Memory allocator alignment fixes
These changes remove some assumptions about heap allocator alignment that may not be true on unusual platforms. * src/alloc.c (POWER_OF_2): New macro. (ROUNDUP): Use it. (BLOCK_ALIGN): Verify that it is a power of 2. (aligned_alloc): Check that alignment passed to posix_memalign satisfies POSIX restrictions. (lisp_align_malloc): Check that size passed to aligned_alloc satisfies C11 restrictions. (MALLOC_IS_GC_ALIGNED): Check that GCALIGNMENT is 8, since the code has not been verified to work with other GCALIGNMENT values and the ice is thin here. On GNU/Linux, malloc can return a value that is a multiple of 8 but not 16, even though __alignof__ (max_align_t) is 16. See: https://gcc.gnu.org/ml/gcc-patches/2016-08/msg01902.html (lmalloc) [USE_ALIGNED_ALLOC]: Use aligned_alloc only if size is a multiple of alignment, since C11 says the behavior is undefined otherwise. (lmalloc, lrealloc): Don't use INT_ADD_WRAPV on size_t, as in general this macro is restricted to signed types. Remove assertion that the result is a multiple of GCALIGNMENT, as that need not be true.
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