[libc] Make libc symbols weak by default for bazel

[gchatelet]

Summary

This helps override llvm libc functions for experiment purposes.

[sivachandra]

Do we really want all of the symbols to be weak or only the memory functions?

[gchatelet]

Let's start with the memory functions and we can expand / shrink as needed.

[gchatelet]

• rebase and restrict weak attributes to memory functions

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

 )
 
 libc_function(
     name = "memcpy",
     srcs = ["src/string/memcpy.cpp"],
     hdrs = ["src/string/memcpy.h"],
     copts = ["-mllvm --tail-merge-threshold=0"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "memset",
     srcs = ["src/string/memset.cpp"],
     hdrs = ["src/string/memset.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "memmove",
     srcs = ["src/string/memmove.cpp"],
     hdrs = ["src/string/memmove.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "bcopy",
     srcs = ["src/string/bcopy.cpp"],
     hdrs = ["src/string/bcopy.h"],
     features = no_sanitize_features,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "memcmp",
     srcs = ["src/string/memcmp.cpp"],
     hdrs = ["src/string/memcmp.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":__support_integer_operations",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "bcmp",
     srcs = ["src/string/bcmp.cpp"],
     hdrs = ["src/string/bcmp.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "bzero",
     srcs = ["src/string/bzero.cpp"],
     hdrs = ["src/string/bzero.h"],
     features = no_sanitize_features,
+    weak = True,
     deps = [
         ":__support_common",
         ":string_memory_utils",
     ],
 )
 
 libc_function(
     name = "memrchr",

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

 # This file is licensed under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 """LLVM libc starlark rules for building individual functions."""
 
 load(":platforms.bzl", "PLATFORM_CPU_ARM64", "PLATFORM_CPU_X86_64")
 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
     "name". LLVM libc implementations of libc functions are in C++. So, this
     rule internally generates a C wrapper for the C++ implementation and adds
     it to the source list of the cc_library. This way, the C++ implementation
     and the C wrapper are both available in the cc_library.
 
     Args:
       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.
     """
     deps = deps or []
     deps.append(LIBC_ROOT_TARGET)
     copts = copts or []
     copts.append("-O3")
     copts.append("-fno-builtin")
 
     # We compile the code twice, the first target is suffixed with ".__internal__" and contains the
     # C++ functions in the "__llvm_libc" namespace. This allows us to test the function in the
     # presence of another libc.
     native.cc_library(
         name = name + INTERNAL_SUFFIX,
         srcs = srcs,
         deps = deps,
         copts = copts,
         linkstatic = 1,
         **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,
         deps = deps,
         copts = copts,
         linkstatic = 1,
         **kwargs
     )