[libc++] Handle Newlib and picolibc using options #152968
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Previously, we relied on _NEWLIB_VERSION being defined to detect if we are building with Newlib or its derivative picolibc. This is somewhat unreliable, because whether or not _NEWLIB_VERSION is defined depends on if any headers from the C library have been included or not. This could lead to inconsistent definitions, for example as observed in bug llvm#152763. The code contained a workaround that detected picolibc by looking for its header file, with a TODO to replace this logic with a different mechanism. This change addresses that TODO by handling Newlib and picolibc similarly to how we handle musl: by providing a LIBCXX_HAS_*LIBC option and a corresponding _LIBCPP_HAS_* macro to be used in code. Specifically: - For Newlibc, we provide LIBCXX_HAS_NEWLIB_LIBC and _LIBCPP_HAS_NEWLIB_LIBC. - For picolibc, we provide LIBCXX_HAS_PICOLIBC and _LIBCPP_HAS_PICOLIBC. Existing code has been rewritten so that instead of checking for defined(_NEWLIB_VERSION) it checks for _LIBCPP_HAS_NEWLIB_LIBC || _LIBCPP_HAS_PICOLIBC.
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Previously, we relied on _NEWLIB_VERSION being defined to detect if we are building with Newlib or its derivative picolibc. This is somewhat unreliable, because whether or not NEWLIB_VERSION is defined depends on if any headers from the C library have been included or not. This could lead to inconsistent definitions, for example as observed in bug #152763. The code contained a workaround that detected picolibc by looking for its header file, with a TODO to replace this logic with a different mechanism. This change addresses that TODO by handling Newlib and picolibc similarly to how we handle musl: by providing a LIBCXX_HASLIBC option and a corresponding LIBCPP_HAS macro to be used in code.
Specifically:
For Newlibc, we provide LIBCXX_HAS_NEWLIB_LIBC and _LIBCPP_HAS_NEWLIB_LIBC.
For picolibc, we provide LIBCXX_HAS_PICOLIBC and _LIBCPP_HAS_PICOLIBC.
Existing code has been rewritten so that instead of checking for defined(_NEWLIB_VERSION) it checks for
_LIBCPP_HAS_NEWLIB_LIBC || _LIBCPP_HAS_PICOLIBC.