Modernise and tidy the CMake a bit:
- Use generator expressions for a lot of conditionals so that things
are more reliable with multi-config generators (and less verbose).
- Remove C as a needed language. None of the code was C but we were
using C to test if headers worked. This was fragile because a build
with `CMAKE_CXX_COMPILER` set might have checked things compiled with
the system C compiler and then failed when the specified C++ compiler
used different headers.
- Rename the `BACKTRACE_HEADER` macro to `SNMALLOC_BACKTRACE_HEADER`.
This is exposed into code that consumes snmalloc and so should be
'namespaced' (to the degree that's possible with C macros).
- Clean up the options and use dependent options to hide options
that are not always relevant.
- Use functions instead of macros for better variable scoping.
- Factor out some duplicated bits into functions.
- Update to the latest way of telling CMake to use C++17 or C++20.
- Migrate everything that's setting global properties to setting only
per-target properties.
- Link with -nostdlib++ if it's available. If it isn't, fall back to
enabling the C language and linking with the C compiler.
- Make the per-test log messages verbose outputs. These kept scrolling
important messages off the top of the screen for me.
- Make building as a header-only library a public option.
- Add install targets that install all of the headers and provide a
config option. This works with the header-only configuration for
integration with things like vcpkg.
- Fix a missing `#endif` in the `malloc_useable_size` check. This was
failing co compile on all platforms because of the missing `#endif`.
- Bump the minimum version to 3.14 so that we have access to
target_link_options. This is necessary to use generator expressions
for linker flags.
- Make the linker error if the shim libraries depend on symbols that
are not defined in the explicitly-provided libraries.
- Make the old-Ubuntu CI jobs use C++17 explicitly (previously CMake
was silently ignoring the fact that the compiler didn't support C++20)
- Fix errors found by the more aggressive linking mode.
With these changes, it's now possible to install snmalloc and then, in
another project, do something like this:
```cmake
find_package(snmalloc CONFIG REQUIRED)
target_link_libraries(t1 snmalloc::snmalloc)
target_link_libraries(t2 snmalloc::snmallocshim-static)
```
In this example, `t1` gets all of the compile flags necessary to include
snmalloc headers for its build configuration. `t2` is additionally
linked to the snmalloc static shim library.
# Pagemap
The Pagemap now stores all the meta-data for the object allocation. The meta-data in the pagemap is effectively a triple of the sizeclass, the remote allocator, and a pointer to a 64 byte block of meta-data for this chunk of memory. By storing the pointer to a block, it allows the pagemap to handle multiple slab sizes without branching on the fast path. There is one entry in the pagemap per 16KiB of address space, but by using the same entry in the pagemap for 4 adjacent entries, then we can treat a 64KiB range can be treated as a single slab of allocations.
This change also means there is almost no capability amplification required by the implementation on CHERI for finding meta-data. The only amplification is required, when we change the way a chunk is used to a size of object allocation.
# Backend
There is a second major aspect of the refactor that there is now a narrow API that abstracts the Pagemap, PAL and address space management. This should better enable the compartmentalisation and makes it easier to produce alternative backends for various research directions. This is a template parameter that can be used to specialised by the front-end in different ways.
# Thread local state
The thread local state has been refactored into two components, one (called 'localalloc') that is stored directly in the TLS and is constant initialised, and one that is allocated in the address space (called 'coreallloc') which is lazily created and pooled.
# Difference
This removes Superslabs/Medium slabs as there meta-data is now part of the pagemap.
* Fix sized delete of nullptr
The core snmalloc code assumes if you know the size, then it is not
nullptr. However, the C++ delete operator can be called with nullptr.
This change checks for that case.
