Directly use TCMalloc rather than the system malloc on Linux.

This improve the toolchain's performance by about 10%.

It will also allow us to leverage TCMalloc's extensions to do heap
profiling and get other information about how efficiently we're using
the heap.

Note that currently this causes all of our builds to produce a warning
due to an issue with `rules_python` and multiple modules registering
python toolchains:
bazelbuild/rules_python#1818

This is also only enabled on Linux as there is no support for other OSes
at the moment.

.bazelrc

@@ -110,6 +110,9 @@ build --action_env=LANG=en_US.UTF-8
 build --enable_platform_specific_config
 build:linux --define=pfm=1
 
+# Enable TCMalloc on Linux as well.
+build:linux --custom_malloc=@tcmalloc//tcmalloc
+
 # Allow users to override any of the flags desired by importing a user-specific
 # RC file here if present.
 try-import %workspace%/user.bazelrc

MODULE.bazel

@@ -43,6 +43,17 @@ archive_override(
     urls = ["https://sourceforge.net/projects/perfmon2/files/libpfm4/libpfm-{0}.tar.gz".format(libpfm_version)],
 )
 
+# The registry has a snapshot, but upstream is active and not regularly marking
+# releases. We start with the BCR snapshot to avoid a miss and then override it
+# with a more recent commit.
+bazel_dep(name = "tcmalloc", version = "0.0.0-20240411-5ed309d", dev_dependency = True)
+git_override(
+    module_name = "tcmalloc",
+    # HEAD as of 2024-07-14.
+    commit = "923df94c922e0cd2d0512c1662d374f63c2c0c96",
+    remote = "https://github.com/google/tcmalloc.git",
+)
+
 bazel_dep(name = "rules_bison", version = "0.2.2")
 bazel_dep(name = "rules_flex", version = "0.2.1")
 bazel_dep(name = "rules_m4", version = "0.2.3")

bazel/cc_toolchains/clang_cc_toolchain_config.bzl

@@ -1076,10 +1076,13 @@ def _impl(ctx):
         target_system_name = identifier,
         target_cpu = ctx.attr.target_cpu,
 
+        # This is used to expose a "flag" that `config_setting` rules can use to
+        # determine if the compiler is Clang.
+        compiler = "clang",
+
         # These attributes aren't meaningful at all so just use placeholder
         # values.
         target_libc = "local",
-        compiler = "local",
         abi_version = "local",
         abi_libc_version = "local",