[libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (#143187)

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Author: lntue

libc/cmake/modules/LLVMLibCCompileOptionRules.cmake

@@ -106,6 +106,10 @@ function(_get_compile_options_from_config output_var)
     list(APPEND config_options "-DLIBC_MATH=${LIBC_CONF_MATH_OPTIMIZATIONS}")
   endif()
 
+  if(LIBC_CONF_ERRNO_MODE)
+    set(APPEND config_options "-DLIBC_ERRNO_MODE=${LIBC_CONF_ERRNO_MODE}")
+  endif()
+
   set(${output_var} ${config_options} PARENT_SCOPE)
 endfunction(_get_compile_options_from_config)
 

libc/config/config.json

@@ -2,7 +2,7 @@
   "errno": {
     "LIBC_CONF_ERRNO_MODE": {
       "value": "LIBC_ERRNO_MODE_DEFAULT",
-      "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, and LIBC_ERRNO_MODE_SYSTEM."
+      "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, LIBC_ERRNO_MODE_SYSTEM, and LIBC_ERRNO_MODE_SYSTEM_INLINE."
     }
   },
   "printf": {

libc/docs/dev/code_style.rst

@@ -101,7 +101,7 @@ test infrastructure itself can be affected. To avoid perturbing the unit test
 infrastructure around the setting of ``errno``, the following rules are to be
 followed:
 
-#. A special macro named ``libc_errno`` defined in ``src/errno/libc_errno.h``
+#. A special macro named ``libc_errno`` defined in ``src/__support/libc_errno.h``
    should be used when setting ``errno`` from libc runtime code. For example,
    code to set ``errno`` to ``EINVAL`` should be:
 
@@ -117,7 +117,7 @@ followed:
    `ErrorOr <https://github.com/llvm/llvm-project/blob/main/libc/src/__support/error_or.h>`_
    to return error values.
 
-#. The header file ``src/errno/libc_errno.h`` is shipped as part of the target
+#. The header file ``src/__support/libc_errno.h`` is shipped as part of the target
    corresponding to the ``errno`` entrypoint ``libc.src.errno.errno``. We do
    not in general allow dependencies between entrypoints. However, the ``errno``
    entrypoint is the only exceptional entrypoint on which other entrypoints

libc/shared/fp_bits.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_SHARED_FP_BITS_H
 #define LLVM_LIBC_SHARED_FP_BITS_H
 
+#include "libc_common.h"
 #include "src/__support/FPUtil/FPBits.h"
 
 namespace LIBC_NAMESPACE_DECL {

libc/shared/libc_common.h

@@ -0,0 +1,26 @@
+//===-- Common defines for sharing LLVM libc with LLVM projects -*- C++ -*-===//
+//
+// Part of the LLVM Project, 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_LIBC_COMMON_H
+#define LLVM_LIBC_SHARED_LIBC_COMMON_H
+
+// Use system errno.
+#ifdef LIBC_ERRNO_MODE
+#if LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM_INLINE
+#error                                                                         \
+    "LIBC_ERRNO_MODE was set to something different from LIBC_ERRNO_MODE_SYSTEM_INLINE."
+#endif // LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM_INLINE
+#else
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM_INLINE
+#endif // LIBC_ERRNO_MODE
+
+#ifndef LIBC_NAMESPACE
+#define LIBC_NAMESPACE __llvm_libc
+#endif // LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SHARED_LIBC_COMMON_H

libc/shared/rpc_server.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_SHARED_RPC_SERVER_H
 #define LLVM_LIBC_SHARED_RPC_SERVER_H
 
+#include "libc_common.h"
 #include "src/__support/RPC/rpc_server.h"
 
 namespace LIBC_NAMESPACE_DECL {

libc/shared/str_to_float.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_SHARED_STR_TO_FLOAT_H
 #define LLVM_LIBC_SHARED_STR_TO_FLOAT_H
 
+#include "libc_common.h"
 #include "src/__support/str_to_float.h"
 
 namespace LIBC_NAMESPACE_DECL {

libc/shared/str_to_integer.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_SHARED_STR_TO_INTEGER_H
 #define LLVM_LIBC_SHARED_STR_TO_INTEGER_H
 
+#include "libc_common.h"
 #include "src/__support/str_to_integer.h"
 
 namespace LIBC_NAMESPACE_DECL {

libc/src/__support/CMakeLists.txt

@@ -1,6 +1,15 @@
 add_subdirectory(CPP)
 add_subdirectory(macros)
 
+add_header_library(
+  libc_errno
+  HDRS
+    libc_errno.h
+  DEPENDS
+    libc.hdr.errno_macros
+    libc.src.__support.macros.config
+)
+
 add_header_library(
   block
   HDRS

libc/src/__support/FPUtil/FEnvImpl.h

@@ -12,10 +12,10 @@
 #include "hdr/fenv_macros.h"
 #include "hdr/math_macros.h"
 #include "hdr/types/fenv_t.h"
+#include "src/__support/libc_errno.h"
 #include "src/__support/macros/attributes.h" // LIBC_INLINE
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/properties/architectures.h"
-#include "src/errno/libc_errno.h"
 
 #if defined(LIBC_TARGET_ARCH_IS_AARCH64) && defined(__ARM_FP)
 #if defined(__APPLE__)