[cfe-dev] Smart Pointer Lifetime Optimizations

[Richard Smith via cfe-dev]

On Mon, 8 Jun 2020 at 19:52, John McCall via cfe-dev <cfe-dev at lists.llvm.org>
wrote:

> On 8 Jun 2020, at 21:13, Richard Smith wrote:
>
> On Mon, 8 Jun 2020 at 00:22, John McCall via cfe-dev <
> cfe-dev at lists.llvm.org>
> wrote:
>
> You wouldn’t be the first person to be surprised by the result of this sort
> of analysis, but I’m afraid it’s what we’re working with.
>
> Unfortunately, there’s really no way to eliminate this one without either
> interprocedural information or language changes. trivial_abi eliminates
> the other one because it changes the convention for passing by value, but
> to
> pass an “immutably borrowed” value in C++ we have to pass by reference,
> which
> allows the reference to be escaped and accessed (and even mutated, if the
> original object wasn’t declared const) as long as those accesses happen
> before destruction.
>
> Perhaps we should expose LLVM's nocapture attribute to the source level?
>
> I think we have with __attribute__((noescape)). Of course, adopting it
> systematically would be hugely invasive.
>
> *> 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. *
>
>
> We could actually do this for trivial_abi types too. If we added a builtin
> type trait to check if a type has the trivial_abi attribute, libc++ could
> conditionally give unique_ptr the trivial_abi attribute if its base type
> also had the attribute. Additionally, we could add a config macro that
> would do this globally when libc++ is in unstable ABI mode.
>
> Hmm. That doesn’t just fall out from any analysis I see. trivial_abi
> is an existing, ABI-stable attribute, so changing the ABI of
> std::unique_ptr
> for types that are already trivial_abi is just as much of an ABI break
> as changing it in general would be. You could try to justify it by saying
> that there just aren’t very many trivial_abi types yet, or that
> trivial_abi
> is a vendor-specific attribute that’s unlikely to be used on a type with a
> stable ABI because non-Clang implementations wouldn’t be able to compile
> it compatibly, but those aren’t terribly convincing arguments to me.
>
> I guess I should finish https://reviews.llvm.org/D63748 at some point.
> (Though I think we probably shouldn't enable it in libc++ unstable ABI
> configurations by default, since it also changes observable program
> semantics due to altering destruction order, and is arguably non-conforming
> for the same reason.)
>
> It definitely changes observable semantics, but it’s not *obviously*
> non-conforming; [expr.call]p7 gives us a lot of flexibility here:
>
> It is implementation-defined whether the lifetime of a parameter
> ends when the function in which it is defined returns or at the
> end of the enclosing full-expression.
>
This is the non-conformance I'm referring to: https://godbolt.org/z/cgf5_2

Even given [expr.call]p7, we are still required to destroy
automatic-storage-duration objects in reverse construction order by
[stmt.jump]p2:

"On exit from a scope (however accomplished), objects with automatic
storage duration (6.7.5.3) that have been constructed in that scope are
destroyed in the reverse order of their construction."

> And note that MSVC has traditionally destroyed parameters in the callee.
> IIRC the standard actually originally specified that parameters were
> always destroyed at the end of the call and only changed it due to
> Itanium doing otherwise.
>
When targeting the MS ABI, we initialize parameters right-to-left to
compensate for them being destroyed left-to-right. I think we could fix the
non-conformance by initializing [[trivial_abi]] parameters last (and in the
correct order, whatever we decide that is) when forming a function call.
(This is only a partial fix: there are cases where the evaluation order of
function arguments is mandated. But those cases make all callee-cleanup
ABIs non-conforming, which I don't think is intended, so it's probably
reasonable to chalk that up as a standard bug and ignore it.)

> Now, it’s possible that the copy-elision rules have an unfortunate
> impact here. IIRC an object initialized with an elided copy is supposed
> to take on the longer of the two natural lifetimes. Does that mean that
> if you have a parameter initialized by an elided copy from a temporary,
> the parameter needs to live until the end of the calling full-expression
> like the temporary would have? If so, you either wouldn’t be able to
> use a callee-destroy ABI or you wouldn’t be allowed to elide copies
> into parameters, and the latter seems unacceptable.
>
That case can no longer arise when initializing a function parameter under
the new guaranteed copy-elision rules, so this problem is gone at least in
C++17 onwards. But yes, historically I believe it was that case that an
implementation that did callee cleanup was effectively not permitted to
perform copy elision for function parameters (though of course
implementations did perform copy elision anyway).

> Even if it’s conforming, I’m sure there are bugs about e.g. the proper
> ordering with things like function-try-blocks and exception specifications.
>
Probably :)

> John.
>
> John.
>
> Best,
>
> Zoe
>
> On Sat, Jun 6, 2020 at 2:07 PM John McCall <rjmccall at apple.com> wrote:
>
> 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 typedefs), 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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