Currently MCA takes instruction properties from scheduling model.
However, some instructions may execute differently depending on external
factors - for example, latency of memory instructions may vary
differently depending on whether the load comes from L1 cache, L2 or
DRAM. While MCA as a static analysis tool cannot model such differences
(and currently takes some static decision, e.g. all memory ops are
treated as L1 accesses), it makes sense to allow manual modification of
instruction properties to model different behavior (e.g. sensitivity of
code performance to cache misses in particular load instruction). This
patch addresses this need.
The library modification is intentionally generic - arbitrary
modifications to InstrDesc are allowed. The tool support is currently
limited to changing instruction latencies (single number applies to all
output arguments and MaxLatency) via coments in the input assembler
code; the format is the like this:
add (%eax), eax // LLVM-MCA-LATENCY:100
Users of MCA library can already make additional customizations; command
line tool can be extended in the future.
Note that InstructionView currently shows per-instruction information
according to scheduling model and is not affected by this change.
See https://github.com/llvm/llvm-project/issues/133429 for additional
clarifications (including explanation why existing customization
mechanisms do not provide required functionality)
---------
Co-authored-by: Min-Yih Hsu <min@myhsu.dev>
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 llvm/docs/
sphinx-build -b linkcheck . _build/lintcheck/
# report will be generated in _build/lintcheck/output.txt
Doxygen page Output
==============
Install doxygen <https://www.doxygen.nl/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
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