>> this would be possible, though (with sufficient analysis).
>>
>>
>> Last, an optimization that I hope to do is lowering the unique_ptr to 
>> a raw
>> pointer if all lifetime paths are known. I think removing this layer 
>> of
>> abstraction would make it easier for other optimization passes to be
>> successful. Eventually, we may even be able to specialize functions 
>> that
>> used to take a unique_ptr to now take a raw pointer, if the 
>> argument's
>> lifetime was also able to be fully analyzed.
>>
>>
>> *Lifetime Annotations*
>>
>>
>> Right now, the pass relies on (pre-inlined) function calls to 
>> generate
>> ownership information. Another approach would be to add ownership
>> annotations, such as the lifetime intrinsics (i.e. 
>> llvm.lifetime.start).
>>
>>
>> *ARC Optimizations*
>>
>>
>> There are a huge number of large and small ARC optimizations already 
>> in
>> LLVM. For unique_ptr specifically, I'm not sure these are of any 
>> benefit
>> because unique_ptr doesn't actually do any reference counting. But, 
>> later
>> on, when I start working on generalizing this pass to support more 
>> smart
>> pointers (specifically shared_ptr) I think the ARC optimization pass, 
>> and
>> especially the utilities it contains, could be very beneficial. If 
>> anyone
>> has experience with ARC optimizations, I'd love to hear your thoughts 
>> on
>> extending them to other reference counted objects.
>>
>>
>> *trivial_abi and Hidden References*
>>
>>
>> Arthur O'Dwyer made a good point, which is that a lot of these
>> optimizations can be applied when with the trivial_abi attribute. 
>> However,
>> given that's not a standard attribute and these optimizations only 
>> *happen*
>> to work with trivial_abi (i.e., in a more complicated program, they 
>> may not
>> continue to work). I think lifetime utilities and specific lifetime
>> optimization passes are still beneficial (especially if they can be 
>> applied
>> to other smart pointers in the future).
>>
>>
>> Because all smart pointers have non-trivial destructors, they are 
>> always
>> passed by hidden references. With unique_ptr, this is as simple as
>> bit-casting the pointer member to unique_ptr, which would allow for 
>> it to
>> be lowered to a single raw pointer instead of a stack-allocated 
>> object.
>> Even without the trival_abi attribute, I think this is an 
>> optimization that
>> could be done.
>>
>>
>> *Results*
>>
>>
>> Here's the unique_ptr pass I've been talking about: ⚙ D81288 Opt 
>> Smart
>> pointer lifetime optimizations pass. 
>> <https://reviews.llvm.org/D81288>
>>
>> For reference, here are the before and after results:
>>
>> Clang trunk (four branches): Compiler Explorer
>> <https://godbolt.org/z/bsJFty>
>>
>> With optimizations (branchless): https://pastebin.com/raw/mQ2r6pru
>>
>> Unfortunately, these are not legal optimizations for your test case:
>>
>>    -
>>
>>    guaranteed is permitted to escape a reference (or pointer) to the
>>    object it was passed. Tat references and pointers remain valid
>>    until the object goes out of scope.
>>    -
>>
>>    The object can be mutated through that reference because the 
>> underlying
>>    object is not const. Being passed a const reference is not a
>>    semantic contract in C++.
>>    -
>>
>>    Through a combination of the above, the call to owner may change
>>    the value of p, and so the caller may not rely on it still being
>>    in a trivially-destructible state after that call.
>>    -
>>
>>    owner may leave the value of its parameter object in a
>>    non-trivially-destructible state, and under the Itanium C++ ABI,
>>    cleaning
>>    up that object is the caller’s responsibility. I agree that this 
>> is a
>>    bad rule for optimization purposes, but it’s the rule. This can 
>> only be
>>    optimized with a more global, interprocedural optimization that 
>> shifts
>>    responsibility to owner to destroy its argument.
>>
>> John.
>>


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