Hi folks-
*tl;dr **plarena.h has been replaced, stop using it.*
In bug 943156 [0] I recently landed [1] a templatized arena allocator,
mozilla::ArenaAllocator, that we can used as a drop-in replacement for
PLArenaPool. This has the benefit of getting rid of the odd "#define this
thing before including plarena.h" paradigm that is spread throughout the
code base, additionally we can build files that use ArenaAllocator in
unified mode.
Initial micro-benchmarking indicates that the performance of ArenaAllocator
is as good or better than PLArenaPool.
At this point we shouldn't need to use plarena.h. I've gone ahead and
converted all existing usage of PLArenaPool over to ArenaAllocator [2, 3,
4, 5].
*Further details:*
The usage is pretty straightforward. Here's an example of defining an
allocator what uses a 4K arena size and 8-byte alignment to allocate 10
bytes:
#include "mozilla/ArenaAllocator.h"
mozilla::ArenaAllocator<4096, 8> a;
// By default allocation is infallible:
void* x = a.Allocate(10); // kaboom on OOM
// But you can also allocate fallibly:
void* y = a.Allocate(10, mozilla::fallible);
if (!y)
return NS_ERROR_OUT_OF_MEMORY;
Additionally if you find yourself using an arena to manage strings, you may
find the arena allocator extensions [6] helpful:
#include "mozilla/ArenaAllocatorExtensions.h"
char* fromCStr = mozilla::ArenaStrdup("a string", a);
There are versions for nsAString and nsACString as well.
-e
[0] https://bugzilla.mozilla.org/show_bug.cgi?id=943156
[1] https://hg.mozilla.org/mozilla-central/rev/f09969186032
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1351732
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=1351801
[4] https://bugzilla.mozilla.org/show_bug.cgi?id=1351804
[5] https://bugzilla.mozilla.org/show_bug.cgi?id=1351904
[6]
https://hg.mozilla.org/mozilla-central/file/tip/xpcom/ds/ArenaAllocatorExtensions.h
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