[LLVMdev] [PATCH][RFC] HSAIL Target

[Matt Arsenault]

Hi,

AMD would like to propose including an LLVM backend for the HSAIL 
target. Patches for review are attached and can also be found at 
https://github.com/HSAFoundation/HLC-HSAIL-Development-LLVM/ 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.

First, background and a description of the backend:

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 
http://www.hsafoundation.com/standards/. 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.

With the current set of features this HSAIL backend is able to execute 
C++AMP programs by following the directions here: 
https://bitbucket.org/multicoreware/cppamp-driver-ng/wiki/HSA%20Support%20Status. 
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.

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.

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 https://github.com/HSAFoundation/HSAIL-Tools.

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.

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.

Thanks

Matt Arsenault
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