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clang_configuration.bzl
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clang_configuration.bzl
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# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
# Exceptions. See /LICENSE for license information.
# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
"""Starlark repository rules to configure Clang (and LLVM) toolchain for Bazel.
These rules should be run from the `WORKSPACE` file to substitute appropriate
configured values into a `clang_detected_variables.bzl` file that can be used
by the actual toolchain configuration.
"""
def _run(repository_ctx, cmd):
"""Runs the provided `cmd`, checks for failure, and returns the result."""
exec_result = repository_ctx.execute(cmd)
if exec_result.return_code != 0:
fail("Unable to run command successfully: %s" % str(cmd))
return exec_result
def _detect_system_clang(repository_ctx):
"""Detects whether the system-provided clang can be used.
Returns a tuple of (is_clang, environment).
"""
# If the user provides an explicit `CC` environment variable, use that as
# the compiler. This should point at the `clang` executable to use.
cc = repository_ctx.os.environ.get("CC")
if cc:
version_output = _run(repository_ctx, [cc, "--version"]).stdout
if not "clang" in version_output:
fail("The `CC` environment variable is not a Clang compiler.")
return repository_ctx.path(cc)
# Try looking on the path. We only check for the normal name.
system_clang = repository_ctx.which("clang")
if not system_clang:
fail("Unable to find a `clang` executable on the system path.")
return system_clang
def _compute_clang_resource_dir(repository_ctx, clang):
"""Runs the `clang` binary to get its resource dir."""
output = _run(
repository_ctx,
[clang, "-no-canonical-prefixes", "--print-resource-dir"],
).stdout
# The only line printed is this path.
return output.splitlines()[0]
def _compute_mac_os_sysroot(repository_ctx):
"""Runs `xcrun` to extract the correct sysroot."""
xcrun = repository_ctx.which("xcrun")
if not xcrun:
fail("`xcrun` not found: is Xcode installed?")
output = _run(repository_ctx, [xcrun, "--show-sdk-path"]).stdout
return output.splitlines()[0]
def _compute_clang_cpp_include_search_paths(repository_ctx, clang, sysroot):
"""Runs the `clang` binary and extracts the include search paths.
Returns the resulting paths as a list of strings.
"""
# The only way to get this out of Clang currently is to parse the verbose
# output of the compiler when it is compiling C++ code.
cmd = [
clang,
# Avoid canonicalizing away symlinks.
"-no-canonical-prefixes",
# Extract verbose output.
"-v",
# Just parse the input, don't generate outputs.
"-fsyntax-only",
# Force the language to be C++.
"-x",
"c++",
# Read in an empty input file.
"/dev/null",
# Always use libc++.
"-stdlib=libc++",
]
# We need to use a sysroot to correctly represent a run on macOS.
if repository_ctx.os.name.lower().startswith("mac os"):
if not sysroot:
fail("Must provide a sysroot on macOS!")
cmd.append("--sysroot=" + sysroot)
# Note that verbose output is on stderr, not stdout!
output = _run(repository_ctx, cmd).stderr.splitlines()
# Return the list of directories printed for system headers. These are the
# only ones that Bazel needs us to manually provide. We find these by
# searching for a begin and end marker. We also have to strip off a leading
# space from each path.
include_begin = output.index("#include <...> search starts here:") + 1
include_end = output.index("End of search list.", include_begin)
return [
repository_ctx.path(s.lstrip(" "))
for s in output[include_begin:include_end]
]
def _configure_clang_toolchain_impl(repository_ctx):
# First just symlink in the untemplated parts of the toolchain repo.
repository_ctx.symlink(repository_ctx.attr._clang_toolchain_build, "BUILD")
repository_ctx.symlink(
repository_ctx.attr._clang_cc_toolchain_config,
"cc_toolchain_config.bzl",
)
# Find a Clang C++ compiler, and where it lives. We need to walk symlinks
# here as the other LLVM tools may not be symlinked into the PATH even if
# `clang` is. We also insist on finding the basename of `clang++` as that is
# important for C vs. C++ compiles.
clang = _detect_system_clang(repository_ctx)
clang = clang.realpath.dirname.get_child("clang++")
# Compute the various directories used by Clang.
resource_dir = _compute_clang_resource_dir(repository_ctx, clang)
sysroot_dir = None
if repository_ctx.os.name.lower().startswith("mac os"):
sysroot_dir = _compute_mac_os_sysroot(repository_ctx)
include_dirs = _compute_clang_cpp_include_search_paths(
repository_ctx,
clang,
sysroot_dir,
)
repository_ctx.template(
"clang_detected_variables.bzl",
repository_ctx.attr._clang_detected_variables_template,
substitutions = {
"{LLVM_BINDIR}": str(clang.dirname),
"{CLANG_RESOURCE_DIR}": resource_dir,
"{CLANG_INCLUDE_DIRS_LIST}": str(
[str(path) for path in include_dirs],
),
"{SYSROOT}": str(sysroot_dir),
},
executable = False,
)
configure_clang_toolchain = repository_rule(
implementation = _configure_clang_toolchain_impl,
configure = True,
local = True,
attrs = {
"_clang_toolchain_build": attr.label(
default = Label("//bazel/cc_toolchains:clang_toolchain.BUILD"),
allow_single_file = True,
),
"_clang_cc_toolchain_config": attr.label(
default = Label(
"//bazel/cc_toolchains:clang_cc_toolchain_config.bzl",
),
allow_single_file = True,
),
"_clang_detected_variables_template": attr.label(
default = Label(
"//bazel/cc_toolchains:clang_detected_variables.tpl.bzl",
),
allow_single_file = True,
),
},
environ = ["CC"],
)