[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/skylark/drake_cc.bzl

@@ -110,6 +110,20 @@ BASE_COPTS = select({
     ("//conditions:default", []),
 ]))
 
+# The _base_linkopts() are used for all drake_cc_{binary,library,test} rules.
+def _base_linkopts():
+    is_debug = select({
+        "//tools/cc_toolchain:debug": True,
+        "//conditions:default": False,
+    })
+    is_linux = select({
+        "@drake//tools/skylark:linux": True,
+        "//conditions:default": False,
+    })
+    if is_linux and is_debug:
+        return ["-fuse-ld=mold"]
+    return []
+
 def _platform_copts(rule_copts, rule_gcc_copts, rule_clang_copts, cc_test = 0):
     """Returns the concatenation of Drake's platform-specific BASE_COPTS,
     plus the passed-in rule_copts, plus the passed-in rule_{cc}_copts iff
@@ -596,6 +610,7 @@ def drake_cc_library(
         copts = [],
         clang_copts = [],
         gcc_copts = [],
+        linkopts = [],
         linkstatic = 1,
         internal = False,
         compile_once_per_scalar = False,
@@ -649,6 +664,7 @@ 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 +706,7 @@ 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 +800,7 @@ 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 +819,7 @@ 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 +857,7 @@ def drake_cc_test(
         copts = [],
         gcc_copts = [],
         clang_copts = [],
+        linkopts = [],
         allow_network = None,
         display = False,
         num_threads = None,
@@ -868,6 +887,7 @@ 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 +902,7 @@ 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.