On platforms that support low-memory notifications register callbacks
that perform lazy decommit. This allows idle processes to return memory
to the OS. Without incurring the cost of constantly committing and
decommitting memory.
Code review and CI changes
* Fixed test to use a template to make constexpr magic work
* Factored out basic notification mechanism so can be reused on other
platforms.
If the external thread statics are used, then
we don't need to include some C++ runtime
concepts. This refactoring moves some global initialization under
conditional compilation.
The PAL API previously allowed for returning more memory than asked for.
This was when the PAL performed the alignment work, now this is done in
large alloc, so removing from the PAL.
Fixes a few places where Clang complains about Windows specific code,
and also uses macros supported by Clang on Windows. A few places
separating platform and compiler specific code, as MSVC and WIN32 were
used interchangably previously.
HEADER_GLOBAL was using non-standard attributes to achieve what C++17
now permits with a keyword. Use the standard formulation.
Update the README to note that gcc is still not recommended, but because
of its poor codegen for 128-bit atomic compare and exchange, rather than
because it doesn't support the attribute used for HEADER_GLOBAL.
Pull out a generic POSIX PAL as a superclass for the Linux and generic
BSD PALs. Now we have FreeBSD and Linux adding OS-specific behaviour,
OpenBSD as a named subclass of the generic BSD PAL that doesn't add any
behaviour.
I believe a NetBSD PAL should now be identical to the OpenBSD one -
patches welcome if anyone wants to test one!
If the operating system will allocate private pages on demand for the
pagemap then use the FlatPageMap by default as it generates better code
for deallocation.
We were passing an argument less than 4K to the MAP_ALIGNED macro, which
caused an undefined shift. The compiler helpfully propagated the undef
values back to earlier in the code and gave us some exciting nonsense.
This complains if a non-const parameter is not modified. In the PALs,
the size parameter is modified only by some implementations, so we can
make it const in the ones where it isn't.
