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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/171123">171123</a></dd>
<dt>Author</dt> <dd>benjamin@webkit.org</dd>
<dt>Date</dt> <dd>2014-07-15 16:59:14 -0700 (Tue, 15 Jul 2014)</dd>
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<h3>Log Message</h3>
<pre>Reduce the overhead of updating the AssemblerBuffer
https://bugs.webkit.org/show_bug.cgi?id=134659
Reviewed by Gavin Barraclough.
In <a href="http://trac.webkit.org/projects/webkit/changeset/164548">r164548</a>, the linker was changed to allow the LinkBuffer to survive its MacroAssembler.
That feature is useful for JSC to get offsets inside a linked buffer in order to jump directly
there.
On ARM, we use branch compaction and we need to keep the "compaction offset" somewher to be able
to get the real address of a lable. That is done by reusing the memory of AssemblerData.
To share the memory between LinkBuffer and the Assembler, <a href="http://trac.webkit.org/projects/webkit/changeset/164548">r164548</a> moved the AssemblerData into
a ref-counted object. Unfortunately, the extra complexity related to the new AssemblerData was enough
to make clang give up a bunch of optimizations.
This patch solve (some of) the problems by making AssemblerBuffer and AssemblerData super low overhead structures.
In particular, the grow() function becomes 8 Thumb instructions, which is easily inlined everywhere it is used.
Instead of sharing ownership between the Assembler and LinkBuffer, LinkBuffer now takes full ownership of
the AssemblerData. I feel this is also safer since LinkBuffer is reusing the AssemblerData is a very
specific way that would make it unusable for the Assembler.
-- Technical details --