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The OpenACC standard specifies an `atomic` construct in section 2.12 (of 3.3 spec), used to ensure that a specific location is accessed or updated atomically. Four different clauses are allowed: `read`, `write`, `update`, or `capture`. If no clause appears, it is as if `update` is used. The OpenMP specification defines the same clauses for `omp atomic`. The types of expression and the clauses in the OpenACC spec match the OpenMP spec exactly. The main difference is that the OpenMP specification is a superset - it includes clauses for `hint` and `memory order`. It also allows conditional expression statements. But otherwise, the expression definition matches. Thus, for OpenACC, we refactor and reuse the OpenMP implementation as follows: * The atomic operations are duplicated in OpenACC dialect. This is preferable so that each language's semantics are precisely represented even if specs have divergence. * However, since semantics overlap, a common interface between the atomic operations is being added. The semantics for the interfaces are not generic enough to be used outside of OpenACC and OpenMP, and thus new folders were added to hold common pieces of the two dialects. * The atomic interfaces define common accessors (such as getting `x` or `v`) which match the OpenMP and OpenACC specs. It also adds common verifiers intended to be called by each dialect's operation verifier. * The OpenMP write operation was updated to use `x` and `expr` to be consistent with its other operations (that use naming based on spec). The frontend lowering necessary to generate the dialect can also be reused. This will be done in a follow up change.
Introduction
Warning The Bazel build is experimental and best-effort, supported in line with the policy for LLVM's peripheral support tier. LLVM's official build system is CMake. If in doubt use that. If you make changes to LLVM, you're expected to update the CMake build but you don't need to update Bazel build files. Reviewers should not ask authors to update Bazel build files unless the author has opted in to support Bazel. Keeping the Bazel build files up-to-date is on the people who use the Bazel build.
Bazel is a multi-language build system focused on reproducible builds to enable dependency analysis and caching for fast incremental builds.
The main motivation behind the existence of an LLVM Bazel build is that a number of projects that depend on LLVM use Bazel, and Bazel works best when it knows about the whole source tree (as opposed to installing artifacts coming from another build system). Community members are also welcome to use Bazel for their own development as long as they continue to maintain the official CMake build system. See also, the proposal for adding this configuration.
Quick Start
git clone https://github.com/llvm/llvm-project.git; cd llvm-projectif you don't have a checkout yet.- Install Bazel at the version indicated by .bazelversion,
following the official instructions, if you don't have it installed yet:
https://docs.bazel.build/versions/main/install.html.
- You can also install and use bazelisk which automates downloading the proper bazel version
cd utils/bazelbazel build --config=generic_clang @llvm-project//...- If you're using clang, it's expected that lld is also available
- If you're using MSVC or gcc, instead of
--config=generic_clang, pass--config=generic_gccor--config=msvc - To specify a specific local compiler to use, add the following bazel
flag:
--repo_env=CC=/usr/bin/clang--config=generic_clang/--config=generic_gccby default set--repo_env=CC=clang/--repo_env=CC=gcc, usingclang/gccon thePATH
Configuration
The repository .bazelrc will import user-specific settings from a
user.bazelrc file (in addition to the standard locations). Adding your typical
config setting is recommended.
build --config=generic_clang
You can enable disk caching, which will cache build results
build --disk_cache=~/.cache/bazel-disk-cache
You can instruct Bazel to use a ramdisk for its sandboxing operations via --sandbox_base, which can help avoid IO bottlenecks for the symlink strategy used for sandboxing. This is especially important with many inputs and many cores (see https://github.com/bazelbuild/bazel/issues/11868):
build --sandbox_base=/dev/shm
Bear in mind that this requires that your ramdisk is of sufficient size to hold any temporary files. Anecdotally, 1GB should be sufficient.
Coverage
The LLVM, MLIR, and Clang subprojects have configurations for Linux (Clang and GCC), Mac (Clang and GCC), and Windows (MSVC). Configuration options that are platform-specific are selected for in defines. Many are also hardcoded to the values currently used by all supported configurations. If there is a configuration you'd like to use that isn't supported, please send a patch.
Continuous Testing
A Buildkite pipeline runs the full Bazel build on every commit to the main branch. Notifications of failures are sent to the llvm-bazel-alerts google group, which anyone is free to join. Currently, the behavior is just to send an email on each failure using Buildkite's built-in notification system, so if you subscribe, it is highly recommended that you set up email filters or some other mechanism to not flood your inbox. More sophisticated notifications, e.g. only on status change or routed based on blamelist are TODO (contributions welcome).
Pre-merge Testing
A Buildkite pipeline runs the full Bazel build as a pre-merge test using the LLVM pre-merge testing. It is triggered on all changes to the utils/bazel directory and when the patch author is a member of the Bazel Phabricator project. If you use or benefit from the Bazel build, please join the project so that you can help keep it green. As a bonus, it runs in under 5 minutes, much faster than any of the other pre-merge builds.
Usage in Downstream Projects
To use in dependent projects using Bazel, you can import LLVM and then use the
provided configuration rule. See example usage in the examples/ directory.