build: Allow to use atomic builtins from compiler-rt instead of libat…

…omic

compiler-rt, when it's built with the `COMPILER_RT_EXCLUDE_ATOMIC_BUILTIN`
option disabled, provides all atomic builtins and serves as a full
replacement for libatomic.

By default, that option is enabled[0], meaning that default builds of
compiler-rt, followed by the most of Linux distributions (Fedora,
openSUSE, Ubuntu), do **not** provide all necessary atomics.

However, FreeBSD[1] and Gentoo (with LLVM-based profiles)[2] disable it,
therefore providing compiler-rt with atomic builtins.

That difference can be detected by checking the symbol table of
compiler-rt:

```
nm $(clang -print-libgcc-file-name) | grep __atomic
```

No matching symbols indicate lack of atomic builtins and necessity of
linking libatomic. Matching symbols indicate a possibility to not link
libatomic.

Given that difference, provide the `--use-compiler-rt-atomics` option
in `configure.py`. When enabled, the configure script checks whether
compiler-rt provides atomics and, if yes, does not link libatomic. By
default, without that option enabled, a build with clang links libatomic.

For more context, see the discussion on Gentoo bugzilla[3].

[0] https://github.com/llvm/llvm-project/blob/llvmorg-19.1.6/compiler-rt/lib/builtins/CMakeLists.txt#L227-L229
[1] https://cgit.freebsd.org/src/commit/?id=7b67d47c70cca47f65fbbc9d8607b7516c2a82ee
[2] gentoo/gentoo@63b4ae7
[3] https://bugs.gentoo.org/911340

configure.py

@@ -136,6 +136,11 @@
     dest='use_clang',
     default=None,
     help='use clang instead of gcc')
+parser.add_argument('--use-compiler-rt-atomics',
+    action='store_true',
+    dest='use_compiler_rt_atomics',
+    default=None,
+    help='use compiler-rt atomic builtins instead of libatomic')
 
 parser.add_argument('--dest-os',
     action='store',
@@ -1086,6 +1091,31 @@ def try_check_compiler(cc, lang):
   return (True, is_clang, clang_version, gcc_version)
 
 
+def try_check_compiler_rt_atomics(cc):
+  try:
+    proc = subprocess.Popen(shlex.split(cc) + ['-print-libgcc-file-name'],
+                            stdout=subprocess.PIPE)
+    with proc:
+      rt_file_name = to_utf8(proc.communicate()[0]).strip()
+  except OSError as e:
+    return (False, False, False, False)
+
+  nm = shutil.which('nm') or shutil.which('llvm-nm')
+  if not nm:
+    return (True, False, False, False)
+
+  try:
+    proc = subprocess.Popen(shlex.split(nm) + shlex.split(rt_file_name),
+                            stdout=subprocess.PIPE)
+    with proc:
+      symbols = to_utf8(proc.communicate()[0]).strip()
+  except OSError as e:
+    return (True, True, False, False)
+
+  has_atomics = '__atomic' in symbols
+  return (True, True, True, has_atomics)
+
+
 #
 # The version of asm compiler is needed for building openssl asm files.
 # See deps/openssl/openssl.gypi for detail.
@@ -1417,6 +1447,23 @@ def configure_node(o):
   if options.use_clang:
     o['variables']['clang'] = 1
 
+  # Allow using compiler-rt atomic builtins instead of libatomic.
+  if options.use_compiler_rt_atomics:
+    if not o['variables']['clang'] == 1:
+      error('--use-compiler-rt-atomics can be used only with clang')
+    cc_ok, found_nm, nm_ok, has_atomics = try_check_compiler_rt_atomics(CC)
+    if not cc_ok:
+      error('''Failed to execute `''' + CC + ''' -print-libgcc-file-name` to
+            find the runtime library.''')
+    if not found_nm:
+      error('''Could not find nm or llvm-nm.''')
+    if not nm_ok:
+      error('''Failed to execute nm.''')
+    if not has_atomics:
+      error('''compiler-rt does not have atomics.''')
+    if has_atomics:
+      o['variables']['compiler_rt_atomics'] = 1
+
   cross_compiling = (options.cross_compiling
                      if options.cross_compiling is not None
                      else target_arch != host_arch)

node.gyp

@@ -510,7 +510,7 @@
           '-Wl,-bnoerrmsg',
         ],
       }],
-      ['OS=="linux" and clang==1', {
+      ['OS=="linux" and clang==1 and compiler_rt_atomics!=1', {
         'libraries': ['-latomic'],
       }],
     ],

tools/v8_gypfiles/v8.gyp

@@ -1298,7 +1298,7 @@
         }],
         # Platforms that don't have Compare-And-Swap (CAS) support need to link atomic library
         # to implement atomic memory access
-        ['v8_current_cpu in ["mips64", "mips64el", "ppc", "arm", "riscv64", "loong64"]', {
+        ['compiler_rt_atomics!=1 and v8_current_cpu in ["mips64", "mips64el", "ppc", "arm", "riscv64", "loong64"]', {
           'link_settings': {
             'libraries': ['-latomic', ],
           },