[libc][bazel] Use Bazel aspects to implement libc_release_library.

[vonosmas]

Instead of creating hundreds of implicit "filegroup" targets to keep track of sources and textual headers required to build each libc function or helper library, use Bazel aspects (see https://bazel.build/versions/8.0.0/extending/aspects), which enable transparent collection of transitive sources / textual headers while walking the dependency DAG, and minimizes the Starlark overhead.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Alexey Samsonov (vonosmas)

Changes

Instead of creating hundreds of implicit "filegroup" targets to keep track of sources and textual headers required to build each libc function or helper library, use Bazel aspects (see https://bazel.build/versions/8.0.0/extending/aspects), which enable transparent collection of transitive sources / textual headers while walking the dependency DAG, and minimizes the Starlark overhead.

---

Full diff: https://github.com/llvm/llvm-project/pull/134948.diff

1 Files Affected:

• (modified) utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl (+112-53)

diff --git a/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl b/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
index 4e73170be1e81..5640bf02392fb 100644
--- a/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
+++ b/utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl
@@ -10,8 +10,9 @@ load(":libc_configure_options.bzl", "LIBC_CONFIGURE_OPTIONS")
 load(":libc_namespace.bzl", "LIBC_NAMESPACE")
 load(":platforms.bzl", "PLATFORM_CPU_X86_64")
 
+# TODO: Remove this helper function once all donwstream users are migrated.
 def libc_internal_target(name):
-    return name + ".__internal__"
+    return name
 
 def libc_common_copts():
     root_label = Label(":libc")
@@ -44,84 +45,134 @@ def libc_release_copts():
     })
     return copts + platform_copts
 
-def _libc_library(name, copts = [], deps = [], local_defines = [], **kwargs):
+def _libc_library(name, **kwargs):
     """Internal macro to serve as a base for all other libc library rules.
 
     Args:
       name: Target name.
-      copts: The special compiler options for the target.
-      deps: The list of target dependencies if any.
-      local_defines: The list of target local_defines if any.
       **kwargs: All other attributes relevant for the cc_library rule.
     """
 
+    for attr in ["copts", "local_defines"]:
+        if attr in kwargs:
+            fail("disallowed attribute: '{}' in rule: '{}'".format(attr, name))
     native.cc_library(
         name = name,
-        copts = copts + libc_common_copts(),
-        local_defines = local_defines + LIBC_CONFIGURE_OPTIONS,
-        deps = deps,
+        copts = libc_common_copts(),
+        local_defines = LIBC_CONFIGURE_OPTIONS,
         linkstatic = 1,
         **kwargs
     )
 
-def _libc_library_filegroups(
-        name,
-        is_function,
-        srcs = [],
-        hdrs = [],
-        textual_hdrs = [],
-        deps = [],
-        # We're not using kwargs, but instead explicitly list all possible
-        # arguments that can be passed to libc_support_library or
-        # libc_function macros. This is done to limit the configurability
-        # and ensure the consistent and tightly controlled set of flags
-        # (see libc_common_copts and libc_release_copts above) is used to build
-        # libc code both for tests and for release configuration.
-        target_compatible_with = None):  # @unused
-    """Internal macro to collect sources and headers required to build a library.
-    """
-
-    # filegroups created from "libc_function" macro has an extra "_fn" in their
-    # name to ensure that no other libc target can depend on libc_function.
-    prefix = name + ("_fn" if is_function else "")
-    native.filegroup(
-        name = prefix + "_srcs",
-        srcs = srcs + hdrs + [dep + "_srcs" for dep in deps],
-    )
-    native.filegroup(
-        name = prefix + "_textual_hdrs",
-        srcs = textual_hdrs + [dep + "_textual_hdrs" for dep in deps],
-    )
-
 # A convenience function which should be used to list all libc support libraries.
 # Any library which does not define a public function should be listed with
 # libc_support_library.
 def libc_support_library(name, **kwargs):
     _libc_library(name = name, **kwargs)
-    _libc_library_filegroups(name = name, is_function = False, **kwargs)
 
 def libc_function(name, **kwargs):
     """Add target for a libc function.
 
     This macro creates an internal cc_library that can be used to test this
-    function, and creates filegroups required to include this function into
-    a release build of libc.
+    function.
 
     Args:
-      name: Target name. It is normally the name of the function this target is
-            for.
+      name: Target name. Typically the name of the function this target is for.
       **kwargs: Other attributes relevant for a cc_library. For example, deps.
     """
 
-    # Build "internal" library with a function, the target has ".__internal__" suffix and contains
-    # C++ functions in the "LIBC_NAMESPACE" namespace. This allows us to test the function in the
+    # Builds "internal" library with a function, exposed as a C++ function in
+    # the "LIBC_NAMESPACE" namespace. This allows us to test the function in the
     # presence of another libc.
     _libc_library(
         name = libc_internal_target(name),
         **kwargs
     )
 
-    _libc_library_filegroups(name = name, is_function = True, **kwargs)
+# LibcLibraryInfo is used to collect all sources and textual headers required
+# to build a particular libc_function or libc_support_library.
+LibcLibraryInfo = provider(
+    fields = ["srcs", "textual_hdrs"],
+)
+
+def _get_libc_info_aspect_impl(target, ctx):
+    maybe_srcs = getattr(ctx.rule.attr, "srcs", [])
+    maybe_hdrs = getattr(ctx.rule.attr, "hdrs", [])
+    maybe_textual_hdrs = getattr(ctx.rule.attr, "textual_hdrs", [])
+    maybe_deps = getattr(ctx.rule.attr, "deps", [])
+    return LibcLibraryInfo(
+        srcs = depset(
+            [
+                f
+                for src in maybe_srcs + maybe_hdrs
+                for f in src.files.to_list()
+                if f.is_source
+            ],
+            transitive = [
+                dep[LibcLibraryInfo].srcs
+                for dep in maybe_deps
+                if LibcLibraryInfo in dep
+            ],
+        ),
+        textual_hdrs = depset(
+            [
+                f
+                for hdr in maybe_textual_hdrs
+                for f in hdr.files.to_list()
+                if f.is_source
+            ],
+            transitive = [
+                dep[LibcLibraryInfo].textual_hdrs
+                for dep in maybe_deps
+                if LibcLibraryInfo in dep
+            ],
+        ),
+    )
+
+_get_libc_info_aspect = aspect(
+    implementation = _get_libc_info_aspect_impl,
+    attr_aspects = ["deps"],
+)
+
+def _get_libc_srcs_impl(ctx):
+    return DefaultInfo(
+        files = depset(transitive = [
+            fn[LibcLibraryInfo].srcs
+            for fn in ctx.attr.libs
+        ]),
+    )
+
+# get_libc_srcs returns the list of sources required to build all
+# specified libraries.
+get_libc_srcs = rule(
+    implementation = _get_libc_srcs_impl,
+    attrs = {
+        "libs": attr.label_list(
+            mandatory = True,
+            aspects = [_get_libc_info_aspect],
+        ),
+    },
+)
+
+def _get_libc_textual_hdrs_impl(ctx):
+    return DefaultInfo(
+        files = depset(transitive = [
+            fn[LibcLibraryInfo].textual_hdrs
+            for fn in ctx.attr.libs
+        ]),
+    )
+
+# get_libc_textual_hdrs returns the list of textual headers required to compile
+# all specified libraries.
+get_libc_textual_hdrs = rule(
+    implementation = _get_libc_textual_hdrs_impl,
+    attrs = {
+        "libs": attr.label_list(
+            mandatory = True,
+            aspects = [_get_libc_info_aspect],
+        ),
+    },
+)
 
 def libc_release_library(
         name,
@@ -138,15 +189,18 @@ def libc_release_library(
         **kwargs: Other arguments relevant to cc_library.
     """
 
-    # Combine all sources into a single filegroup to avoid repeated sources error.
-    native.filegroup(
+    get_libc_srcs(
         name = name + "_srcs",
-        srcs = [function + "_fn_srcs" for function in libc_functions],
+        libs = libc_functions,
     )
 
+    get_libc_textual_hdrs(
+        name = name + "_textual_hdrs",
+        libs = libc_functions,
+    )
     native.cc_library(
         name = name + "_textual_hdr_library",
-        textual_hdrs = [function + "_fn_textual_hdrs" for function in libc_functions],
+        textual_hdrs = [":" + name + "_textual_hdrs"],
     )
 
     weak_attributes = [
@@ -175,15 +229,20 @@ def libc_header_library(name, hdrs, deps = [], **kwargs):
       **kwargs: All other attributes relevant for the cc_library rule.
     """
 
-    # Combine sources from dependencies to create a single cc_library target.
-    native.filegroup(
+    get_libc_srcs(
         name = name + "_hdr_deps",
-        srcs = [dep + "_srcs" for dep in deps],
+        libs = deps,
+    )
+
+    get_libc_textual_hdrs(
+        name = name + "_textual_hdrs",
+        libs = deps,
     )
     native.cc_library(
         name = name + "_textual_hdr_library",
-        textual_hdrs = [dep + "_textual_hdrs" for dep in deps],
+        textual_hdrs = [":" + name + "_textual_hdrs"],
     )
+
     native.cc_library(
         name = name,
         # Technically speaking, we should put _hdr_deps in srcs, as they are