[libc] Make libc memory functions weak by default for bazel

This helps override llvm libc functions for experiment purposes.

Differential Revision: https://reviews.llvm.org/D138999

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

@@ -1096,6 +1096,7 @@ libc_function(
     hdrs = ["src/string/memcpy.h"],
     copts = ["-mllvm --tail-merge-threshold=0"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
@@ -1107,6 +1108,7 @@ libc_function(
     srcs = ["src/string/memset.cpp"],
     hdrs = ["src/string/memset.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
@@ -1118,6 +1120,7 @@ libc_function(
     srcs = ["src/string/memmove.cpp"],
     hdrs = ["src/string/memmove.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
@@ -1140,6 +1143,7 @@ libc_function(
     srcs = ["src/string/memcmp.cpp"],
     hdrs = ["src/string/memcmp.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":__support_integer_operations",
@@ -1152,6 +1156,7 @@ libc_function(
     srcs = ["src/string/bcmp.cpp"],
     hdrs = ["src/string/bcmp.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
@@ -1163,6 +1168,7 @@ libc_function(
     srcs = ["src/string/bzero.cpp"],
     hdrs = ["src/string/bzero.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",

utils/bazel/llvm-project-overlay/libc/libc_build_rules.bzl

@@ -10,7 +10,7 @@ load("@bazel_skylib//lib:selects.bzl", "selects")
 LIBC_ROOT_TARGET = ":libc_root"
 INTERNAL_SUFFIX = ".__internal__"
 
-def libc_function(name, srcs, deps = None, copts = None, **kwargs):
+def libc_function(name, srcs, weak = False, deps = None, copts = None, **kwargs):
     """Add target for a libc function.
 
     The libc function is eventually available as a cc_library target by name
@@ -23,6 +23,7 @@ def libc_function(name, srcs, deps = None, copts = None, **kwargs):
       name: Target name. It is normally the name of the function this target is
             for.
       srcs: The .cpp files which contain the function implementation.
+      weak: Whether the symbol is marked weak.
       deps: The list of target dependencies if any.
       copts: The list of options to add to the C++ compilation command.
       **kwargs: Other attributes relevant for a cc_library. For example, deps.
@@ -47,6 +48,8 @@ def libc_function(name, srcs, deps = None, copts = None, **kwargs):
 
     # This second target is the llvm libc C function.
     copts.append("-DLLVM_LIBC_PUBLIC_PACKAGING")
+    if weak:
+        copts.append("-DLLVM_LIBC_FUNCTION_ATTR=__attribute__((weak))")
     native.cc_library(
         name = name,
         srcs = srcs,