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Hi,<br>
<br>
AMD would like to propose including an LLVM backend for the HSAIL
target. Patches for review are attached and can also be found at
<a
href="https://github.com/HSAFoundation/HLC-HSAIL-Development-LLVM/">https://github.com/HSAFoundation/HLC-HSAIL-Development-LLVM/</a>
on the hsail-review branch. Most of the recent work is visible on
the hsail-1.0f branch, which is based on an LLVM commit
approximately 1 month before 3.6 branched. The hsail-review branch
is the content of hsail-1.0f with the minimal changes required to
port it to trunk in the approximate squashed form I plan on
eventually committing to the LLVM repository.<br>
<br>
First, background and a description of the backend:<br>
<br>
HSAIL is a virtual target machine similar in spirit to AMDIL and
NVPTX. A language compiler produces HSAIL text (or the binary
format, called BRIG) and an HSA implementation provides an HSAIL
finalizer which produces a binary for execution on a physical
device. This backend implements the HSA 1.0 standard published at
<a href="http://www.hsafoundation.com/standards/">http://www.hsafoundation.com/standards/</a>.
The AMD OpenCL 2.0 implementation uses HSAIL via LLVM when targeting
any GPU device, supporting Sea Islands GPUs and later. This backend
is unrelated to the R600 / AMDGPU target which is already in tree.
This HSAIL backend is multivendor, and capable of supporting
multiple generations of ISA from any one vendor.
About 8 months ago, the HSAIL target was forked off of the internal
version and I've been working since then to catch it up to LLVM
trunk and into a state acceptable for upstreaming. To expedite this
process, various features (such as image support, debug info, and a
few optimizations) were dropped, so this version is currently not as
full featured as the internal version. The intention is to gradually
merge that functionality back into this version
once upstream, and eventually base
AMD’s internal version on the upstream target and be minimally
divergent. There is still some work I believe needs to be done
before it is ready to be committed, but at this point I think it is
ready for others to start providing feedback on areas where I should
be focusing to get there.<br>
<br>
With the current set of features this HSAIL backend is able to
execute C++AMP programs by following the directions here:
<a
href="https://bitbucket.org/multicoreware/cppamp-driver-ng/wiki/HSA%20Support%20Status">https://bitbucket.org/multicoreware/cppamp-driver-ng/wiki/HSA%20Support%20Status</a>.
Over time, we intend to evolve to a full C++17 compiler on the LLVM
HSAIL back end with the ability to target the GPU for parallel
regions.
Other languages will also use this path over time.<br>
<br>
One noteworthy difference from how other targets are structured is
there are two code emission paths in the backend. The first path,
which is the original used by the internal OpenCL compiler, uses a
third party library (libHSAIL) for code emission plugged into the
AsmPrinter. The reason for this is partially legacy, and partially
because the HSA specification defines its own object format, BRIG,
which is unlike any of the supported object formats such as ELF.
Supporting BRIG in MC would be a challenge (largely because it is
not really streamable), but attempting to emit it using the standard
infrastructure may be a consideration in the future. This path
supports emitting object files and text output via libHSAIL's
disassembler. The second path which I've implemented over the past
few months uses a normal AsmPrinter emitting HSAIL text using the
standard MC infrastructure, and does not support object output. The
two paths have similar pass rates on the C++ AMP conformance test
suite, and should produce the same output except for some whitespace
and comment differences.<br>
<br>
The backend currently has a cmake option to control whether libHSAIL
will be used or not, defaulting to using it if libHSAIL is found.
Developers building LLVM with or without HSAIL support will not need
libHSAIL to build the backend or run tests, all of which pass using
either AsmPrinter. libHSAIL and associated assembler / disassembler
can be found at
<a href="https://github.com/HSAFoundation/HSAIL-Tools">https://github.com/HSAFoundation/HSAIL-Tools</a>.<br>
<br>
Currently I have left it as an experimental target, not built by
default. I have only implemented the build support necessary for
cmake. I'm not planning on adding the autotools support, since
hopefully that will be gone soon enough.<br>
<br>
My highest priority is to get the backend upstreamed as soon as
possible, so I would appreciate feedback about any kinds of blocking
issues on that.<br>
<br>
Thanks<br>
<br>
Matt Arsenault
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