Added support for _POSIX_MAPPED_FILES if defined in source

[CoderRC]

I have created my own library for mingw32 that includes the POSIX functions that the source needs to compile. That is how I will support llama.cpp for windows using mingw32 compilation in mys2. Compiled successfully using:
make LDFLAGS='-D_POSIX_MAPPED_FILES -lmingw32_extended'

[anzz1]

Since you're one of the few using mingw32 , could you please look into this first ? #551

Where does the _POSIX_MAPPED_FILES get defined exactly?

[jart]

I'd be reluctant to merge this because mmap() is a mandatory feature.

   _POSIX_MAPPED_FILES
           Support for memory mapped files is mandatory in POSIX.1-2017.

           This facility provides for the mapping of regular files into the
           process address space.

           Both this facility and the Shared Memory Objects option provide
           shared access to memory objects in the process address space. The
           mmap() and munmap() functions provide the functionality of
           existing practice for mapping regular files. This functionality
           was deemed unnecessary, if not inappropriate, for embedded systems
           applications and is expected to be optional in subprofiles.

I have a working mmap() implementation now for MSVC which I intend to push soon. Would it be sufficient for your purposes if we just got rid of the MINGW #ifdef and assumed that Mingw users will supply their own mmap()?

[CoderRC]

Yes, mmap() is a mandatory feature in POSIX.1-2017, but not in older POSIX versions, and MSVC does not fully support POSIX.1-2017 and _POSIX_MAPPED_FILES is not defined in MSVC.

[jart]

Would you be interested in sharing your feedback on the mmap() polyfill I've just added to the mmap branch? https://github.com/ggerganov/llama.cpp/blob/mmap/mmap.c

[CoderRC]

Sucessfully compiled mmap branch and sucessfully compiled resolved conflicts.
In msys2 with mingw32 gcc compiler using:
make LDFLAGS='-D_POSIX_MAPPED_FILES -lmingw32_extended'

[CoderRC]

My real goal of this pull request is if mmap exists then _POSIX_MAPPED_FILES will be defined due to POSIX and it overrides any implementation in mmap.h.

[jart]

OK let's do it. Thank you for this change.

[CoderRC]

Thank you, I use https://github.com/CoderRC/libmingw32_extended to compile this, thanks.