[tools] Use mold linker on Linux debug builds

Author: rpoyner-tri

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

@@ -1,6 +1,7 @@
 curl
 kcov
 libstdc++6-10-dbg
+mold
 python3-dateutil
 python3-dbg
 python3-flask

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/lint/library_lint_reporter.py

@@ -63,7 +63,8 @@ def main():
         # Filter out false positives.  All C++ code is OK to depend on these.
         item for item in extra_deps
         if not (item.startswith("//tools/cc_toolchain:")
-                or "@bazel_tools//" in item)
+                or "@bazel_tools//" in item
+                or item.startswith("//tools/skylark"))
     ]
     if extra_deps:
         print(("ERROR: Extra deps in {}'s drake_cc_package_library.").format(

tools/skylark/BUILD.bazel

@@ -1,8 +1,14 @@
+load("@bazel_skylib//lib:selects.bzl", "selects")
 load("//tools/lint:lint.bzl", "add_lint_tests")
 load("//tools/skylark:drake_py.bzl", "drake_py_binary", "drake_py_unittest")
 
 package(default_visibility = ["//visibility:public"])
 
+config_setting(
+    name = "debug",
+    values = {"compilation_mode": "dbg"},
+)
+
 config_setting(
     name = "linux",
     constraint_values = ["@platforms//os:linux"],
@@ -13,6 +19,14 @@ config_setting(
     constraint_values = ["@platforms//os:osx"],
 )
 
+selects.config_setting_group(
+    name = "linux_debug",
+    match_all = [
+        ":linux",
+        ":debug",
+    ],
+)
+
 drake_py_binary(
     name = "rewrite_osx_ar_hidden",
     srcs = ["rewrite_osx_ar_hidden.py"],

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/skylark:linux_debug": ["-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,
@@ -783,6 +792,7 @@ 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,
@@ -801,7 +811,7 @@ 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.
@@ -839,6 +849,7 @@ def drake_cc_test(
         copts = [],
         gcc_copts = [],
         clang_copts = [],
+        linkopts = [],
         allow_network = None,
         display = False,
         num_threads = None,
@@ -868,6 +879,7 @@ 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,
@@ -882,6 +894,7 @@ 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.