[tools] Use mold linker on Linux debug builds

setup/ubuntu/source_distribution/packages-noble-test-only.txt

@@ -1,5 +1,6 @@
 curl
 libstdc++6-10-dbg
+mold
 python3-dateutil
 python3-dbg
 python3-flask

tools/cc_toolchain/BUILD.bazel

@@ -1,5 +1,10 @@
 load("@bazel_skylib//lib:selects.bzl", "selects")
-load("@bazel_skylib//rules:common_settings.bzl", "int_flag", "string_flag")
+load(
+    "@bazel_skylib//rules:common_settings.bzl",
+    "bool_flag",
+    "int_flag",
+    "string_flag",
+)
 load("//tools/lint:lint.bzl", "add_lint_tests")
 load("//tools/skylark:sh.bzl", "sh_binary")
 
@@ -98,6 +103,33 @@ config_setting(
     values = {"compilation_mode": "dbg"},
 )
 
+# (Internal use only.)  When this is set to True, some Drake builds may use the
+# mold linker, primarily for its improved handling of DWARF debug
+# information. It should be False (the default) for all builds initiated via
+# Drake's CMake wrappers. See the other "mold_linker" settings below and issue
+# #21836.
+bool_flag(
+    name = "allow_mold_linker",
+    build_setting_default = False,
+)
+
+config_setting(
+    name = "mold_linker_allowed",
+    flag_values = {":allow_mold_linker": "True"},
+)
+
+selects.config_setting_group(
+    name = "use_mold_linker",
+    match_all = [
+        # TODO(rpoyner-tri): consider removing the "noble" requirement when
+        # Drake's support of "jammy" ends. Jammy is excluded because its
+        # version of the mold linker is too old.
+        "//tools:ubuntu_noble",
+        ":debug",
+        ":mold_linker_allowed",
+    ],
+)
+
 # In our developer builds and CMake installs, we use this as an optional hint
 # for configuring compiler warnings. It's not strictly necessary to set this,
 # but doing so might reduce warning spam during the build.

tools/cc_toolchain/bazel.rc

@@ -9,6 +9,11 @@ build --action_env=CCACHE_DISABLE=1
 # command line, or promote *any* warnings even from externals to errors via
 # --copt=-Werror.
 #
-# When compiilng Drake as an external package, this rcfile is not loaded and we
+# When compiling Drake as an external package, this rcfile is not loaded and we
 # won't promote warnings to errors by default.
 build --@drake//tools/cc_toolchain:error_severity=error
+
+# Similarly, only allow using the mold linker when Drake is the main
+# module. Other conditions will be checked as well; see
+# //tools/cc_toolchain:use_mold_linker for more details.
+build --@drake//tools/cc_toolchain:allow_mold_linker=true

tools/skylark/drake_cc.bzl

@@ -129,6 +129,12 @@ def _platform_copts(rule_copts, rule_gcc_copts, rule_clang_copts, cc_test = 0):
         "//conditions:default": [],
     })
 
+# The BASE_LINKOPTS are used for all drake_cc_{binary,library,test} rules.
+BASE_LINKOPTS = select({
+    "@drake//tools/cc_toolchain:use_mold_linker": ["-fuse-ld=mold"],
+    "//conditions:default": [],
+})
+
 def _check_library_deps_blacklist(name, deps):
     """Report an error if a library should not use something from deps."""
     if not deps:
@@ -596,6 +602,7 @@ def drake_cc_library(
         copts = [],
         clang_copts = [],
         gcc_copts = [],
+        linkopts = [],
         linkstatic = 1,
         internal = False,
         compile_once_per_scalar = False,
@@ -649,6 +656,7 @@ def drake_cc_library(
     should be surrounded with `#if DRAKE_ONCE_PER_SCALAR_PHASE == 0`.
     """
     new_copts = _platform_copts(copts, gcc_copts, clang_copts)
+    new_linkopts = BASE_LINKOPTS + linkopts
     new_tags = kwargs.pop("tags", None) or []
     if internal:
         if install_hdrs_exclude != []:
@@ -690,6 +698,7 @@ def drake_cc_library(
         deps = deps + add_deps,
         implementation_deps = implementation_deps,
         copts = new_copts,
+        linkopts = new_linkopts,
         linkstatic = linkstatic,
         declare_installed_headers = declare_installed_headers,
         install_hdrs_exclude = install_hdrs_exclude,
@@ -783,11 +792,13 @@ def drake_cc_binary(
     defaults using test_rule_args=["-f", "--bar=42"] or test_rule_size="baz".
     """
     new_copts = _platform_copts(copts, gcc_copts, clang_copts)
+    new_linkopts = BASE_LINKOPTS + linkopts
     new_srcs, add_deps = _maybe_add_pruned_private_hdrs_dep(
         base_name = name,
         srcs = srcs,
         deps = deps,
         copts = new_copts,
+        linkopts = new_linkopts,
         testonly = testonly,
         **kwargs
     )
@@ -801,7 +812,7 @@ def drake_cc_binary(
         testonly = testonly,
         linkshared = linkshared,
         linkstatic = linkstatic,
-        linkopts = linkopts,
+        linkopts = new_linkopts,
         features = [
             # We should deduplicate symbols while linking (for a ~6% reduction
             # in disk use), to conserve space in CI; see #18545 for details.
@@ -820,6 +831,7 @@ def drake_cc_binary(
             data = data + test_rule_data,
             deps = deps + add_deps,
             copts = copts,
+            linkopts = new_linkopts,
             gcc_copts = gcc_copts,
             size = test_rule_size,
             timeout = test_rule_timeout,
@@ -839,6 +851,7 @@ def drake_cc_test(
         copts = [],
         gcc_copts = [],
         clang_copts = [],
+        linkopts = [],
         allow_network = None,
         display = False,
         num_threads = None,
@@ -868,11 +881,13 @@ def drake_cc_test(
     kwargs = incorporate_display(kwargs, display = display)
     kwargs = incorporate_num_threads(kwargs, num_threads = num_threads)
     new_copts = _platform_copts(copts, gcc_copts, clang_copts, cc_test = 1)
+    new_linkopts = BASE_LINKOPTS + linkopts
     new_srcs, add_deps = _maybe_add_pruned_private_hdrs_dep(
         base_name = name,
         srcs = srcs,
         deps = deps,
         copts = new_copts,
+        linkopts = new_linkopts,
         **kwargs
     )
     cc_test(
@@ -882,6 +897,7 @@ def drake_cc_test(
         args = args,
         deps = deps + add_deps,
         copts = new_copts,
+        linkopts = new_linkopts,
         features = [
             # We should deduplicate symbols while linking (for a ~6% reduction
             # in disk use), to conserve space in CI; see #18545 for details.