86 lines
3.6 KiB
ReStructuredText
86 lines
3.6 KiB
ReStructuredText
Convention for implementing entrypoints
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=======================================
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LLVM-libc entrypoints are defined in the entrypoints document. In this document,
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we explain how the entrypoints are implemented. The source layout document
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explains that, within the high level ``src`` directory, there exists one
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directory for every public header file provided by LLVM-libc. The
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implementations of related group of entrypoints will also live in a directory of
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their own. This directory will have a name indicative of the related group of
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entrypoints, and will be under the directory corresponding to the header file of
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the entrypoints. For example, functions like ``fopen`` and ``fclose`` cannot be
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tested independent of each other and hence will live in a directory named
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``src/stdio/file_operations``. On the other hand, the implementation of the
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``round`` function from ``math.h`` can be tested by itself, so it will live in
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the directory of its own named ``src/math/round/``.
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Implementation of entrypoints can span multiple ``.cpp`` and ``.h`` files, but
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there will be atleast one header file with name of the form
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``<entrypoint name>.h`` for every entrypoint. This header file is called as the
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implementation header file. For the ``round`` function, the path to the
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implementation header file will be ``src/math/round/round.h``. The rest of this
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document explains the structure of implementation header files and ``.cpp``
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files.
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Implementation Header File Structure
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------------------------------------
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We will use the ``round`` function from the public ``math.h`` header file as an
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example. The ``round`` function will be declared in an internal header file
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``src/math/round/round.h`` as follows::
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// --- round.h --- //
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#ifndef LLVM_LIBC_SRC_MATH_ROUND_ROUND_H
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#define LLVM_LIBC_SRC_MATH_ROUND_ROUND_H
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namespace __llvm_libc {
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double round(double);
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} // namespace __llvm_libc
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#endif LLVM_LIBC_SRC_MATH_ROUND_ROUND_H
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Notice that the ``round`` function declaration is nested inside the namespace
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``__llvm_libc``. All implementation constructs in LLVM-libc are declared within
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the namespace ``__llvm_libc``.
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``.cpp`` File Structure
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-----------------------
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The implementation can span multiple ``.cpp`` files. However, the signature of
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the entrypoint function should make use of a special macro. For example, the
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``round`` function from ``math.h`` should be defined as follows, say in the file
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``src/math/math/round.cpp``::
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// --- round.cpp --- //
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namespace __llvm_libc {
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double LLVM_LIBC_ENTRYPOINT(round)(double d) {
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// ... implementation goes here.
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}
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} // namespace __llvm_libc
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Notice the use of the macro ``LLVM_LIBC_ENTRYPOINT``. This macro helps us define
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an C alias symbol for the C++ implementation. The C alias need not be added by
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the macro by itself. For example, for ELF targets, the macro is defined as
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follows::
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#define ENTRYPOINT_SECTION_ATTRIBUTE(name) \
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__attribute__((section(".llvm.libc.entrypoint."#name)))
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#define LLVM_LIBC_ENTRYPOINT(name) ENTRYPOINT_SECTION_ATTRIBUTE(name) name
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The macro places the C++ function in a unique section with name
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``.llvm.libc.entrypoint.<function name>``. This allows us to add a C alias using
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a post build step. For example, for the ``round`` function, one can use
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``objcopy`` to add an alias symbol as follows::
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objcopy --add-symbol round=.llvm.libc.entrypoint.round:0,function round.o
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NOTE: We use a post build ``objcopy`` step to add an alias instead of using
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the ``__attribute__((alias))``. For C++, this ``alias`` attribute requires
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mangled names of the referees. Using the post build ``objcopy`` step helps
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us avoid putting mangled names with ``alias`` attributes.
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