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160 lines
6.4 KiB
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===============
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Opaque Pointers
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===============
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The Opaque Pointer Type
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=======================
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Traditionally, LLVM IR pointer types have contained a pointee type. For example,
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``i32 *`` is a pointer that points to an ``i32`` somewhere in memory. However,
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due to a lack of pointee type semantics and various issues with having pointee
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types, there is a desire to remove pointee types from pointers.
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The opaque pointer type project aims to replace all pointer types containing
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pointee types in LLVM with an opaque pointer type. The new pointer type is
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tentatively represented textually as ``ptr``.
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Address spaces are still used to distinguish between different kinds of pointers
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where the distinction is relevant for lowering (e.g. data vs function pointers
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have different sizes on some architectures). Opaque pointers are not changing
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anything related to address spaces and lowering. For more information, see
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`DataLayout <LangRef.html#langref-datalayout>`_.
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Issues with explicit pointee types
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==================================
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LLVM IR pointers can be cast back and forth between pointers with different
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pointee types. The pointee type does not necessarily actually represent the
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actual underlying type in memory. In other words, the pointee type contains no
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real semantics.
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Lots of operations do not actually care about the underlying type. These
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operations, typically intrinsics, usually end up taking an ``i8 *``. This causes
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lots of redundant no-op bitcasts in the IR to and from a pointer with a
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different pointee type. The extra bitcasts take up space and require extra work
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to look through in optimizations. And more bitcasts increases the chances of
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incorrect bitcasts, especially in regards to address spaces.
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Some instructions still need to know what type to treat the memory pointed to by
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the pointer as. For example, a load needs to know how many bytes to load from
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memory. In these cases, instructions themselves contain a type argument. For
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example the load instruction from older versions of LLVM
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.. code-block:: llvm
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load i64* %p
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becomes
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.. code-block:: llvm
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load i64, ptr %p
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A nice analogous transition that happened earlier in LLVM is integer signedness.
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There is no distinction between signed and unsigned integer types, rather the
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integer operations themselves contain what to treat the integer as. Initially,
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LLVM IR distinguished between unsigned and signed integer types. The transition
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from manifesting signedness in types to instructions happened early on in LLVM's
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life to the betterment of LLVM IR.
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I Still Need Pointee Types!
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===========================
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The frontend should already know what type each operation operates on based on
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the input source code. However, some frontends like Clang may end up relying on
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LLVM pointer pointee types to keep track of pointee types. The frontend needs to
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keep track of frontend pointee types on its own.
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For optimizations around frontend types, pointee types are not useful due their
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lack of semantics. Rather, since LLVM IR works on untyped memory, for a frontend
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to tell LLVM about frontend types for the purposes of alias analysis, extra
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metadata is added to the IR. For more information, see `TBAA
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<LangRef.html#tbaa-metadata>`_.
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Some specific operations still need to know what type a pointer types to. For
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the most part, this is codegen and ABI specific. For example, `byval
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<LangRef.html#parameter-attributes>`_ arguments are pointers, but backends need
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to know the underlying type of the argument to properly lower it. In cases like
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these, the attributes contain a type argument. For example,
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.. code-block:: llvm
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call void @f(ptr byval(i32) %p)
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signifies that ``%p`` as an argument should be lowered as an ``i32`` passed
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indirectly.
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If you have use cases that this sort of fix doesn't cover, please email
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llvm-dev.
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Transition Plan
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===============
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LLVM currently has many places that depend on pointee types. Each dependency on
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pointee types needs to be resolved in some way or another. This essentially
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translates to figuring out how to remove all calls to
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``PointerType::getElementType`` and ``Type::getPointerElementType()``.
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Making everything use opaque pointers in one huge commit is infeasible. This
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needs to be done incrementally. The following steps need to be done, in no
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particular order:
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* Introduce the opaque pointer type
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* Already done
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* Remove remaining in-tree users of pointee types
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* There are many miscellaneous uses that should be cleaned up individually
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* Some of the larger use cases are mentioned below
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* Various ABI attributes and instructions that rely on pointee types need to be
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modified to specify the type separately
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* This has already happened for all instructions like loads, stores, GEPs,
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and various attributes like ``byval``
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* More cases may be found as work continues
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* Remove calls to and deprecate ``IRBuilder`` methods that rely on pointee types
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* For example, some of the ``IRBuilder::CreateGEP()`` methods use the pointer
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operand's pointee type to determine the GEP operand type
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* Some methods are already deprecated with ``LLVM_ATTRIBUTE_DEPRECATED``, such
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as some overloads of ``IRBuilder::CreateLoad()``
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* Allow bitcode auto-upgrade of legacy pointer type to the new opaque pointer
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type (not to be turned on until ready)
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* To support legacy bitcode, such as legacy stores/loads, we need to track
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pointee types for all values since legacy instructions may infer the types
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from a pointer operand's pointee type
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* Migrate frontends to not keep track of frontend pointee types via LLVM pointer
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pointee types
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* This is mostly Clang, see ``clang::CodeGen::Address::getElementType()``
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* Add option to internally treat all pointer types opaque pointers and see what
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breaks, starting with LLVM tests, then run Clang over large codebases
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* We don't want to start mass-updating tests until we're fairly confident that opaque pointers won't cause major issues
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* Replace legacy pointer types in LLVM tests with opaque pointer types
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Frontend Migration Steps
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========================
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If you have your own frontend, there are a couple of things to do after opaque
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pointer types fully work.
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* Don't rely on LLVM pointee types to keep track of frontend pointee types
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* Migrate away from LLVM IR instruction builders that rely on pointee types
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* For example, ``IRBuilder::CreateGEP()`` has multiple overloads; make sure to
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use one where the source element type is explicitly passed in, not inferred
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from the pointer operand pointee type
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