Extended Description

This is used by some glibc headers.

From: http://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html#Common-Predefined-Macros

USER_LABEL_PREFIX
This macro expands to a single token which is the prefix applied to user labels (symbols visible to C code) in assembly. For example, in the m68k-aout environment it expands to an `_', but in the m68k-coff environment it expands to nothing.

This macro will have the correct definition even if -f(no-)underscores is in use, but it will not be correct if target-specific options that adjust this prefix are used (e.g. the OSF/rose -mno-underscores option). 

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