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Leonard Chan c24d9d2b01 [llvm][IR] Add dso_local_equivalent Constant
The `dso_local_equivalent` constant is a wrapper for functions that represents a
value which is functionally equivalent to the global passed to this. That is, if
this accepts a function, calling this constant should have the same effects as
calling the function directly. This could be a direct reference to the function,
the `@plt` modifier on X86/AArch64, a thunk, or anything that's equivalent to the
resolved function as a call target.

When lowered, the returned address must have a constant offset at link time from
some other symbol defined within the same binary. The address of this value is
also insignificant. The name is leveraged from `dso_local` where use of a function
or variable is resolved to a symbol in the same linkage unit.

In this patch:
- Addition of `dso_local_equivalent` and handling it
- Update Constant::needsRelocation() to strip constant inbound GEPs and take
  advantage of `dso_local_equivalent` for relative references

This is useful for the [Relative VTables C++ ABI](https://reviews.llvm.org/D72959)
which makes vtables readonly. This works by replacing the dynamic relocations for
function pointers in them with static relocations that represent the offset between
the vtable and virtual functions. If a function is externally defined,
`dso_local_equivalent` can be used as a generic wrapper for the function to still
allow for this static offset calculation to be done.

See [RFC](http://lists.llvm.org/pipermail/llvm-dev/2020-August/144469.html) for more details.

Differential Revision: https://reviews.llvm.org/D77248
2020-11-19 10:26:17 -08:00
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LLVM Documentation
==================

LLVM's documentation is written in reStructuredText, a lightweight
plaintext markup language (file extension `.rst`). While the
reStructuredText documentation should be quite readable in source form, it
is mostly meant to be processed by the Sphinx documentation generation
system to create HTML pages which are hosted on <https://llvm.org/docs/> and
updated after every commit. Manpage output is also supported, see below.

If you instead would like to generate and view the HTML locally, install
Sphinx <http://sphinx-doc.org/> and then do:

    cd <build-dir>
    cmake -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_HTML=true <src-dir>
    make -j3 docs-llvm-html
    $BROWSER <build-dir>/docs//html/index.html

The mapping between reStructuredText files and generated documentation is
`docs/Foo.rst` <-> `<build-dir>/docs//html/Foo.html` <-> `https://llvm.org/docs/Foo.html`.

If you are interested in writing new documentation, you will want to read
`SphinxQuickstartTemplate.rst` which will get you writing documentation
very fast and includes examples of the most important reStructuredText
markup syntax.

Manpage Output
===============

Building the manpages is similar to building the HTML documentation. The
primary difference is to use the `man` makefile target, instead of the
default (which is `html`). Sphinx then produces the man pages in the
directory `<build-dir>/docs/man/`.

    cd <build-dir>
    cmake -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_MAN=true <src-dir>
    make -j3 docs-llvm-man
    man -l >build-dir>/docs/man/FileCheck.1

The correspondence between .rst files and man pages is
`docs/CommandGuide/Foo.rst` <-> `<build-dir>/docs//man/Foo.1`.
These .rst files are also included during HTML generation so they are also
viewable online (as noted above) at e.g.
`https://llvm.org/docs/CommandGuide/Foo.html`.

Checking links
==============

The reachability of external links in the documentation can be checked by
running:

    cd docs/
    make -f Makefile.sphinx linkcheck

Doxygen page Output
==============

Install doxygen <http://www.stack.nl/~dimitri/doxygen/download.html> and dot2tex <https://dot2tex.readthedocs.io/en/latest>.

    cd <build-dir>
    cmake -DLLVM_ENABLE_DOXYGEN=On <llvm-top-src-dir>
    make doxygen-llvm # for LLVM docs
    make doxygen-clang # for clang docs

It will generate html in

    <build-dir>/docs/doxygen/html # for LLVM docs
    <build-dir>/tools/clang/docs/doxygen/html # for clang docs