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otherwise, shrink in-place. as explained in the description of commit
3e16313f8fe2ed143ae0267fd79d63014c24779f, the split here is valid
without holding split_merge_lock because all chunks involved are in
the in-use state.
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commit 3e16313f8fe2ed143ae0267fd79d63014c24779f introduced this bug by
making the copy case reachable with n (new size) smaller than n0
(original size). this was left as the only way of shrinking an
allocation because it reduces fragmentation if a free chunk of the
appropriate size is available. when that's not the case, another
approach may be better, but any such improvement would be independent
of fixing this bug.
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it both malloc and aligned_alloc have been replaced but the internal
aligned_alloc still gets called, the replacement is a wrapper of some
sort. it's not clear if this usage should be officially supported, but
it's at least a plausibly interesting debugging usage, and easy to do.
it should not be relied upon unless it's documented as supported at
some later time.
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a new weak predicate function replacable by the malloc implementation,
__malloc_allzerop, is introduced. by default it's always false; the
default version will be used when static linking if the bump allocator
was used (in which case performance doesn't matter) or if malloc was
replaced by the application. only if the real internal malloc is
linked (always the case with dynamic linking) does the real version
get used.
if malloc was replaced dynamically, as indicated by __malloc_replaced,
the predicate function is ignored and conditional-memset is always
performed.
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it's not part of the malloc implementation but glue with musl dynamic
linker.
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abstractly, calloc is completely malloc-implementation-independent;
it's malloc followed by memset, or as we do it, a "conditional memset"
that avoids touching fresh zero pages.
previously, calloc was kept separate for the bump allocator, which can
always skip memset, and the version of calloc provided with the full
malloc conditionally skipped the clearing for large direct-mmapped
allocations. the latter is a moderately attractive optimization, and
can be added back if needed. however, further consideration to make it
correct under malloc replacement would be needed.
commit b4b1e10364c8737a632be61582e05a8d3acf5690 documented the
contract for malloc replacement as allowing omission of calloc, and
indeed that worked for dynamic linking, but for static linking it was
possible to get the non-clearing definition from the bump allocator;
if not for that, it would have been a link error trying to pull in
malloc.o.
the conditional-clearing code for the new common calloc is taken from
mal0_clear in oldmalloc, but drops the need to access actual page size
and just uses a fixed value of 4096. this avoids potentially needing
access to global data for the sake of an optimization that at best
marginally helps archs with offensively-large page sizes.
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this sets the stage for replacement, and makes it practical to keep
oldmalloc around as a build option for a while if that ends up being
useful.
only the files which are actually part of the implementation are
moved. memalign and posix_memalign are entirely generic. in theory
calloc could be pulled out too, but it's useful to have it tied to the
implementation so as to optimize out unnecessary memset when
implementation details make it possible to know the memory is already
clear.
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