llvm dev - Sep 2012 - [LLVMdev] [cfe-dev] SPIR provisional specifciation is now available in the Khronos website

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> On Behalf Of James Molloy
> Sent: Tuesday, September 11, 2012 8:49 AM
> To: Ouriel, Boaz
> Cc: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] SPIR provisional specifciation is now available
> in the Khronos website
> 
> Hi Boaz,
> 
> I have a couple of specific questions:
> 
> (a) You mention special calling conventions and adding them to LLVM.
> What are their semantics? And what is their purpose?
[Villmow, Micah] One purpose is to differentiate between kernel and device
functions.
Another is to differentiate between the standard calling conventions that have
device specific assumptions built into them.
> 
> (b) Why disallow type conversion for vector types? (ss. 3.3)
[Villmow, Micah] Type conversions in OpenCL between vector types is doing
via builtin functions and not via implicit conversions, so there is no OpenCL
code that can generate these conversions directly(OpenCL spec 6.2.1). In order
to be portable, library functions cannot be lowered to their IR equivalent until
after the device is known.
> 
> Cheers,
> 
> James
> 
> On Tue, 2012-09-11 at 12:56 +0100, Ouriel, Boaz wrote:
> > Hi All,
> >
> > In continuation of the previous SPIR introduction email here is a link
> to the specification:
> > http://www.khronos.org/registry/cl/specs/spir_spec-1.0-provisional.pdf
> >
> > The first topic which we would like to discuss is "SPIR
portability".
> > I will send soon an additional mail which will help in leading the
> discussion on this topic.
> >
> > Thanks and happy reading,
> > Boaz
> >
> > -----Original Message-----
> > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> > On Behalf Of Ouriel, Boaz
> > Sent: Thursday, September 06, 2012 22:06
> > To: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> > Subject: [LLVMdev] "SPIR" - A Standard Portable IR for
OpenCL Kernel
> > Language
> >
> > 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 :), Boaz
> > ---------------------------------------------------------------------
> > Intel Israel (74) Limited
> >
> > This e-mail and any attachments may contain confidential material for
> > the sole use of the intended recipient(s). Any review or distribution
> > by others is strictly prohibited. If you are not the intended
> > recipient, please contact the sender and delete all copies.
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > ---------------------------------------------------------------------
> > Intel Israel (74) Limited
> >
> > This e-mail and any attachments may contain confidential material for
> > the sole use of the intended recipient(s). Any review or distribution
> > by others is strictly prohibited. If you are not the intended
> > recipient, please contact the sender and delete all copies.
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> 
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Micah,
>> (a) You mention special calling conventions and adding them to LLVM.
>> What are their semantics? And what is their purpose?
> [Villmow, Micah] One purpose is to differentiate between kernel and device
functions.
> Another is to differentiate between the standard calling conventions that
have
> device specific assumptions built into them.
Do you have an example of such a device-specific assumption? Why
wouldn't the default (no explicit) calling convention do for a
storage-only format like SPIR?

When it does come to codegen (for a CPU target), an LLVM backend would
be forced to change the calling convention back to something standard
anyway. So what's the advantage of adding a semanticless calling
convention over metadata?
>> (b) Why disallow type conversion for vector types? (ss. 3.3)
> [Villmow, Micah] Type conversions in OpenCL between vector types is doing
> via builtin functions and not via implicit conversions, so there is no
OpenCL
> code that can generate these conversions directly(OpenCL spec 6.2.1). In
order
> to be portable, library functions cannot be lowered to their IR equivalent
until
> after the device is known.
Is SPIR meant to be storage-only, or to allow optimisations to be done
on it (valid SPIR -> opt -> valid SPIR)?

If you allow scalar type conversions, and you allow vectors, it
follows that an optimiser may turn scalar type conversions into vector
type conversions. Why explicitly disallow this even though there is no
corollary directly from CL-C source code?

Cheers,

James

On 11 September 2012 16:54, Villmow, Micah <Micah.Villmow at amd.com>
wrote:
>
>
>> -----Original Message-----
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
>> On Behalf Of James Molloy
>> Sent: Tuesday, September 11, 2012 8:49 AM
>> To: Ouriel, Boaz
>> Cc: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
>> Subject: Re: [LLVMdev] SPIR provisional specifciation is now available
>> in the Khronos website
>>
>> Hi Boaz,
>>
>> I have a couple of specific questions:
>>
>> (a) You mention special calling conventions and adding them to LLVM.
>> What are their semantics? And what is their purpose?
> [Villmow, Micah] One purpose is to differentiate between kernel and device
functions.
> Another is to differentiate between the standard calling conventions that
have
> device specific assumptions built into them.
>>
>> (b) Why disallow type conversion for vector types? (ss. 3.3)
> [Villmow, Micah] Type conversions in OpenCL between vector types is doing
> via builtin functions and not via implicit conversions, so there is no
OpenCL
> code that can generate these conversions directly(OpenCL spec 6.2.1). In
order
> to be portable, library functions cannot be lowered to their IR equivalent
until
> after the device is known.
>>
>> Cheers,
>>
>> James
>>
>> On Tue, 2012-09-11 at 12:56 +0100, Ouriel, Boaz wrote:
>> > Hi All,
>> >
>> > In continuation of the previous SPIR introduction email here is a
link
>> to the specification:
>> >
http://www.khronos.org/registry/cl/specs/spir_spec-1.0-provisional.pdf
>> >
>> > The first topic which we would like to discuss is "SPIR
portability".
>> > I will send soon an additional mail which will help in leading the
>> discussion on this topic.
>> >
>> > Thanks and happy reading,
>> > Boaz
>> >
>> > -----Original Message-----
>> > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
>> > On Behalf Of Ouriel, Boaz
>> > Sent: Thursday, September 06, 2012 22:06
>> > To: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
>> > Subject: [LLVMdev] "SPIR" - A Standard Portable IR for
OpenCL Kernel
>> > Language
>> >
>> > 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 :), Boaz
>> >
---------------------------------------------------------------------
>> > Intel Israel (74) Limited
>> >
>> > This e-mail and any attachments may contain confidential material
for
>> > the sole use of the intended recipient(s). Any review or
distribution
>> > by others is strictly prohibited. If you are not the intended
>> > recipient, please contact the sender and delete all copies.
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
---------------------------------------------------------------------
>> > Intel Israel (74) Limited
>> >
>> > This e-mail and any attachments may contain confidential material
for
>> > the sole use of the intended recipient(s). Any review or
distribution
>> > by others is strictly prohibited. If you are not the intended
>> > recipient, please contact the sender and delete all copies.
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>>
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

> -----Original Message-----
> From: mankeyrabbit at gmail.com [mailto:mankeyrabbit at gmail.com] On
Behalf
> Of James Molloy
> Sent: Tuesday, September 11, 2012 1:45 PM
> To: Villmow, Micah
> Cc: James Molloy; Ouriel, Boaz; cfe-dev at cs.uiuc.edu; llvmdev at
cs.uiuc.edu
> Subject: Re: [cfe-dev] [LLVMdev] SPIR provisional specifciation is now
> available in the Khronos website
> 
> Hi Micah,
> 
> >> (a) You mention special calling conventions and adding them to
LLVM.
> >> What are their semantics? And what is their purpose?
> > [Villmow, Micah] One purpose is to differentiate between kernel and
> device functions.
> > Another is to differentiate between the standard calling conventions
> > that have device specific assumptions built into them.
> 
> Do you have an example of such a device-specific assumption? Why
> wouldn't the default (no explicit) calling convention do for a storage-
> only format like SPIR?
[Villmow, Micah] It is my understanding, and correct me if I'm wrong, clang
generates code based on the calling convention and the device. We don't want
any assumptions made about the device calling convention until SPIR loading time
and
the cleanest way to do that is to introduce our own. Also the default calling
convention in LLVM is C, which
does not have the same semantics as ours(for example varargs is illegal except
for printf, kernels and functions are different,
etc...).
> 
> When it does come to codegen (for a CPU target), an LLVM backend would
> be forced to change the calling convention back to something standard
> anyway. So what's the advantage of adding a semanticless calling
> convention over metadata?
[Villmow, Micah] I wouldn't say it is semantic-less, just that its semantics
are different than the calling conventions that LLVM currently
supports.
> 
> >> (b) Why disallow type conversion for vector types? (ss. 3.3)
> > [Villmow, Micah] Type conversions in OpenCL between vector types is
> > doing via builtin functions and not via implicit conversions, so there
> > is no OpenCL code that can generate these conversions directly(OpenCL
> > spec 6.2.1). In order to be portable, library functions cannot be
> > lowered to their IR equivalent until after the device is known.
> 
> Is SPIR meant to be storage-only, or to allow optimisations to be done
> on it (valid SPIR -> opt -> valid SPIR)?
[Villmow, Micah] While you can optimize SPIR, you run the chance of reducing
portability by optimizing in a non-portable manner. The SPIR spec does not
specify how the SPIR is generated or what is done to the binary format before
generation, only what is and isn't valid. It is quite possible to generate
valid SPIR that is non-portable, but in this case, there is no reason for using
SPIR.
> 
> If you allow scalar type conversions, and you allow vectors, it follows
> that an optimiser may turn scalar type conversions into vector type
> conversions. Why explicitly disallow this even though there is no
> corollary directly from CL-C source code?
[Villmow, Micah] SPIR in its current form is limited to OpenCL C(being an OpenCL
extension). So things that are disallowed in OpenCL C are disallowed in SPIR at
this time.
> 
> Cheers,
> 
> James
> 
> On 11 September 2012 16:54, Villmow, Micah <Micah.Villmow at amd.com>
> wrote:
> >
> >
> >> -----Original Message-----
> >> From: llvmdev-bounces at cs.uiuc.edu
> >> [mailto:llvmdev-bounces at cs.uiuc.edu]
> >> On Behalf Of James Molloy
> >> Sent: Tuesday, September 11, 2012 8:49 AM
> >> To: Ouriel, Boaz
> >> Cc: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> >> Subject: Re: [LLVMdev] SPIR provisional specifciation is now
> >> available in the Khronos website
> >>
> >> Hi Boaz,
> >>
> >> I have a couple of specific questions:
> >>
> >> (a) You mention special calling conventions and adding them to
LLVM.
> >> What are their semantics? And what is their purpose?
> > [Villmow, Micah] One purpose is to differentiate between kernel and
> device functions.
> > Another is to differentiate between the standard calling conventions
> > that have device specific assumptions built into them.
> >>
> >> (b) Why disallow type conversion for vector types? (ss. 3.3)
> > [Villmow, Micah] Type conversions in OpenCL between vector types is
> > doing via builtin functions and not via implicit conversions, so there
> > is no OpenCL code that can generate these conversions directly(OpenCL
> > spec 6.2.1). In order to be portable, library functions cannot be
> > lowered to their IR equivalent until after the device is known.
> >>
> >> Cheers,
> >>
> >> James
> >>
> >> On Tue, 2012-09-11 at 12:56 +0100, Ouriel, Boaz wrote:
> >> > Hi All,
> >> >
> >> > In continuation of the previous SPIR introduction email here
is a
> >> > link
> >> to the specification:
> >> >
http://www.khronos.org/registry/cl/specs/spir_spec-1.0-provisional.
> >> > pdf
> >> >
> >> > The first topic which we would like to discuss is "SPIR
> portability".
> >> > I will send soon an additional mail which will help in
leading the
> >> discussion on this topic.
> >> >
> >> > Thanks and happy reading,
> >> > Boaz
> >> >
> >> > -----Original Message-----
> >> > From: llvmdev-bounces at cs.uiuc.edu
> >> > [mailto:llvmdev-bounces at cs.uiuc.edu]
> >> > On Behalf Of Ouriel, Boaz
> >> > Sent: Thursday, September 06, 2012 22:06
> >> > To: cfe-dev at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> >> > Subject: [LLVMdev] "SPIR" - A Standard Portable IR
for OpenCL
> >> > Kernel Language
> >> >
> >> > 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 :), Boaz
> >> >
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> >> > Intel Israel (74) Limited
> >> >
> >> > This e-mail and any attachments may contain confidential
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> >> > for the sole use of the intended recipient(s). Any review or
> >> > distribution by others is strictly prohibited. If you are not
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> >> > intended recipient, please contact the sender and delete all
> copies.
> >> >
> >> >
> >> > _______________________________________________
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> >> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >
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> >> > intended recipient, please contact the sender and delete all
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> >> >
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> >>
> >>
> >>
> >>
> >> _______________________________________________
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