POC fast_llvm toolchain

MODULE.bazel

@@ -280,6 +280,24 @@ llvm.toolchain(
     llvm_version = "22.1.7",
     stdlib = {"": "stdc++"},
 )
+
+# The default `toolchains_llvm` setup downloads the LLVM distribution through
+# its own generated repository. Use our repository rule instead so Bazel
+# downloads and extracts the known upstream archive directly.
+fast_llvm_repo = use_repo_rule("//:fast_llvm_repo.bzl", "fast_llvm_repo")
+
+# Create `@llvm_toolchain_llvm`, matching the repository name that
+# toolchains_llvm normally generates. Keep this version equal to
+# `llvm.toolchain(llvm_version = ...)`.
+fast_llvm_repo(
+    name = "llvm_toolchain_llvm",
+    llvm_version = "22.1.7",
+)
+
+# Point the existing LLVM toolchain configuration at the replacement
+# repository. This changes only where LLVM is obtained; its public target
+# layout remains compatible with toolchains_llvm's generated repository.
+llvm.toolchain_root(label = "@llvm_toolchain_llvm//:BUILD")
 llvm.sysroot(
     name = "llvm_toolchain",
     label = "@ubuntu24_04_sysroot_amd64//sysroot",
@@ -290,7 +308,10 @@ llvm.sysroot(
     label = "@ubuntu24_04_sysroot_arm64//sysroot",
     targets = ["linux-aarch64"],
 )
-use_repo(llvm, "llvm_toolchain", "llvm_toolchain_llvm")
+
+# `llvm_toolchain` contains the configured compiler toolchain. The custom
+# repository above provides `llvm_toolchain_llvm` under its original name.
+use_repo(llvm, "llvm_toolchain")
 
 bazel_dep(name = "score_toolchains_rust", version = "0.10.0", dev_dependency = True)
 

fast_llvm_repo.bzl

@@ -0,0 +1,164 @@
+# The supported LLVM releases. Each entry maps a requested version and host
+# architecture to the upstream archive Bazel downloads and verifies.
+_LLVM_DISTRIBUTIONS = {
+    "19.1.0": {
+        "aarch64": {
+            # Official prebuilt LLVM package for 64-bit ARM Linux hosts.
+            "url": "https://github.com/llvm/llvm-project/releases/download/llvmorg-19.1.0/clang+llvm-19.1.0-aarch64-linux-gnu.tar.xz",
+            # SHA-256 pinned by toolchains_llvm for this exact upstream archive.
+            "sha256": "7bb54afd330fe1a1c2d4c593fa1e2dbe2abd9bf34fb3597994ff41e443cf144b",
+        },
+        "x86_64": {
+            # Official prebuilt LLVM package for 64-bit x86 Linux hosts.
+            "url": "https://github.com/llvm/llvm-project/releases/download/llvmorg-19.1.0/LLVM-19.1.0-Linux-X64.tar.xz",
+            # SHA-256 of that archive. `ctx.download` rejects altered or corrupt
+            # downloads instead of unpacking them.
+            "sha256": "cee77d641690466a193d9b88c89705de1c02bbad46bde6a3b126793c0a0f2923",
+        },
+    },
+    "19.1.1": {
+        "aarch64": {
+            # Official prebuilt LLVM package for 64-bit ARM Linux hosts.
+            "url": "https://github.com/llvm/llvm-project/releases/download/llvmorg-19.1.1/clang+llvm-19.1.1-aarch64-linux-gnu.tar.xz",
+            # SHA-256 pinned by toolchains_llvm for this exact upstream archive.
+            "sha256": "414d2ebef10c5035e9df10a224e81b484dbe17d319373050d0c1b3b1467040d2",
+        },
+        "x86_64": {
+            # Official prebuilt LLVM package for 64-bit x86 Linux hosts.
+            "url": "https://github.com/llvm/llvm-project/releases/download/llvmorg-19.1.1/LLVM-19.1.1-Linux-X64.tar.xz",
+            # SHA-256 pinned by toolchains_llvm for this exact upstream archive.
+            "sha256": "8204de000b6a6921f0572e038336601e3225898e9a253c8aaa43b0a5fae8a4ce",
+        },
+    },
+    "22.1.7": {
+        "aarch64": {
+            # Official prebuilt LLVM package for 64-bit ARM Linux hosts.
+            "url": "https://github.com/llvm/llvm-project/releases/download/llvmorg-22.1.7/LLVM-22.1.7-Linux-ARM64.tar.xz",
+            # SHA-256 from toolchains_llvm's distribution metadata for this
+            # exact upstream archive. `ctx.download` rejects altered or corrupt
+            # downloads.
+            "sha256": "118ca2d3ad9da34367e05735317854e7977db45dc4c02a32af58da64c23b8789",
+        },
+        "x86_64": {
+            # Official prebuilt LLVM package for 64-bit x86 Linux hosts.
+            "url": "https://github.com/llvm/llvm-project/releases/download/llvmorg-22.1.7/LLVM-22.1.7-Linux-X64.tar.xz",
+            "sha256": "edb0522b41e261819c06ea437d249f9b8acfa413d3805bc9920eec6fb76ff830",
+        },
+    },
+}
+
+_ARCH_ALIASES = {
+    "aarch64": "aarch64",
+    "amd64": "x86_64",
+    "arm64": "aarch64",
+    "x86_64": "x86_64",
+}
+
+def _log(message):
+    print("[fast_llvm_repo] " + message)
+
+# Implementation of `fast_llvm_repo`. Repository rules run while Bazel is
+# preparing external dependencies, before analysing or building project code.
+def _fast_llvm_repo_impl(ctx):
+    # Select the archive native to the host which will execute this toolchain.
+    host_arch = _ARCH_ALIASES.get(ctx.os.arch)
+    if not host_arch:
+        fail("Unsupported host architecture: %s" % ctx.os.arch)
+
+    # Look up the requested release and its host-specific archive.
+    distributions = _LLVM_DISTRIBUTIONS.get(ctx.attr.llvm_version)
+    dist = distributions.get(host_arch) if distributions else None
+
+    # Fail with a useful message if MODULE.bazel asks for a version that has no
+    # URL and checksum entry yet.
+    if not dist:
+        fail("Unsupported LLVM version/host architecture: %s/%s" % (ctx.attr.llvm_version, host_arch))
+
+    # Choose a temporary file inside this external repository for the archive.
+    archive = ctx.path("llvm.tar.xz")
+
+    # Download the archive and verify its SHA-256 before using it.
+    _log("downloading LLVM %s for %s" % (ctx.attr.llvm_version, host_arch))
+    ctx.download(
+        url = dist["url"],
+        output = archive,
+        sha256 = dist["sha256"],
+    )
+    _log("download completed")
+
+    # Use the host's `tar` executable to unpack the xz archive. `ctx.which`
+    # returns None instead of guessing when `tar` is unavailable.
+    tar = ctx.which("tar")
+    if not tar:
+        fail("tar not found in PATH")
+
+    # LLVM's archive has many independently decompressible XZ blocks. Let xz
+    # decode them in parallel, then stream the resulting tar file directly to
+    # tar; materializing the 8+ GiB uncompressed archive would be wasteful.
+    # Keep the regular tar invocation for minimal Linux hosts without xz.
+    xz = ctx.which("xz")
+    if xz:
+        _log("extracting LLVM with xz -T0 and tar")
+        sh = ctx.which("sh")
+        if not sh:
+            fail("sh not found in PATH")
+        result = ctx.execute(
+            [
+                sh,
+                "-c",
+                "set -e; \"$1\" -T0 -dc \"$2\" | \"$3\" -xf - --strip-components=1 -C \"$4\"",
+                "fast_llvm_repo",
+                xz,
+                archive,
+                tar,
+                ctx.path("."),
+            ],
+            # Allow the relatively large archive up to 30 minutes to unpack.
+            timeout = 1800,
+            # Do not suppress command output from Bazel's repository-rule log.
+            quiet = False,
+        )
+    else:
+        _log("xz not found; extracting LLVM with tar fallback")
+        result = ctx.execute(
+            [
+                tar,
+                "-xf",
+                archive,
+                "--strip-components=1",
+                # Extract into the external repository represented by `ctx`.
+                "-C",
+                ctx.path("."),
+            ],
+            timeout = 1800,
+            quiet = False,
+        )
+
+    # Stop repository creation if extraction reported an error.
+    if result.return_code:
+        fail(result.stderr)
+    _log("extraction completed")
+
+    # The archive is no longer needed after extraction; leave only the LLVM
+    # distribution files in the external repository.
+    ctx.delete(archive)
+
+    # Generate the BUILD file expected by toolchains_llvm 1.7. LLVM 16 and
+    # later store compiler resources in a directory named by the major version.
+    _log("generating BUILD.bazel")
+    ctx.template(
+        "BUILD.bazel",
+        Label("@toolchains_llvm//toolchain:BUILD.llvm_repo.tpl"),
+        substitutions = {"{LLVM_VERSION}": ctx.attr.llvm_version.split(".")[0]},
+    )
+    _log("llvm setup completed")
+
+# Public repository rule used from MODULE.bazel. Its only user-facing input
+# is an LLVM version.
+fast_llvm_repo = repository_rule(
+    implementation = _fast_llvm_repo_impl,
+    attrs = {
+        # Required version key used to select an entry in `_LLVM_DISTRIBUTIONS`.
+        "llvm_version": attr.string(mandatory = True),
+    },
+)

quality/visibility_guard/public_targets.golden

@@ -1,3 +1,4 @@
+//:fast_llvm_repo.bzl
 //quality/compiler_warnings/clang:additional_warnings
 //quality/compiler_warnings/clang:default_flags
 //quality/compiler_warnings/clang:minimal_warnings