[cfe-dev] "SPIR" – A Standard Portable IR for OpenCL Kernel Language

Ouriel, Boaz boaz.ouriel at intel.com
Thu Sep 6 12:05:58 PDT 2012

Greetings All,
I am sending this mail on behalf of the OpenCL Khronos members. 

**** Introduction ****
Lately, Khronos has ratified a new provisional specification which is called SPIR. 
This specification standardizes an intermediate representation for the OpenCL kernel language.
It is based on LLVM infrastructure and this is why I am sending this mail to the LLVM mailing list. 
Khronos members would like to initiate a review on the specification with the LLVM community. 

**** What is SPIR? ****
The SPIR specification standardizes an intermediate representation for OpenCL programs, 
which a hypothetical frontend can target to generate binaries that can be consumed and executed by OpenCL drivers supporting SPIR. 
The SPIR specification, however, does not standardize the design and implementation of such a frontend.

**** SPIR and LLVM ****
Khronos members chose SPIR to be layered on top of LLVM. 
Why?  Portability is a key goal of SPIR, and LLVM has proven to be highly portable, given its many backends. 
Defining a robust IR for OpenCL from scratch is difficult and requires skills which are not the core competency of the OpenCL Khronos members. 
In addition, after the IR is defined, implementing the necessary SW stack around it is a huge investment.  LLVM thus provides a time-to-market advantage for SPIR.  
Today, many of the OpenCL vendors base their technology on LLVM. This makes LLVM IR the de facto OpenCL IR and the immediate candidate to be considered by the Khronos members. 
An analysis showed that LLVM IR has its limitations but in general provides a very good solution for SPIR. 

**** Minimal Changes to LLVM ****
When defining SPIR, Khronos set a goal to keep the changes in LLVM minimal. 
Most of the changes made during prototyping were in the frontends that the different OpenCL Khronos members used.
The only changes required by SPIR in LLVM are a new target for SPIR, a new calling convention for regular OpenCL functions, and another one for OpenCL kernels. 
LLVM IR language definition remains unmodified.

**** Why is SPIR important for OpenCL? ****
SPIR offers binary portability between OpenCL implementations, and a stable target for 3rd party compilers without having to go through OpenCL "C".  

Binary compatibility simplifies the support burden for developers delivering applications that use OpenCL.  
The same application can be delivered in fully binary form and work across existing and future OpenCL implementations supporting SPIR. 
This helps the entire OpenCL ecosystem.

Generally speaking OpenCL is a JIT environment and as such deserves and requires an intermediate representation like other major JIT environments already have.

Also, some developers using OpenCL have requested portability at binary level.  Today OpenCL offers portability only at the source level with OpenCL “C”. 
They are concerned with protecting their IP by meeting “Digital Millennium Copyright Act” requirements. 
Today, those companies are forced to distribute their OpenCL code using device specific binaries. This leads to many difficulties for SW developers and end users.
 In addition, the binaries are not guaranteed to be functionally working as new devices and vendors appear in the market. 
This constraint places OpenCL standard in a disadvantage compared to other standards which already have a portable binary distribution form. 
>From discussions with some of the companies which raised the request, LLVM IR meets their requirements.
SPIR doesn’t guarantee any security / obfuscation mechanisms. It just provides a portable IR definition.

Khronos members also believe that SPIR will enable data parallel domain specific languages which will generate directly to SPIR and execute on top of OpenCL runtimes. 

**** SPIR Portability vs. OpenCL “C” ****
Portability is one of SPIR’s goals. However, SPIR does not attempt to solve inherent portability issues, which exist in OpenCL “C” or in C99. 
It is clear that OpenCL programs could be written in a way which make them non portable and very device specific. 
Such programs will never be portable. In addition, some corner case scenarios which have been identified by Khronos have been disallowed in SPIR. 
SPIR does not guarantee performance portability across devices. This is also true for OpenCL “C”.

**** Is this the final version of SPIR specification (set in stone?) ****
The short answer is “NO”, it is not final.

All along the definition stage of SPIR, Khronos had the goal of reviewing this proposal and collecting feedback on its content with LLVM community.
This feedback is not a “nice to have” but rather “a must have”. 
So why didn’t we define the specification with the community right from the start? The answer to that has two aspects. 
The first is that Khronos members wanted to do their homework and make sure that the proposal is mature enough to start discussions based on it. 
The due diligence includes full implementation of the specification by a few members within Khronos. 
The second aspect is the legal part which prevented Khronos from sharing this information publicly until the specification is ratified inside Khronos. 
The current version of SPIR specification which is shared with LLVM community is a provisional specification. 
The main goal of this version of the specification is to collect feedback from LLVM community, apply the changes and shape the specification to its final version. 

**** Suggested review process ****
SPIR introduces an intermediate language for OpenCL and hence is a very large specification with many details and a lot of topics to discuss.
Khronos will share the specification with the LLVM community as a reference. 
However, Khronos believes that the right approach is to review it in parts:  by peeling the different layers and aspects of the specification, layer by layer (the “onion” way), 
going from top to bottom and topic by topic. 

Each such topic would be contained in an email thread in LLVM mailing list. Since SPIR specification deals with the “HOW”, and not with the “WHY”, 
each topic will be associated with a short document that aims at providing insights into the considerations and goals behind the way it was defined in SPIR specification. 
Some of the discussions would be accompanied by pieces of code in CLANG or LLVM that demonstrate what has been implemented by Khronos members. 
A successful discussion would result with a decision acceptable by both LLVM community and Khronos.  
We expect that many discussions will move to LLVM Bugzilla for resolution.  This should improve convergence.

We do not want to fork LLVM.  We plan to evolve SPIR in response to LLVM community feedback. 
In addition, where applicable - Khronos members would like to upstream the relevant changes to LLVM and not wait for the entire review of the specification to be completed.
Khronos members do realize that applying changes to the LLVM code will not always be possible since some discussions depend on other discussions. 

Why not review the entire specification as a whole? Doing the review on the entire specification would make the discussions unfocused and difficult to track. 
We expect discussions will be more effective and converge better by a piecemeal approach. 
That being said, we will try to keep the proposal coherent at a high level.

**** clang as a sample OpenCL SPIR generator ****
Even though SPIR does not standardize the generation process, the Khronos working group would like clang to eventually become the sample OpenCL SPIR generator.  
So why only make it a sample generator? Khronos wanted to permit the different OpenCL vendors to choose their own frontend technology and not require them to use CLANG.  

Also, we avoid using clang as a reference generator because any discrepancy between SPIR outputs generated by clang and the SPIR spec will be resolved in favor of the spec.  
That is, implementers of other SPIR generators would not be required to maintain bug compatibility with clang.

**** Suggested Topics to discuss ****

This is the list of suggested topics to discuss:
1.    SPIR specification introduction and scope (this mail)
2.    SPIR Portability
     a.    32 / 64bits architectures (pointers, size_t, ptrdiff_t, intptr_t, uintptr_t)
     b.    Endianness in OpenCL “C”
3.    OpenCL built-ins in SPIR
     a.    OpenCL Built-ins & LLVM Built-ins
     b.    Name Mangling
4.    OpenCL Metadata Arrangement
5.    OpenCL Specific items
    a.    OpenCL special data types (events, images, samplers) as opaque data types
    b.    Null and zeroinitializer
    c.    Local Memory and alloca’s 
    d.    Others

**** Where can I find SPIR specification? ****
Khronos is now working on making SPIR specification available through the Khronos website. 
Once available, we will send a link to the document in this mailing list.

I am sure this is going to be a lot of fun :),
Intel Israel (74) Limited

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