[libcxx] Allow checking for newlib without using _NEWLIB_VERSION.

[abidh]

Summary

Existing code in libc++ uses _NEWLIB_VERSION to check for the presence
of newlib. But this macro is only set when a library header has been
included before that brings in its definition. In some cases, this macro
can't be used as a library header has not been included before it use.

This patch adds a CMake variable and corresponding macro that is used
to check for the presence of newlib in the following 2 cases:

1. The __config header is the generally the first header included. So

_NEWLIB_VERSION used in it will not be set. We check for
_LIBCPP_HAS_NEWLIB_LIBC and include newlib.h that will make sure that
_NEWLIB_VERSION is correctly defined.

2. The nasty_macros.h also has similar issue so _NEWLIB_VERSION can't be

used in it. We check for _LIBCPP_HAS_NEWLIB_LIBC to stub out a
problematic macro for newlib.

The new variable for newlib is quite similar to existing variable for MUSL.

[abidh]

ping.

[ldionne]

I see a couple issues with this:

1. I don't want to add a new CMake option just to detect an underlying libc. We should be able to detect it through other means. Actually why do we even need _LIBCPP_HAS_NEWLIB_LIBC?
2. We should have testers for this configuration. Newlib is not currently supported (well.. not officially at least), and we need some kind of testing if we pretend we're supporting it.

[abidh]

In D89040#2344267, @ldionne wrote:

Thanks for the review.

I see a couple issues with this:

Actually why do we even need _LIBCPP_HAS_NEWLIB_LIBC?

Consider example of __config, it already has some code conditional on _NEWLIB_VERSION. But this macro will always be undefined in this file as nothing is bringing in its definition. Note that _NEWLIB_VERSION is not defined by the compiler but it needs newlib.h to be included either directly or indirectly through some other library header. This is what _LIBCPP_HAS_NEWLIB_LIBC was trying to do. Detect newlib where we could not use _NEWLIB_VERSION.

1. I don't want to add a new CMake option just to detect an underlying libc. We should be able to detect it through other means.

One alternate I could think of what to use -include newlib.h in my compiler flags. That works but the problem with nasty_macros.h still remains unless I use an override of configure_compile_flags_header_includes in my customized config file to skip the check for nasty_macros.h. This can work but it will require all newlib users to handle these things in their own config files.

2. We should have testers for this configuration. Newlib is not currently supported (well.. not officially at least), and we need some kind of testing if we pretend we're supporting it.

You mentioned in https://reviews.llvm.org/D88825 that you will write some instruction. I will be happy to setup one.

libcxx/CMakeLists.txt

   "Build libc++ with support for a monotonic clock.
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
 option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)
+option(LIBCXX_HAS_NEWLIB_LIBC "Build libc++ with support for the Newlib C library" OFF)
 option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXX_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32 thread API" OFF)
 option(LIBCXX_HAS_EXTERNAL_THREAD_API

 config_define_if(LIBCXX_HAS_WIN32_THREAD_API _LIBCPP_HAS_THREAD_API_WIN32)
 config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
+config_define_if(LIBCXX_HAS_NEWLIB_LIBC _LIBCPP_HAS_NEWLIB_LIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define_if(LIBCXX_ENABLE_PARALLEL_ALGORITHMS _LIBCPP_HAS_PARALLEL_ALGORITHMS)
 

libcxx/include/__config

 #  define _LIBCPP_USING_DEV_RANDOM
 #endif
 
+// Include newlib.h to get _NEWLIB_VERSION
+#if defined(_LIBCPP_HAS_NEWLIB_LIBC)
+#  include <newlib.h>
+#endif
+
 #if !defined(_LIBCPP_LITTLE_ENDIAN) && !defined(_LIBCPP_BIG_ENDIAN)
 #  include <endian.h>
 #  if __BYTE_ORDER == __LITTLE_ENDIAN

libcxx/include/__config_site.in

 #cmakedefine _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 #cmakedefine _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
+#cmakedefine _LIBCPP_HAS_NEWLIB_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine _LIBCPP_HAS_THREAD_API_WIN32

libcxx/test/support/nasty_macros.h

 #endif
 
 #define __output NASTY_MACRO
+// The newlib uses __input in stdlib.h, so don't define it when newlib is
+// in use.
+#ifndef _LIBCPP_HAS_NEWLIB_LIBC
 #define __input NASTY_MACRO
+#endif
 
 #define __acquire NASTY_MACRO
 #define __release NASTY_MACRO