forked from OSchip/llvm-project
150 lines
4.9 KiB
ReStructuredText
150 lines
4.9 KiB
ReStructuredText
=============================================
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Enable std::unique_ptr [[clang::trivial_abi]]
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=============================================
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Background
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==========
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Consider the follow snippets
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.. code-block:: cpp
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void raw_func(Foo* raw_arg) { ... }
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void smart_func(std::unique_ptr<Foo> smart_arg) { ... }
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Foo* raw_ptr_retval() { ... }
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std::unique_ptr<Foo*> smart_ptr_retval() { ... }
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The argument ``raw_arg`` could be passed in a register but ``smart_arg`` could not, due to current
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implementation.
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Specifically, in the ``smart_arg`` case, the caller secretly constructs a temporary ``std::unique_ptr``
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in its stack-frame, and then passes a pointer to it to the callee in a hidden parameter.
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Similarly, the return value from ``smart_ptr_retval`` is secretly allocated in the caller and
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passed as a secret reference to the callee.
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Goal
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===================
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``std::unique_ptr`` is passed directly in a register.
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Design
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======
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* Annotate the two definitions of ``std::unique_ptr`` with ``clang::trivial_abi`` attribute.
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* Put the attribute behind a flag because this change has potential compilation and runtime breakages.
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This comes with some side effects:
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* ``std::unique_ptr`` parameters will now be destroyed by callees, rather than callers.
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It is worth noting that destruction by callee is not unique to the use of trivial_abi attribute.
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In most Microsoft's ABIs, arguments are always destroyed by the callee.
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Consequently, this may change the destruction order for function parameters to an order that is non-conforming to the standard.
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For example:
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.. code-block:: cpp
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struct A { ~A(); };
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struct B { ~B(); };
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struct C { C(A, unique_ptr<B>, A) {} };
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C c{{}, make_unique<B>, {}};
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In a conforming implementation, the destruction order for C::C's parameters is required to be ``~A(), ~B(), ~A()`` but with this mode enabled, we'll instead see ``~B(), ~A(), ~A()``.
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* Reduced code-size.
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Performance impact
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------------------
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Google has measured performance improvements of up to 1.6% on some large server macrobenchmarks, and a small reduction in binary sizes.
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This also affects null pointer optimization
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Clang's optimizer can now figure out when a `std::unique_ptr` is known to contain *non*-null.
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(Actually, this has been a *missed* optimization all along.)
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.. code-block:: cpp
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struct Foo {
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~Foo();
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};
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std::unique_ptr<Foo> make_foo();
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void do_nothing(const Foo&)
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void bar() {
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auto x = make_foo();
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do_nothing(*x);
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}
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With this change, ``~Foo()`` will be called even if ``make_foo`` returns ``unique_ptr<Foo>(nullptr)``.
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The compiler can now assume that ``x.get()`` cannot be null by the end of ``bar()``, because
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the deference of ``x`` would be UB if it were ``nullptr``. (This dereference would not have caused
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a segfault, because no load is generated for dereferencing a pointer to a reference. This can be detected with ``-fsanitize=null``).
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Potential breakages
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-------------------
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The following breakages were discovered by enabling this change and fixing the resulting issues in a large code base.
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- Compilation failures
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- Function definitions now require complete type ``T`` for parameters with type ``std::unique_ptr<T>``. The following code will no longer compile.
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.. code-block:: cpp
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class Foo;
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void func(std::unique_ptr<Foo> arg) { /* never use `arg` directly */ }
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- Fix: Remove forward-declaration of ``Foo`` and include its proper header.
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- Runtime Failures
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- Lifetime of ``std::unique_ptr<>`` arguments end earlier (at the end of the callee's body, rather than at the end of the full expression containing the call).
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.. code-block:: cpp
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util::Status run_worker(std::unique_ptr<Foo>);
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void func() {
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std::unique_ptr<Foo> smart_foo = ...;
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Foo* owned_foo = smart_foo.get();
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// Currently, the following would "work" because the argument to run_worker() is deleted at the end of func()
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// With the new calling convention, it will be deleted at the end of run_worker(),
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// making this an access to freed memory.
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owned_foo->Bar(run_worker(std::move(smart_foo)));
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^
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// <<<Crash expected here
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}
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- Lifetime of local *returned* ``std::unique_ptr<>`` ends earlier.
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Spot the bug:
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.. code-block:: cpp
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std::unique_ptr<Foo> create_and_subscribe(Bar* subscriber) {
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auto foo = std::make_unique<Foo>();
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subscriber->sub([&foo] { foo->do_thing();} );
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return foo;
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}
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One could point out this is an obvious stack-use-after return bug.
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With the current calling convention, running this code with ASAN enabled, however, would not yield any "issue".
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So is this a bug in ASAN? (Spoiler: No)
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This currently would "work" only because the storage for ``foo`` is in the caller's stackframe.
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In other words, ``&foo`` in callee and ``&foo`` in the caller are the same address.
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ASAN can be used to detect both of these.
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