On Jun 9, 2009, at 2:38 PM, Akos Kiss wrote:
Dear Community,
Today, we realized that there is a new ARM JIT port for WebKit. (http://trac.webkit.org/changeset/44514
) Congratulations on getting this working!, great job.
Hi Akos,
Thank you! Just to clarify, we have just landed a ARMv7 architecture
(thumb2) JIT backend into ToT. I say ARMv7 to distinguish this port
from the ARM application instruction set found in the ARMv6
architecture and earlier (as I believe would be a common understanding
of the term ARM, and and which I believe your port targets). Thumb2
in ARMv7 is, of course, a very different instruction set to the
traditional 32-bit instructions found in ARM – with completely
different machine encodings, and significantly different capabilities
(e.g. two operand versus three operand instructions, sizes of
immediate operands, and options for instruction predication). For the
JIT to be able to run on both ARM and ARMv7 platforms, it needs to be
ported to both architectures – in much the same way the the JIT is
ported to both the x86 and x86-64 platforms.
Obviously there is a great deal of similarity on the surface between
ARM and ARMv7, but in terms of the JIT implementation that may well
only be true above the level of the MacroAssembler interface. There
may be some limited opportunities to share code within the Assembler
classes (register numbering enums, and possibly types describing
immediate operands), but since the assembler is primarily concerned
with formatting machine instructions, and since the instruction
encodings are different, it seems likely the bulk of the code will
have to remain separate. Again, the differences in instruction
selection options available on the two architectures will likely make
it hard to share code within the MacroAssember (different numbers of
operands to many common instructions, and the options when working
with large immediate values particularly spring to mind). We would
certainly want to share code and avoid any duplication where ever it
makes sense to do so.
I cannot conceal how disappointed I am, as is the whole team at
Szeged.
I am very sorry to hear this. If you look at the patches that landed
into ToT there were very few changes made outside of the new assembler
classes which, for the reasons described above, I think are highly
unlikely to have much in common on the two platforms. The changes
that have been made to common code outside of the assemblers should
only help in removing x86 dependencies and assumptions that had
existed in the code. I strongly urge you to review the changes that
have been made, as I hope and believe you will find that they will
assist the team in integrating your ARM port.
Of course, we've felt that you were reluctant to accept our
implementation.
We were (and remain) reluctant to accept a duplicate of the JIT into
the tree, rather than a port of the existing JIT utilizing the
MacroAssembler abstraction. We are concerned that it would be
extremely difficult to continue to maintain such a port as we move the
JIT technology forwards. Beyond that, they key barrier to the ARM JIT
being accepted into WebKit is that there simply haven't been any
patches put forwards for us to review! (I'm sorry, I'm aware you have
provided a link to an external git repository, but I'm afraid we
really can't seek through version control systems to find changes to
review – we do need contributors to attach patches to bugs, and we
need a review flag setting to indicate when the contributor believes
their patch is ready. If there is any uncertainty as to the
procedure, please see http://webkit.org/coding/contributing.html .)
- Are you still looking for patches, bug reports, feature requests,
etc., or is it all in vain - you will get everything done in house?
Yes! Please do so, this is only way your changes will get into the
tree.
- Should we ask for credits in the new files, as it was done by you
when we first published our JIT implementation? I'm quite sure that
we can state the same argument: "a number of the new files appear to
have taken large chunks of logic from existing jit files".
The new files were derived from their x86 counterparts, with reference
to the ARMv7 manuals. As such the existing copyright notifications
within the files from which they were derived have been retained.
(Apologies for the slow reply, we have a busy week with WWDC on!)
cheers,
G.
Best regards,
Akos
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