llvm dev - May 2019 - [cfe-dev] [RFC] Expose user provided vector function for auto-vectorization.

Francesco, there won't be any duplication. Most of the declarative OpenMP
directives are represented as attributes internally, so, I think, it will be
natural to use an attribute here rather than pragma.

Best regards,
Alexey Bataev
> 31 мая 2019 г., в 13:32, Francesco Petrogalli <Francesco.Petrogalli at
arm.com> написал(а):
> 
> 
> 
>> On May 31, 2019, at 12:00 PM, Alexey Bataev via cfe-dev <cfe-dev at
lists.llvm.org> wrote:
>> 
>> Hi Francesco, did you think about adding the attribute instead of the
pragma? It is a common way to express such constructs as function attributes in
clang/GCC rather than as pragma.
>> 
> 
> Yes, I thought about it, I believe that GCC plans to use attributes.
> 
> In
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
there is a description of the “simd” attribute, but I couldn’t find an example
of how it is used.
> 
> Also, I cannot find an attribute that is equivalent to the “declare
variant” directive. Maybe it is planned for future releases.
> 
> The idea of using a `clang` equivalent of the `omp` directive was to avoid
duplication in terms of handling both the attribute and the omp directive, as
both directive will share much of the infrastructure.
> 
>> Best regards,
>> Alexey Bataev
>> 
>>> 31 мая 2019 г., в 12:18, Francesco Petrogalli via cfe-dev
<cfe-dev at lists.llvm.org> написал(а):
>>> 
>>> Hi All,
>>> 
>>> Thank you for the feedback so far.
>>> 
>>> I am replying to all your questions/concerns/suggestions in this
single email. Please let me know if I have missed any.
>>> 
>>> I will update the RFC accordingly to what we end up deciding here.
>>> 
>>> Kind regards,
>>> 
>>> Francesco
>>> 
>>> 
>>> # TOPIC 1: concerns about name mangling
>>> 
>>> I understand that there are concerns in using the mangling scheme I
proposed, and that it would be preferred to have a mangling scheme that is based
on (and standardized by) OpenMP. I hear the argument on having some common
ground here. In fact, there is already common ground between the x86 and aarch64
backend, who have based their respective Vector Function ABI specifications on
OpenMP.
>>> 
>>> In fact, the mangled name grammar can be summarized as follows:
>>> 
>>> _ZGV<isa><masking><VLEN><parameter
type>_<scalar name>
>>> 
>>> Across vector extensions the only <token> that will differ is
the <isa> token.
>>> 
>>> This might lead people to think that we could drop the
_ZGV<isa> prefix and consider the <masking><VLEN><parameter
type>_<scalar name> part as a sort of unofficial OpenMP mangling
scheme: in fact, the signature of an “unmasked 2-lane vector vector of `sin`”
will always be `<2 x double>(2 x double>).
>>> 
>>> The problem with this choice is the number of vector version
available for a target is not unique.
>>> 
>>> In particular, the following declaration generates multiple vector
versions, depending on the target:
>>> 
>>> #pragma omp declare simd simdlen(2) notinbranch
>>> double foo(double) {…};
>>> 
>>> On x86, this generates at least 4 symbols (one for SSE, one for
AVX, one for AVX2, and one for AVX512: https://godbolt.org/z/TLYXPi)
>>> 
>>> On aarch64, the same declaration generates a unique symbol, as
specified in the Vector Function ABI.
>>> 
>>> This means that the attribute (or metadata) that carries the
information on the available vector version needs to deal also with things that
are not usually visible at IR level, but that might still need to be provided to
be able to decide which particular instruction set/ vector extension needs to be
targeted.
>>> 
>>> I used an example based on `declare simd` instead of `declare
variant` because the attribute/metadata needed for `declare variant` is a
modification of the one needed for `declare simd`, which has already been agreed
in a previous RFC proposed by Intel [1], and for which Intel has already
provided an implementation [2]. The changes proposed in this RFC are fully
compatible with the work that is being don for the VecClone pass in [2].
>>> 
>>> [1] http://lists.llvm.org/pipermail/cfe-dev/2016-March/047732.html
>>> [2] VecCLone pass: https://reviews.llvm.org/D22792
>>> 
>>> The good news is that as far as AArch64 and x86 are concerned, the
only thing that will differ in the mangled name is the “<isa>” token. As
far as I can tell, the mangling scheme of the rest of the vector name is the
same, therefore a lot of infrastructure in terms of mangling and demangling can
be reused. In fact, the `mangleVectorParameters` function in
https://clang.llvm.org/doxygen/CGOpenMPRuntime_8cpp_source.html#l09918 could
already be shared among x86 and aarch64.
>>> 
>>> TOPIC 2: metadata vs attribute
>>> 
>>> From a functionality point of view, I don’t care whether we use
metadata or attributes. The VecClone pass mentioned in TOPIC 1 uses the
following:
>>> 
>>> attributes #0 = { nounwind uwtable
“vector-variants"="_ZGVbM4vv_vec_sum,_ZGVbN4vv_vec_sum,_ZGVcM8vv_vec_sum,_ZGVcN8vv_vec_sum,_ZGVdM8vv_vec_sum,_ZGVdN8vv_vec_sum,_ZGVeM16vv_vec_sum,_ZGVeN16”}
>>> 
>>> This is an attribute (I though it was metadata?), I am happy to
reword the RFC using the right terminology (sorry for messing this up).
>>> 
>>> Also, @Renato expressed concern that metadata might be dropped by
optimization passes - would using attributes prevent that?
>>> 
>>> TOPIC 3: "There is no way to notify the backend how conformant
the SIMD versions are.”
>>> 
>>> @Shawn, I am afraid I don’t understand what you mean by
“conformant” here. Can you elaborate with an example?
>>> 
>>> TOPIC 3: interaction of the `omp declare variant` with `clang
declare variant`
>>> 
>>> I believe this is described in the `Option behavior, and
interaction with OpenMP`. The option `-fclang-declare-variant` is there to make
the OpenMP based one orthogonal. Of course, we might decide to make
-fclang-declare-variant on/off by default, and have default behavior when
interacting with -fopenmp-simd. For the sake of compatibility with other
compilers, we might need to require -fno-clang-declare-variant when targeting
-fopenmp-[simd].
>>> 
>>> TOPIC 3: "there are no special arguments / flags / status regs
that are used / changed in the vector version that the compiler will have to
"just know”
>>> 
>>> I believe that this concern is raised by the problem of handling FP
exceptions? If that’s the case, the compiler is not allowed to do any assumption
on the vector function about that, and treat it with the same knowledge of any
other function, depending on the visibility it has in the compilation unit.
@Renato, does this answer your question?
>>> 
>>> TOPIC 4: attribute in function declaration vs attribute function
call site
>>> 
>>> We discussed this in the previous version of the proposal. Having
it in the call sites guarantees that incompatible vector version are used when
merging modules compiled for different targets. I don’t have a use case for
this, if I remember correctly this was asked by @Hideki Saito. Hideki, any
comment on this?
>>> 
>>> TOPIC 5: overriding system header (the discussion on #pragma
omp/clang/system variants initiated by @Hal Finkel).
>>> 
>>> I though that the split among #pragma clang declare variant and
#pragma omp declare variant was already providing the orthogonality between
system header and user header. Meaning that a user should always prefer the omp
version (for portability to other compilers) instead of the #pragma clang one,
which would be relegated to system headers and headers provided by the compiler.
Am I missing something? If so, I am happy to add a “system” version of the
directive, as it would be quite easy to do given most of the parsing
infrastructure will be shared.
>>> 
>>> 
>>>> On May 30, 2019, at 12:53 PM, Philip Reames <listmail at
philipreames.com> wrote:
>>>> 
>>>> 
>>>>>> On 5/30/19 9:05 AM, Doerfert, Johannes wrote:
>>>>>>> On 05/29, Finkel, Hal J. via cfe-dev wrote:
>>>>>>>> On 5/29/19 1:52 PM, Philip Reames wrote:
>>>>>>>>> On 5/28/19 7:55 PM, Finkel, Hal J. wrote:
>>>>>>>>> On 5/28/19 3:31 PM, Philip Reames via
cfe-dev wrote:
>>>>>>>>> I generally like the idea of having support
in IR for vectorization of
>>>>>>>>> custom functions.  I have several use cases
which would benefit from this.
>>>>>>>>> 
>>>>>>>>> I'd suggest a couple of reframings to
the IR representation though.
>>>>>>>>> 
>>>>>>>>> First, this should probably be specified as
metadata/attribute on a
>>>>>>>>> function declaration.  Allowing the
callsite variant is fine, but it
>>>>>>>>> should primarily be a property of the
called function, not of the call
>>>>>>>>> site.  Being able to specify it once per
declaration is much cleaner.
>>>>>>>> I agree. We should support this both on the
function declaration and on
>>>>>>>> the call sites.
>>>>>>>> 
>>>>>>>> 
>>>>>>>>> Second, I really don't like the
mangling use here.  We need a better way
>>>>>>>>> to specify the properties of the function
then it's mangled name.  One
>>>>>>>>> thought to explore is to directly use the
Value of the function
>>>>>>>>> declaration (since this is metadata and we
can do that), and then tie
>>>>>>>>> the properties to the function declaration
in some way?  Sorry, I don't
>>>>>>>>> really have a specific suggestion here.
>>>>>>>> Is the problem the mangling or the fact that
the mangling is
>>>>>>>> ABI/target-specific? One option is to use
LLVM's mangling scheme (the
>>>>>>>> one we use for intrinsics) and then provide
some backend infrastructure
>>>>>>>> to translate later.
>>>>>>> Well, both honestly.  But mangling with a
non-target specific scheme is
>>>>>>> a lot better, so I might be okay with that.   Good
idea.
>>>>>> 
>>>>>> I liked your idea of directly encoding the signature in
the metadata,
>>>>>> but I think that we want to continue to use attributes,
and not
>>>>>> metadata, and the options for attributes seem more
limited - unless we
>>>>>> allow attributes to take metadata arguments - maybe
that's an
>>>>>> enhancement worth considering.
>>>>> I recently talked to people in the OpenMP language
committee meeting
>>>>> about this and, thinking forward to the actual
implementation/use of the
>>>>> OpenMP 5.x declare variant feature, I'd say:
>>>>> 
>>>>> - We will need a mangling scheme if we want to allow
variants on
>>>>> declarations that are defined elsewhere.
>>>>> - We will need a (OpenMP) standardized mangling scheme if
we want
>>>>> interoperability between compilers.
>>>>> 
>>>>> I assume we want both so I think we will need both.
>>>> If I'm reading this correctly, this describes a need for
the frontend to
>>>> have a mangling scheme.  Nothing in here would seem to prevent
the
>>>> frontend for generating a declaration for a mangled external
symbol and
>>>> then referencing that declaration.  Am I missing something?
>>>>> 
>>>>> That said, I think this should allow us to avoid
attributes/metadata
>>>>> which seems to me like a good thing right now.
>>>>> 
>>>>> Cheers,
>>>>> Johannes
>>>>> 
>>>>> 
>>>>>>>>>> On 5/28/19 12:44 PM, Francesco
Petrogalli via llvm-dev wrote:
>>>>>>>>>> Dear all,
>>>>>>>>>> 
>>>>>>>>>> This RFC is a proposal to provide
auto-vectorization functionality for user provided vector functions.
>>>>>>>>>> 
>>>>>>>>>> The proposal is a modification of an
RFC that I have sent out a couple of months ago, with the title `[RFC]
Re-implementing -fveclib with OpenMP` (see
http://lists.llvm.org/pipermail/llvm-dev/2018-December/128426.html). The
previous RFC is to be considered abandoned.
>>>>>>>>>> 
>>>>>>>>>> The original RFC was proposing to
re-implement the `-fveclib` command line option. This proposal avoids that, and
limits its scope to the mechanics of providing vector function in user code that
the compiler can pick up for auto-vectorization. This narrower scope limits the
impact of changes that are needed in both clang and LLVM.
>>>>>>>>>> 
>>>>>>>>>> Please let me know what you think.
>>>>>>>>>> 
>>>>>>>>>> Kind regards,
>>>>>>>>>> 
>>>>>>>>>> Francesco
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>>
================================================================================>>>>>>>>>>
>>>>>>>>>> Introduction
>>>>>>>>>>
===========>>>>>>>>>>
>>>>>>>>>> This RFC encompasses the proposal of
informing the vectorizer about the
>>>>>>>>>> availability of vector functions
provided by the user. The mechanism is
>>>>>>>>>> based on the use of the directive
`declare variant` introduced in OpenMP
>>>>>>>>>> 5.0 [^1].
>>>>>>>>>> 
>>>>>>>>>> The mechanism proposed has the
following properties:
>>>>>>>>>> 
>>>>>>>>>> 1.  Decouples the compiler front-end
that knows about the availability
>>>>>>>>>>   of vectorized routines, from the
back-end that knows how to make use
>>>>>>>>>>   of them.
>>>>>>>>>> 2.  Enable support for a
developer's own vector libraries without
>>>>>>>>>>   requiring changes to the compiler.
>>>>>>>>>> 3.  Enables other frontends (e.g. f18)
to add scalar-to-vector function
>>>>>>>>>>   mappings as relevant for their own
runtime libraries, etc.
>>>>>>>>>> 
>>>>>>>>>> The implemetation consists of two
separate sets of changes.
>>>>>>>>>> 
>>>>>>>>>> The first set is a set o changes in
`llvm`, and consists of:
>>>>>>>>>> 
>>>>>>>>>> 1.  [Changes in LLVM IR](#llvmIR) to
provide information about the
>>>>>>>>>>   availability of user-defined vector
functions via metadata attached
>>>>>>>>>>   to an `llvm::CallInst`.
>>>>>>>>>> 2.  [An
infrastructure](#infrastructure) that can be queried to retrive
>>>>>>>>>>   information about the available
vector functions associated to a
>>>>>>>>>>   `llvm::CallInst`.
>>>>>>>>>> 3.  [Changes in the
LoopVectorizer](#LV) to use the API to query the
>>>>>>>>>>   metadata.
>>>>>>>>>> 
>>>>>>>>>> The second set consists of the changes
[changes in clang](#clang) that
>>>>>>>>>> are needed too to recognize the
`#pragma clang declare variant`
>>>>>>>>>> directive.
>>>>>>>>>> 
>>>>>>>>>> Proposed changes
>>>>>>>>>>
===============>>>>>>>>>>
>>>>>>>>>> We propose an implementation that uses
`#pragma clang declare variant`
>>>>>>>>>> to inform the backend components about
the availability of vector
>>>>>>>>>> version of scalar functions found in
IR. The mechanism relies in storing
>>>>>>>>>> such information in IR metadata, and
therefore makes the
>>>>>>>>>> auto-vectorization of function calls a
mid-end (`opt`) process that is
>>>>>>>>>> independent on the front-end that
generated such IR metadata.
>>>>>>>>>> 
>>>>>>>>>> This implementation provides a generic
mechanism that the users of the
>>>>>>>>>> LLVM compiler will be able to use for
interfacing their own vector
>>>>>>>>>> routines for generic code.
>>>>>>>>>> 
>>>>>>>>>> The implementation can also expose
vectorization-specific descriptors --
>>>>>>>>>> for example, like the `linear` and
`uniform` clauses of the OpenMP
>>>>>>>>>> `declare simd` directive -- that could
be used to finely tune the
>>>>>>>>>> automatic vectorization of some
functions (think for example the
>>>>>>>>>> vectorization of `double sincos(double
, double *, double *)`, where
>>>>>>>>>> `linear` can be used to give extra
information about the memory layout
>>>>>>>>>> of the 2 pointers parameters in the
vector version).
>>>>>>>>>> 
>>>>>>>>>> The directive `#pragma clang declare
variant` follows the syntax of the
>>>>>>>>>> `#pragma omp declare variant` directive
of OpenMP.
>>>>>>>>>> 
>>>>>>>>>> We define the new directive in the
`clang` namespace instead of using
>>>>>>>>>> the `omp` one of OpenMP to allow the
compiler to perform
>>>>>>>>>> auto-vectorization outside of an OpenMP
SIMD context.
>>>>>>>>>> 
>>>>>>>>>> The mechanism is base on OpenMP to
provide a uniform user experience
>>>>>>>>>> across the two mechanism, and to
maximise the number of shared
>>>>>>>>>> components of the infrastructure needed
in the compiler frontend to
>>>>>>>>>> enable the feature.
>>>>>>>>>> 
>>>>>>>>>> Changes in LLVM IR {#llvmIR}
>>>>>>>>>> ------------------
>>>>>>>>>> 
>>>>>>>>>> The IR is enriched with metadata that
details the availability of vector
>>>>>>>>>> versions of an associated scalar
function. This metadata is attached to
>>>>>>>>>> the call site of the scalar function.
>>>>>>>>>> 
>>>>>>>>>> The metadata takes the form of an
attribute containing a comma separated
>>>>>>>>>> list of vector function mappings. Each
entry has a unique name that
>>>>>>>>>> follows the Vector Function ABI[^2] and
real name that is used when
>>>>>>>>>> generating calls to this vector
function.
>>>>>>>>>> 
>>>>>>>>>>   vfunc_name1(real_name1),
vfunc_name2(real_name2)
>>>>>>>>>> 
>>>>>>>>>> The Vector Function ABI name describes
the signature of the vector
>>>>>>>>>> function so that properties like
vectorisation factor can be queried
>>>>>>>>>> during compilation.
>>>>>>>>>> 
>>>>>>>>>> The `(real name)` token is optional and
assumed to match the Vector
>>>>>>>>>> Function ABI name when omitted.
>>>>>>>>>> 
>>>>>>>>>> For example, the availability of a
2-lane double precision `sin`
>>>>>>>>>> function via SVML when targeting AVX on
x86 is provided by the following
>>>>>>>>>> IR.
>>>>>>>>>> 
>>>>>>>>>>   // ...
>>>>>>>>>>   ... = call double @sin(double) #0
>>>>>>>>>>   // ...
>>>>>>>>>> 
>>>>>>>>>>   #0 = { vector-variant =
{"_ZGVcN2v_sin(__svml_sin2),
>>>>>>>>>>                            
_ZGVdN4v_sin(__svml_sin4),
>>>>>>>>>>                             ..."}
}
>>>>>>>>>> 
>>>>>>>>>> The string
`"_ZGVcN2v_sin(__svml_sin2)"` in this vector-variant
>>>>>>>>>> attribute provides information on the
shape of the vector function via
>>>>>>>>>> the string `_ZGVcN2v_sin`, mangled
according to the Vector Function ABI
>>>>>>>>>> for Intel, and remaps the standard
Vector Function ABI name to the
>>>>>>>>>> non-standard name `__svml_sin2`.
>>>>>>>>>> 
>>>>>>>>>> This metadata is compatible with the
proposal "Proposal for function
>>>>>>>>>> vectorization and loop vectorization
with function calls",[^3] that uses
>>>>>>>>>> Vector Function ABI mangled names to
inform the vectorizer about the
>>>>>>>>>> availability of vector functions. The
proposal extends the original by
>>>>>>>>>> allowing the explicit mapping of the
Vector Function ABI mangled name to
>>>>>>>>>> a non-standard name, which allows the
use of existing vector libraries.
>>>>>>>>>> 
>>>>>>>>>> The `vector-variant` attribute needs to
be attached on a per-call basis
>>>>>>>>>> to avoid conflicts when merging modules
with different vector variants.
>>>>>>>>>> 
>>>>>>>>>> The query infrastructure: SVFS
{#infrastructure}
>>>>>>>>>> ------------------------------
>>>>>>>>>> 
>>>>>>>>>> The Search Vector Function System
(SVFS) is constructed from an
>>>>>>>>>> `llvm::Module` instance so it can
create function definitions. The SVFS
>>>>>>>>>> exposes an API with two methods.
>>>>>>>>>> 
>>>>>>>>>> ### `SVFS::isFunctionVectorizable`
>>>>>>>>>> 
>>>>>>>>>> This method queries the avilability of
a vectorized version of a
>>>>>>>>>> function. The signature of the method
is as follows.
>>>>>>>>>> 
>>>>>>>>>>   bool
isFunctionVectorizable(llvm::CallInst * Call, ParTypeMap Params);
>>>>>>>>>> 
>>>>>>>>>> The method determine the availability
of vector version of the function
>>>>>>>>>> invoked by the `Call` parameter by
looking at the `vector-variant`
>>>>>>>>>> metadata.
>>>>>>>>>> 
>>>>>>>>>> The `Params` argument is a map that
associates the position of a
>>>>>>>>>> parameter in the `CallInst` to its
`ParameterType` descriptor. The
>>>>>>>>>> `ParameterType` descriptor holds
information about the shape of the
>>>>>>>>>> correspondend parameter in the
signature of the vector function. This
>>>>>>>>>> `ParamaterType` is used to query the
SVMS about the availability of
>>>>>>>>>> vector version that have `linear`,
`uniform` or `align` parameters (in
>>>>>>>>>> the sense of OpenMP 4.0 and onwards).
>>>>>>>>>> 
>>>>>>>>>> The method `isFunctionVectorizable`,
when invoked with an empty
>>>>>>>>>> `ParTypeMap`, is equivalent to the
`TargetLibraryInfo` method
>>>>>>>>>> `isFunctionVectorizable(StrinRef
Name)`.
>>>>>>>>>> 
>>>>>>>>>> ### `SVFS::getVectorizedFunction`
>>>>>>>>>> 
>>>>>>>>>> This method returns the vector function
declaration that correspond to
>>>>>>>>>> the needs of the vectorization
technique that is being run.
>>>>>>>>>> 
>>>>>>>>>> The signature of the function is as
follows.
>>>>>>>>>> 
>>>>>>>>>>   std::pair<llvm::FunctionType *,
std::string> getVectorizedFunction(
>>>>>>>>>>     llvm::CallInst * Call, unsigned VF,
bool IsMasked, ParTypeSet Params);
>>>>>>>>>> 
>>>>>>>>>> The `Call` parameter is the call
instance that is being vectorized, the
>>>>>>>>>> `VF` parameter represent the
vectorization factor (how many lanes), the
>>>>>>>>>> `IsMasked` parameter decides whether or
not the signature of the vector
>>>>>>>>>> function is required to have a mask
parameter, the `Params` parameter
>>>>>>>>>> describes the shape of the vector
function as in the
>>>>>>>>>> `isFunctionVectorizable` method.
>>>>>>>>>> 
>>>>>>>>>> The methods uses the `vector-variant`
metadata and returns the function
>>>>>>>>>> signature and the name of the function
based on the input parameters.
>>>>>>>>>> 
>>>>>>>>>> The SVFS can add new function
definitions, in the same module as the
>>>>>>>>>> `Call`, to provide vector functions
that are not present within the
>>>>>>>>>> vector-variant metadata. For example,
if a library provides a vector
>>>>>>>>>> version of a function with a
vectorization factor of 2, but the
>>>>>>>>>> vectorizer is requesting a
vectorization factor of 4, the SVFS is
>>>>>>>>>> allowed to create a definition that
calls the 2-lane version twice. This
>>>>>>>>>> capability applies similarly for
providing masked and unmasked versions
>>>>>>>>>> when the request does not match what is
available in the library.
>>>>>>>>>> 
>>>>>>>>>> This method is equivalent to the TLI
method
>>>>>>>>>> `StringRef
getVectorizedFunction(StringRef F, unsigned VF) const;`.
>>>>>>>>>> 
>>>>>>>>>> Notice that to fully support OpenMP
vectorization we need to think about
>>>>>>>>>> a fuzzy matching mechanism that is able
to select a candidate in the
>>>>>>>>>> calling context. However, this proposal
is intended for scalar-to-vector
>>>>>>>>>> mappings of math-like functions that
are most likely to associate a
>>>>>>>>>> unique vector candidate in most
contexts. Therefore, extending this
>>>>>>>>>> behavior to a generic one is an aspect
of the implementation that will
>>>>>>>>>> be treated in a separate RFC about the
vectorization pass.
>>>>>>>>>> 
>>>>>>>>>> ### Scalable vectorization
>>>>>>>>>> 
>>>>>>>>>> Both methods of the SVFS API will be
extended with a boolean parameter
>>>>>>>>>> to specify whether scalable signatures
are needed by the user of the
>>>>>>>>>> SVFS.
>>>>>>>>>> 
>>>>>>>>>> Changes in clang {#clang}
>>>>>>>>>> ----------------
>>>>>>>>>> 
>>>>>>>>>> We use clang to generate the metadata
described above.
>>>>>>>>>> 
>>>>>>>>>> In the compilation unit, the vector
function definition or declaration
>>>>>>>>>> must be visible and associated to the
scalar version via the
>>>>>>>>>> `#pragma clang declare variant`
according to the rule defined by the
>>>>>>>>>> correspondent `#pragma omp declare
variant` defined in OpenMP 5.0, as in
>>>>>>>>>> the following example.
>>>>>>>>>> 
>>>>>>>>>>   #pragma clang declare
variant(vector_sinf) \
>>>>>>>>>>  
match(construct=simd(simdlen(4),notinbranch), device={isa("simd")})
>>>>>>>>>>   extern float sinf(float);
>>>>>>>>>> 
>>>>>>>>>>   float32x4_t vector_sinf(float32x4_t
x);
>>>>>>>>>> 
>>>>>>>>>> The `construct` set in the directive,
together with the `device` set, is
>>>>>>>>>> used to generate the vector mangled
name to be used in the
>>>>>>>>>> `vector-variant` attribute, for example
`_ZGVnN2v_sin`, when targeting
>>>>>>>>>> AArch64 Advanced SIMD code generation.
The rule for mangling the name of
>>>>>>>>>> the scalar function in the vector name
are defined in the the Vector
>>>>>>>>>> Function ABI specification of the
target.
>>>>>>>>>> 
>>>>>>>>>> The part of the vector-variant
attribute that redirects the call to
>>>>>>>>>> `vector_sinf` is derived from the
`variant-id` specified in the
>>>>>>>>>> `variant` clause.
>>>>>>>>>> 
>>>>>>>>>> Summary
>>>>>>>>>>
======>>>>>>>>>>
>>>>>>>>>> New `clang` directive in clang
>>>>>>>>>> ------------------------------
>>>>>>>>>> 
>>>>>>>>>> `#pragma omp declare variant`, same as
`#pragma omp declare variant`
>>>>>>>>>> restricted to the `simd` context
selector, from OpenMP 5.0+.
>>>>>>>>>> 
>>>>>>>>>> Option behavior, and interaction with
OpenMP
>>>>>>>>>>
--------------------------------------------
>>>>>>>>>> 
>>>>>>>>>> The behavior described below makes sure
that
>>>>>>>>>> `#pragma cland declare variant`
function vectorization and OpenMP
>>>>>>>>>> function vectorization are orthogonal.
>>>>>>>>>> 
>>>>>>>>>> `-fclang-declare-variant`
>>>>>>>>>> 
>>>>>>>>>> :   The `#pragma clang declare variant`
directives are parsed and used
>>>>>>>>>>   to populate the `vector-variant`
attribute.
>>>>>>>>>> 
>>>>>>>>>> `-fopenmp[-simd]`
>>>>>>>>>> 
>>>>>>>>>> :   The `#pragma omp declare variant`
directives are parsed and used to
>>>>>>>>>>   populate the `vector-variant`
attribute.
>>>>>>>>>> 
>>>>>>>>>> `-fopenmp[-simd]`and
`-fno-clang-declare-variant`
>>>>>>>>>> 
>>>>>>>>>> :   The directive `#pragma omp declare
variant` is used to populate the
>>>>>>>>>>   `vector-variant` attribute in IR. The
directive
>>>>>>>>>>   `#pragma   clang declare variant` are
ignored.
>>>>>>>>>> 
>>>>>>>>>> [^1]:
<https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf>
>>>>>>>>>> 
>>>>>>>>>> [^2]: Vector Function ABI for x86:
>>>>>>>>>>  
<https://software.intel.com/en-us/articles/vector-simd-function-abi>.
>>>>>>>>>>   Vector Function ABI for AArch64:
>>>>>>>>>>  
https://developer.arm.com/products/software-development-tools/hpc/arm-compiler-for-hpc/vector-function-abi
>>>>>>>>>> 
>>>>>>>>>> [^3]:
<http://lists.llvm.org/pipermail/cfe-dev/2016-March/047732.html>
>>>>>>>>>> 
>>>>>>>>>>
_______________________________________________
>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>
_______________________________________________
>>>>>>>>> cfe-dev mailing list
>>>>>>>>> cfe-dev at lists.llvm.org
>>>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>> -- 
>>>>>> Hal Finkel
>>>>>> Lead, Compiler Technology and Programming Languages
>>>>>> Leadership Computing Facility
>>>>>> Argonne National Laboratory
>>>>>> 
>>>>>> _______________________________________________
>>>>>> cfe-dev mailing list
>>>>>> cfe-dev at lists.llvm.org
>>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>> 
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>

> On May 31, 2019, at 12:38 PM, Alexey Bataev <a.bataev at hotmail.com>
wrote:
> 
> Francesco, there won't be any duplication. Most of the declarative
OpenMP directives are represented as attributes internally, so, I think, it will
be natural to use an attribute here rather than pragma.
> 
Very nice. I am open to get rid of the `clang` based directive in favor of the
attribute one.

At the moment there is no “declare variant” attribute in the list of common
function attributes at
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes.
What should we do? Define our own, or wait for GCC to publish the attribute?

If we decide to do what GCC does, shall we actively participate in the
discussion with the GCC community to shape the attribute itself? Am I right
thinking that the GCC solution is preferred given that system header files are
more likely to follow whatever GCC comes up with?

> Best regards,
> Alexey Bataev
> 
>> 31 мая 2019 г., в 13:32, Francesco Petrogalli <Francesco.Petrogalli
at arm.com> написал(а):
>> 
>> 
>> 
>>> On May 31, 2019, at 12:00 PM, Alexey Bataev via cfe-dev <cfe-dev
at lists.llvm.org> wrote:
>>> 
>>> Hi Francesco, did you think about adding the attribute instead of
the pragma? It is a common way to express such constructs as function attributes
in clang/GCC rather than as pragma.
>>> 
>> 
>> Yes, I thought about it, I believe that GCC plans to use attributes.
>> 
>> In
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
there is a description of the “simd” attribute, but I couldn’t find an example
of how it is used.
>> 
>> Also, I cannot find an attribute that is equivalent to the “declare
variant” directive. Maybe it is planned for future releases.
>> 
>> The idea of using a `clang` equivalent of the `omp` directive was to
avoid duplication in terms of handling both the attribute and the omp directive,
as both directive will share much of the infrastructure.
>> 
>>> Best regards,
>>> Alexey Bataev
>>> 
>>>> 31 мая 2019 г., в 12:18, Francesco Petrogalli via cfe-dev
<cfe-dev at lists.llvm.org> написал(а):
>>>> 
>>>> Hi All,
>>>> 
>>>> Thank you for the feedback so far.
>>>> 
>>>> I am replying to all your questions/concerns/suggestions in
this single email. Please let me know if I have missed any.
>>>> 
>>>> I will update the RFC accordingly to what we end up deciding
here.
>>>> 
>>>> Kind regards,
>>>> 
>>>> Francesco
>>>> 
>>>> 
>>>> # TOPIC 1: concerns about name mangling
>>>> 
>>>> I understand that there are concerns in using the mangling
scheme I proposed, and that it would be preferred to have a mangling scheme that
is based on (and standardized by) OpenMP. I hear the argument on having some
common ground here. In fact, there is already common ground between the x86 and
aarch64 backend, who have based their respective Vector Function ABI
specifications on OpenMP.
>>>> 
>>>> In fact, the mangled name grammar can be summarized as follows:
>>>> 
>>>> _ZGV<isa><masking><VLEN><parameter
type>_<scalar name>
>>>> 
>>>> Across vector extensions the only <token> that will
differ is the <isa> token.
>>>> 
>>>> This might lead people to think that we could drop the
_ZGV<isa> prefix and consider the <masking><VLEN><parameter
type>_<scalar name> part as a sort of unofficial OpenMP mangling
scheme: in fact, the signature of an “unmasked 2-lane vector vector of `sin`”
will always be `<2 x double>(2 x double>).
>>>> 
>>>> The problem with this choice is the number of vector version
available for a target is not unique.
>>>> 
>>>> In particular, the following declaration generates multiple
vector versions, depending on the target:
>>>> 
>>>> #pragma omp declare simd simdlen(2) notinbranch
>>>> double foo(double) {…};
>>>> 
>>>> On x86, this generates at least 4 symbols (one for SSE, one for
AVX, one for AVX2, and one for AVX512: https://godbolt.org/z/TLYXPi)
>>>> 
>>>> On aarch64, the same declaration generates a unique symbol, as
specified in the Vector Function ABI.
>>>> 
>>>> This means that the attribute (or metadata) that carries the
information on the available vector version needs to deal also with things that
are not usually visible at IR level, but that might still need to be provided to
be able to decide which particular instruction set/ vector extension needs to be
targeted.
>>>> 
>>>> I used an example based on `declare simd` instead of `declare
variant` because the attribute/metadata needed for `declare variant` is a
modification of the one needed for `declare simd`, which has already been agreed
in a previous RFC proposed by Intel [1], and for which Intel has already
provided an implementation [2]. The changes proposed in this RFC are fully
compatible with the work that is being don for the VecClone pass in [2].
>>>> 
>>>> [1]
http://lists.llvm.org/pipermail/cfe-dev/2016-March/047732.html
>>>> [2] VecCLone pass: https://reviews.llvm.org/D22792
>>>> 
>>>> The good news is that as far as AArch64 and x86 are concerned,
the only thing that will differ in the mangled name is the “<isa>” token.
As far as I can tell, the mangling scheme of the rest of the vector name is the
same, therefore a lot of infrastructure in terms of mangling and demangling can
be reused. In fact, the `mangleVectorParameters` function in
https://clang.llvm.org/doxygen/CGOpenMPRuntime_8cpp_source.html#l09918 could
already be shared among x86 and aarch64.
>>>> 
>>>> TOPIC 2: metadata vs attribute
>>>> 
>>>> From a functionality point of view, I don’t care whether we use
metadata or attributes. The VecClone pass mentioned in TOPIC 1 uses the
following:
>>>> 
>>>> attributes #0 = { nounwind uwtable
“vector-variants"="_ZGVbM4vv_vec_sum,_ZGVbN4vv_vec_sum,_ZGVcM8vv_vec_sum,_ZGVcN8vv_vec_sum,_ZGVdM8vv_vec_sum,_ZGVdN8vv_vec_sum,_ZGVeM16vv_vec_sum,_ZGVeN16”}
>>>> 
>>>> This is an attribute (I though it was metadata?), I am happy to
reword the RFC using the right terminology (sorry for messing this up).
>>>> 
>>>> Also, @Renato expressed concern that metadata might be dropped
by optimization passes - would using attributes prevent that?
>>>> 
>>>> TOPIC 3: "There is no way to notify the backend how
conformant the SIMD versions are.”
>>>> 
>>>> @Shawn, I am afraid I don’t understand what you mean by
“conformant” here. Can you elaborate with an example?
>>>> 
>>>> TOPIC 3: interaction of the `omp declare variant` with `clang
declare variant`
>>>> 
>>>> I believe this is described in the `Option behavior, and
interaction with OpenMP`. The option `-fclang-declare-variant` is there to make
the OpenMP based one orthogonal. Of course, we might decide to make
-fclang-declare-variant on/off by default, and have default behavior when
interacting with -fopenmp-simd. For the sake of compatibility with other
compilers, we might need to require -fno-clang-declare-variant when targeting
-fopenmp-[simd].
>>>> 
>>>> TOPIC 3: "there are no special arguments / flags / status
regs that are used / changed in the vector version that the compiler will have
to "just know”
>>>> 
>>>> I believe that this concern is raised by the problem of
handling FP exceptions? If that’s the case, the compiler is not allowed to do
any assumption on the vector function about that, and treat it with the same
knowledge of any other function, depending on the visibility it has in the
compilation unit. @Renato, does this answer your question?
>>>> 
>>>> TOPIC 4: attribute in function declaration vs attribute
function call site
>>>> 
>>>> We discussed this in the previous version of the proposal.
Having it in the call sites guarantees that incompatible vector version are used
when merging modules compiled for different targets. I don’t have a use case for
this, if I remember correctly this was asked by @Hideki Saito. Hideki, any
comment on this?
>>>> 
>>>> TOPIC 5: overriding system header (the discussion on #pragma
omp/clang/system variants initiated by @Hal Finkel).
>>>> 
>>>> I though that the split among #pragma clang declare variant and
#pragma omp declare variant was already providing the orthogonality between
system header and user header. Meaning that a user should always prefer the omp
version (for portability to other compilers) instead of the #pragma clang one,
which would be relegated to system headers and headers provided by the compiler.
Am I missing something? If so, I am happy to add a “system” version of the
directive, as it would be quite easy to do given most of the parsing
infrastructure will be shared.
>>>> 
>>>> 
>>>>> On May 30, 2019, at 12:53 PM, Philip Reames <listmail at
philipreames.com> wrote:
>>>>> 
>>>>> 
>>>>>>> On 5/30/19 9:05 AM, Doerfert, Johannes wrote:
>>>>>>>> On 05/29, Finkel, Hal J. via cfe-dev wrote:
>>>>>>>>> On 5/29/19 1:52 PM, Philip Reames wrote:
>>>>>>>>>> On 5/28/19 7:55 PM, Finkel, Hal J.
wrote:
>>>>>>>>>> On 5/28/19 3:31 PM, Philip Reames via
cfe-dev wrote:
>>>>>>>>>> I generally like the idea of having
support in IR for vectorization of
>>>>>>>>>> custom functions.  I have several use
cases which would benefit from this.
>>>>>>>>>> 
>>>>>>>>>> I'd suggest a couple of reframings
to the IR representation though.
>>>>>>>>>> 
>>>>>>>>>> First, this should probably be
specified as metadata/attribute on a
>>>>>>>>>> function declaration.  Allowing the
callsite variant is fine, but it
>>>>>>>>>> should primarily be a property of the
called function, not of the call
>>>>>>>>>> site.  Being able to specify it once
per declaration is much cleaner.
>>>>>>>>> I agree. We should support this both on the
function declaration and on
>>>>>>>>> the call sites.
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>>> Second, I really don't like the
mangling use here.  We need a better way
>>>>>>>>>> to specify the properties of the
function then it's mangled name.  One
>>>>>>>>>> thought to explore is to directly use
the Value of the function
>>>>>>>>>> declaration (since this is metadata and
we can do that), and then tie
>>>>>>>>>> the properties to the function
declaration in some way?  Sorry, I don't
>>>>>>>>>> really have a specific suggestion here.
>>>>>>>>> Is the problem the mangling or the fact
that the mangling is
>>>>>>>>> ABI/target-specific? One option is to use
LLVM's mangling scheme (the
>>>>>>>>> one we use for intrinsics) and then provide
some backend infrastructure
>>>>>>>>> to translate later.
>>>>>>>> Well, both honestly.  But mangling with a
non-target specific scheme is
>>>>>>>> a lot better, so I might be okay with that.  
Good idea.
>>>>>>> 
>>>>>>> I liked your idea of directly encoding the
signature in the metadata,
>>>>>>> but I think that we want to continue to use
attributes, and not
>>>>>>> metadata, and the options for attributes seem more
limited - unless we
>>>>>>> allow attributes to take metadata arguments - maybe
that's an
>>>>>>> enhancement worth considering.
>>>>>> I recently talked to people in the OpenMP language
committee meeting
>>>>>> about this and, thinking forward to the actual
implementation/use of the
>>>>>> OpenMP 5.x declare variant feature, I'd say:
>>>>>> 
>>>>>> - We will need a mangling scheme if we want to allow
variants on
>>>>>> declarations that are defined elsewhere.
>>>>>> - We will need a (OpenMP) standardized mangling scheme
if we want
>>>>>> interoperability between compilers.
>>>>>> 
>>>>>> I assume we want both so I think we will need both.
>>>>> If I'm reading this correctly, this describes a need
for the frontend to
>>>>> have a mangling scheme.  Nothing in here would seem to
prevent the
>>>>> frontend for generating a declaration for a mangled
external symbol and
>>>>> then referencing that declaration.  Am I missing something?
>>>>>> 
>>>>>> That said, I think this should allow us to avoid
attributes/metadata
>>>>>> which seems to me like a good thing right now.
>>>>>> 
>>>>>> Cheers,
>>>>>> Johannes
>>>>>> 
>>>>>> 
>>>>>>>>>>> On 5/28/19 12:44 PM, Francesco
Petrogalli via llvm-dev wrote:
>>>>>>>>>>> Dear all,
>>>>>>>>>>> 
>>>>>>>>>>> This RFC is a proposal to provide
auto-vectorization functionality for user provided vector functions.
>>>>>>>>>>> 
>>>>>>>>>>> The proposal is a modification of
an RFC that I have sent out a couple of months ago, with the title `[RFC]
Re-implementing -fveclib with OpenMP` (see
http://lists.llvm.org/pipermail/llvm-dev/2018-December/128426.html). The
previous RFC is to be considered abandoned.
>>>>>>>>>>> 
>>>>>>>>>>> The original RFC was proposing to
re-implement the `-fveclib` command line option. This proposal avoids that, and
limits its scope to the mechanics of providing vector function in user code that
the compiler can pick up for auto-vectorization. This narrower scope limits the
impact of changes that are needed in both clang and LLVM.
>>>>>>>>>>> 
>>>>>>>>>>> Please let me know what you think.
>>>>>>>>>>> 
>>>>>>>>>>> Kind regards,
>>>>>>>>>>> 
>>>>>>>>>>> Francesco
>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>>
================================================================================>>>>>>>>>>>
>>>>>>>>>>> Introduction
>>>>>>>>>>>
===========>>>>>>>>>>>
>>>>>>>>>>> This RFC encompasses the proposal
of informing the vectorizer about the
>>>>>>>>>>> availability of vector functions
provided by the user. The mechanism is
>>>>>>>>>>> based on the use of the directive
`declare variant` introduced in OpenMP
>>>>>>>>>>> 5.0 [^1].
>>>>>>>>>>> 
>>>>>>>>>>> The mechanism proposed has the
following properties:
>>>>>>>>>>> 
>>>>>>>>>>> 1.  Decouples the compiler
front-end that knows about the availability
>>>>>>>>>>>  of vectorized routines, from the
back-end that knows how to make use
>>>>>>>>>>>  of them.
>>>>>>>>>>> 2.  Enable support for a
developer's own vector libraries without
>>>>>>>>>>>  requiring changes to the compiler.
>>>>>>>>>>> 3.  Enables other frontends (e.g.
f18) to add scalar-to-vector function
>>>>>>>>>>>  mappings as relevant for their own
runtime libraries, etc.
>>>>>>>>>>> 
>>>>>>>>>>> The implemetation consists of two
separate sets of changes.
>>>>>>>>>>> 
>>>>>>>>>>> The first set is a set o changes in
`llvm`, and consists of:
>>>>>>>>>>> 
>>>>>>>>>>> 1.  [Changes in LLVM IR](#llvmIR)
to provide information about the
>>>>>>>>>>>  availability of user-defined
vector functions via metadata attached
>>>>>>>>>>>  to an `llvm::CallInst`.
>>>>>>>>>>> 2.  [An
infrastructure](#infrastructure) that can be queried to retrive
>>>>>>>>>>>  information about the available
vector functions associated to a
>>>>>>>>>>>  `llvm::CallInst`.
>>>>>>>>>>> 3.  [Changes in the
LoopVectorizer](#LV) to use the API to query the
>>>>>>>>>>>  metadata.
>>>>>>>>>>> 
>>>>>>>>>>> The second set consists of the
changes [changes in clang](#clang) that
>>>>>>>>>>> are needed too to recognize the
`#pragma clang declare variant`
>>>>>>>>>>> directive.
>>>>>>>>>>> 
>>>>>>>>>>> Proposed changes
>>>>>>>>>>>
===============>>>>>>>>>>>
>>>>>>>>>>> We propose an implementation that
uses `#pragma clang declare variant`
>>>>>>>>>>> to inform the backend components
about the availability of vector
>>>>>>>>>>> version of scalar functions found
in IR. The mechanism relies in storing
>>>>>>>>>>> such information in IR metadata,
and therefore makes the
>>>>>>>>>>> auto-vectorization of function
calls a mid-end (`opt`) process that is
>>>>>>>>>>> independent on the front-end that
generated such IR metadata.
>>>>>>>>>>> 
>>>>>>>>>>> This implementation provides a
generic mechanism that the users of the
>>>>>>>>>>> LLVM compiler will be able to use
for interfacing their own vector
>>>>>>>>>>> routines for generic code.
>>>>>>>>>>> 
>>>>>>>>>>> The implementation can also expose
vectorization-specific descriptors --
>>>>>>>>>>> for example, like the `linear` and
`uniform` clauses of the OpenMP
>>>>>>>>>>> `declare simd` directive -- that
could be used to finely tune the
>>>>>>>>>>> automatic vectorization of some
functions (think for example the
>>>>>>>>>>> vectorization of `double
sincos(double , double *, double *)`, where
>>>>>>>>>>> `linear` can be used to give extra
information about the memory layout
>>>>>>>>>>> of the 2 pointers parameters in the
vector version).
>>>>>>>>>>> 
>>>>>>>>>>> The directive `#pragma clang
declare variant` follows the syntax of the
>>>>>>>>>>> `#pragma omp declare variant`
directive of OpenMP.
>>>>>>>>>>> 
>>>>>>>>>>> We define the new directive in the
`clang` namespace instead of using
>>>>>>>>>>> the `omp` one of OpenMP to allow
the compiler to perform
>>>>>>>>>>> auto-vectorization outside of an
OpenMP SIMD context.
>>>>>>>>>>> 
>>>>>>>>>>> The mechanism is base on OpenMP to
provide a uniform user experience
>>>>>>>>>>> across the two mechanism, and to
maximise the number of shared
>>>>>>>>>>> components of the infrastructure
needed in the compiler frontend to
>>>>>>>>>>> enable the feature.
>>>>>>>>>>> 
>>>>>>>>>>> Changes in LLVM IR {#llvmIR}
>>>>>>>>>>> ------------------
>>>>>>>>>>> 
>>>>>>>>>>> The IR is enriched with metadata
that details the availability of vector
>>>>>>>>>>> versions of an associated scalar
function. This metadata is attached to
>>>>>>>>>>> the call site of the scalar
function.
>>>>>>>>>>> 
>>>>>>>>>>> The metadata takes the form of an
attribute containing a comma separated
>>>>>>>>>>> list of vector function mappings.
Each entry has a unique name that
>>>>>>>>>>> follows the Vector Function ABI[^2]
and real name that is used when
>>>>>>>>>>> generating calls to this vector
function.
>>>>>>>>>>> 
>>>>>>>>>>>  vfunc_name1(real_name1),
vfunc_name2(real_name2)
>>>>>>>>>>> 
>>>>>>>>>>> The Vector Function ABI name
describes the signature of the vector
>>>>>>>>>>> function so that properties like
vectorisation factor can be queried
>>>>>>>>>>> during compilation.
>>>>>>>>>>> 
>>>>>>>>>>> The `(real name)` token is optional
and assumed to match the Vector
>>>>>>>>>>> Function ABI name when omitted.
>>>>>>>>>>> 
>>>>>>>>>>> For example, the availability of a
2-lane double precision `sin`
>>>>>>>>>>> function via SVML when targeting
AVX on x86 is provided by the following
>>>>>>>>>>> IR.
>>>>>>>>>>> 
>>>>>>>>>>>  // ...
>>>>>>>>>>>  ... = call double @sin(double) #0
>>>>>>>>>>>  // ...
>>>>>>>>>>> 
>>>>>>>>>>>  #0 = { vector-variant =
{"_ZGVcN2v_sin(__svml_sin2),
>>>>>>>>>>>                           
_ZGVdN4v_sin(__svml_sin4),
>>>>>>>>>>>                           
..."} }
>>>>>>>>>>> 
>>>>>>>>>>> The string
`"_ZGVcN2v_sin(__svml_sin2)"` in this vector-variant
>>>>>>>>>>> attribute provides information on
the shape of the vector function via
>>>>>>>>>>> the string `_ZGVcN2v_sin`, mangled
according to the Vector Function ABI
>>>>>>>>>>> for Intel, and remaps the standard
Vector Function ABI name to the
>>>>>>>>>>> non-standard name `__svml_sin2`.
>>>>>>>>>>> 
>>>>>>>>>>> This metadata is compatible with
the proposal "Proposal for function
>>>>>>>>>>> vectorization and loop
vectorization with function calls",[^3] that uses
>>>>>>>>>>> Vector Function ABI mangled names
to inform the vectorizer about the
>>>>>>>>>>> availability of vector functions.
The proposal extends the original by
>>>>>>>>>>> allowing the explicit mapping of
the Vector Function ABI mangled name to
>>>>>>>>>>> a non-standard name, which allows
the use of existing vector libraries.
>>>>>>>>>>> 
>>>>>>>>>>> The `vector-variant` attribute
needs to be attached on a per-call basis
>>>>>>>>>>> to avoid conflicts when merging
modules with different vector variants.
>>>>>>>>>>> 
>>>>>>>>>>> The query infrastructure: SVFS
{#infrastructure}
>>>>>>>>>>> ------------------------------
>>>>>>>>>>> 
>>>>>>>>>>> The Search Vector Function System
(SVFS) is constructed from an
>>>>>>>>>>> `llvm::Module` instance so it can
create function definitions. The SVFS
>>>>>>>>>>> exposes an API with two methods.
>>>>>>>>>>> 
>>>>>>>>>>> ### `SVFS::isFunctionVectorizable`
>>>>>>>>>>> 
>>>>>>>>>>> This method queries the avilability
of a vectorized version of a
>>>>>>>>>>> function. The signature of the
method is as follows.
>>>>>>>>>>> 
>>>>>>>>>>>  bool
isFunctionVectorizable(llvm::CallInst * Call, ParTypeMap Params);
>>>>>>>>>>> 
>>>>>>>>>>> The method determine the
availability of vector version of the function
>>>>>>>>>>> invoked by the `Call` parameter by
looking at the `vector-variant`
>>>>>>>>>>> metadata.
>>>>>>>>>>> 
>>>>>>>>>>> The `Params` argument is a map that
associates the position of a
>>>>>>>>>>> parameter in the `CallInst` to its
`ParameterType` descriptor. The
>>>>>>>>>>> `ParameterType` descriptor holds
information about the shape of the
>>>>>>>>>>> correspondend parameter in the
signature of the vector function. This
>>>>>>>>>>> `ParamaterType` is used to query
the SVMS about the availability of
>>>>>>>>>>> vector version that have `linear`,
`uniform` or `align` parameters (in
>>>>>>>>>>> the sense of OpenMP 4.0 and
onwards).
>>>>>>>>>>> 
>>>>>>>>>>> The method
`isFunctionVectorizable`, when invoked with an empty
>>>>>>>>>>> `ParTypeMap`, is equivalent to the
`TargetLibraryInfo` method
>>>>>>>>>>> `isFunctionVectorizable(StrinRef
Name)`.
>>>>>>>>>>> 
>>>>>>>>>>> ### `SVFS::getVectorizedFunction`
>>>>>>>>>>> 
>>>>>>>>>>> This method returns the vector
function declaration that correspond to
>>>>>>>>>>> the needs of the vectorization
technique that is being run.
>>>>>>>>>>> 
>>>>>>>>>>> The signature of the function is as
follows.
>>>>>>>>>>> 
>>>>>>>>>>>  std::pair<llvm::FunctionType *,
std::string> getVectorizedFunction(
>>>>>>>>>>>    llvm::CallInst * Call, unsigned
VF, bool IsMasked, ParTypeSet Params);
>>>>>>>>>>> 
>>>>>>>>>>> The `Call` parameter is the call
instance that is being vectorized, the
>>>>>>>>>>> `VF` parameter represent the
vectorization factor (how many lanes), the
>>>>>>>>>>> `IsMasked` parameter decides
whether or not the signature of the vector
>>>>>>>>>>> function is required to have a mask
parameter, the `Params` parameter
>>>>>>>>>>> describes the shape of the vector
function as in the
>>>>>>>>>>> `isFunctionVectorizable` method.
>>>>>>>>>>> 
>>>>>>>>>>> The methods uses the
`vector-variant` metadata and returns the function
>>>>>>>>>>> signature and the name of the
function based on the input parameters.
>>>>>>>>>>> 
>>>>>>>>>>> The SVFS can add new function
definitions, in the same module as the
>>>>>>>>>>> `Call`, to provide vector functions
that are not present within the
>>>>>>>>>>> vector-variant metadata. For
example, if a library provides a vector
>>>>>>>>>>> version of a function with a
vectorization factor of 2, but the
>>>>>>>>>>> vectorizer is requesting a
vectorization factor of 4, the SVFS is
>>>>>>>>>>> allowed to create a definition that
calls the 2-lane version twice. This
>>>>>>>>>>> capability applies similarly for
providing masked and unmasked versions
>>>>>>>>>>> when the request does not match
what is available in the library.
>>>>>>>>>>> 
>>>>>>>>>>> This method is equivalent to the
TLI method
>>>>>>>>>>> `StringRef
getVectorizedFunction(StringRef F, unsigned VF) const;`.
>>>>>>>>>>> 
>>>>>>>>>>> Notice that to fully support OpenMP
vectorization we need to think about
>>>>>>>>>>> a fuzzy matching mechanism that is
able to select a candidate in the
>>>>>>>>>>> calling context. However, this
proposal is intended for scalar-to-vector
>>>>>>>>>>> mappings of math-like functions
that are most likely to associate a
>>>>>>>>>>> unique vector candidate in most
contexts. Therefore, extending this
>>>>>>>>>>> behavior to a generic one is an
aspect of the implementation that will
>>>>>>>>>>> be treated in a separate RFC about
the vectorization pass.
>>>>>>>>>>> 
>>>>>>>>>>> ### Scalable vectorization
>>>>>>>>>>> 
>>>>>>>>>>> Both methods of the SVFS API will
be extended with a boolean parameter
>>>>>>>>>>> to specify whether scalable
signatures are needed by the user of the
>>>>>>>>>>> SVFS.
>>>>>>>>>>> 
>>>>>>>>>>> Changes in clang {#clang}
>>>>>>>>>>> ----------------
>>>>>>>>>>> 
>>>>>>>>>>> We use clang to generate the
metadata described above.
>>>>>>>>>>> 
>>>>>>>>>>> In the compilation unit, the vector
function definition or declaration
>>>>>>>>>>> must be visible and associated to
the scalar version via the
>>>>>>>>>>> `#pragma clang declare variant`
according to the rule defined by the
>>>>>>>>>>> correspondent `#pragma omp declare
variant` defined in OpenMP 5.0, as in
>>>>>>>>>>> the following example.
>>>>>>>>>>> 
>>>>>>>>>>>  #pragma clang declare
variant(vector_sinf) \
>>>>>>>>>>> 
match(construct=simd(simdlen(4),notinbranch), device={isa("simd")})
>>>>>>>>>>>  extern float sinf(float);
>>>>>>>>>>> 
>>>>>>>>>>>  float32x4_t
vector_sinf(float32x4_t x);
>>>>>>>>>>> 
>>>>>>>>>>> The `construct` set in the
directive, together with the `device` set, is
>>>>>>>>>>> used to generate the vector mangled
name to be used in the
>>>>>>>>>>> `vector-variant` attribute, for
example `_ZGVnN2v_sin`, when targeting
>>>>>>>>>>> AArch64 Advanced SIMD code
generation. The rule for mangling the name of
>>>>>>>>>>> the scalar function in the vector
name are defined in the the Vector
>>>>>>>>>>> Function ABI specification of the
target.
>>>>>>>>>>> 
>>>>>>>>>>> The part of the vector-variant
attribute that redirects the call to
>>>>>>>>>>> `vector_sinf` is derived from the
`variant-id` specified in the
>>>>>>>>>>> `variant` clause.
>>>>>>>>>>> 
>>>>>>>>>>> Summary
>>>>>>>>>>>
======>>>>>>>>>>>
>>>>>>>>>>> New `clang` directive in clang
>>>>>>>>>>> ------------------------------
>>>>>>>>>>> 
>>>>>>>>>>> `#pragma omp declare variant`, same
as `#pragma omp declare variant`
>>>>>>>>>>> restricted to the `simd` context
selector, from OpenMP 5.0+.
>>>>>>>>>>> 
>>>>>>>>>>> Option behavior, and interaction
with OpenMP
>>>>>>>>>>>
--------------------------------------------
>>>>>>>>>>> 
>>>>>>>>>>> The behavior described below makes
sure that
>>>>>>>>>>> `#pragma cland declare variant`
function vectorization and OpenMP
>>>>>>>>>>> function vectorization are
orthogonal.
>>>>>>>>>>> 
>>>>>>>>>>> `-fclang-declare-variant`
>>>>>>>>>>> 
>>>>>>>>>>> :   The `#pragma clang declare
variant` directives are parsed and used
>>>>>>>>>>>  to populate the `vector-variant`
attribute.
>>>>>>>>>>> 
>>>>>>>>>>> `-fopenmp[-simd]`
>>>>>>>>>>> 
>>>>>>>>>>> :   The `#pragma omp declare
variant` directives are parsed and used to
>>>>>>>>>>>  populate the `vector-variant`
attribute.
>>>>>>>>>>> 
>>>>>>>>>>> `-fopenmp[-simd]`and
`-fno-clang-declare-variant`
>>>>>>>>>>> 
>>>>>>>>>>> :   The directive `#pragma omp
declare variant` is used to populate the
>>>>>>>>>>>  `vector-variant` attribute in IR.
The directive
>>>>>>>>>>>  `#pragma   clang declare variant`
are ignored.
>>>>>>>>>>> 
>>>>>>>>>>> [^1]:
<https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf>
>>>>>>>>>>> 
>>>>>>>>>>> [^2]: Vector Function ABI for x86:
>>>>>>>>>>> 
<https://software.intel.com/en-us/articles/vector-simd-function-abi>.
>>>>>>>>>>>  Vector Function ABI for AArch64:
>>>>>>>>>>> 
https://developer.arm.com/products/software-development-tools/hpc/arm-compiler-for-hpc/vector-function-abi
>>>>>>>>>>> 
>>>>>>>>>>> [^3]:
<http://lists.llvm.org/pipermail/cfe-dev/2016-March/047732.html>
>>>>>>>>>>> 
>>>>>>>>>>>
_______________________________________________
>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>
_______________________________________________
>>>>>>>>>> cfe-dev mailing list
>>>>>>>>>> cfe-dev at lists.llvm.org
>>>>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>>> -- 
>>>>>>> Hal Finkel
>>>>>>> Lead, Compiler Technology and Programming Languages
>>>>>>> Leadership Computing Facility
>>>>>>> Argonne National Laboratory
>>>>>>> 
>>>>>>> _______________________________________________
>>>>>>> cfe-dev mailing list
>>>>>>> cfe-dev at lists.llvm.org
>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>> 
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>

You can define clang specific attribute and later add GCC alias for it.

Best regards,
Alexey Bataev
> 31 мая 2019 г., в 13:46, Francesco Petrogalli <Francesco.Petrogalli at
arm.com> написал(а):
> 
> 
> 
>> On May 31, 2019, at 12:38 PM, Alexey Bataev <a.bataev at
hotmail.com> wrote:
>> 
>> Francesco, there won't be any duplication. Most of the declarative
OpenMP directives are represented as attributes internally, so, I think, it will
be natural to use an attribute here rather than pragma.
>> 
> 
> Very nice. I am open to get rid of the `clang` based directive in favor of
the attribute one.
> 
> At the moment there is no “declare variant” attribute in the list of common
function attributes at
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes.
What should we do? Define our own, or wait for GCC to publish the attribute?
> 
> If we decide to do what GCC does, shall we actively participate in the
discussion with the GCC community to shape the attribute itself? Am I right
thinking that the GCC solution is preferred given that system header files are
more likely to follow whatever GCC comes up with?
> 
> 
>> Best regards,
>> Alexey Bataev
>> 
>>> 31 мая 2019 г., в 13:32, Francesco Petrogalli
<Francesco.Petrogalli at arm.com> написал(а):
>>> 
>>> 
>>> 
>>>> On May 31, 2019, at 12:00 PM, Alexey Bataev via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>>>> 
>>>> Hi Francesco, did you think about adding the attribute instead
of the pragma? It is a common way to express such constructs as function
attributes in clang/GCC rather than as pragma.
>>>> 
>>> 
>>> Yes, I thought about it, I believe that GCC plans to use
attributes.
>>> 
>>> In
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
there is a description of the “simd” attribute, but I couldn’t find an example
of how it is used.
>>> 
>>> Also, I cannot find an attribute that is equivalent to the “declare
variant” directive. Maybe it is planned for future releases.
>>> 
>>> The idea of using a `clang` equivalent of the `omp` directive was
to avoid duplication in terms of handling both the attribute and the omp
directive, as both directive will share much of the infrastructure.
>>> 
>>>> Best regards,
>>>> Alexey Bataev
>>>> 
>>>>> 31 мая 2019 г., в 12:18, Francesco Petrogalli via cfe-dev
<cfe-dev at lists.llvm.org> написал(а):
>>>>> 
>>>>> Hi All,
>>>>> 
>>>>> Thank you for the feedback so far.
>>>>> 
>>>>> I am replying to all your questions/concerns/suggestions in
this single email. Please let me know if I have missed any.
>>>>> 
>>>>> I will update the RFC accordingly to what we end up
deciding here.
>>>>> 
>>>>> Kind regards,
>>>>> 
>>>>> Francesco
>>>>> 
>>>>> 
>>>>> # TOPIC 1: concerns about name mangling
>>>>> 
>>>>> I understand that there are concerns in using the mangling
scheme I proposed, and that it would be preferred to have a mangling scheme that
is based on (and standardized by) OpenMP. I hear the argument on having some
common ground here. In fact, there is already common ground between the x86 and
aarch64 backend, who have based their respective Vector Function ABI
specifications on OpenMP.
>>>>> 
>>>>> In fact, the mangled name grammar can be summarized as
follows:
>>>>> 
>>>>> _ZGV<isa><masking><VLEN><parameter
type>_<scalar name>
>>>>> 
>>>>> Across vector extensions the only <token> that will
differ is the <isa> token.
>>>>> 
>>>>> This might lead people to think that we could drop the
_ZGV<isa> prefix and consider the <masking><VLEN><parameter
type>_<scalar name> part as a sort of unofficial OpenMP mangling
scheme: in fact, the signature of an “unmasked 2-lane vector vector of `sin`”
will always be `<2 x double>(2 x double>).
>>>>> 
>>>>> The problem with this choice is the number of vector
version available for a target is not unique.
>>>>> 
>>>>> In particular, the following declaration generates multiple
vector versions, depending on the target:
>>>>> 
>>>>> #pragma omp declare simd simdlen(2) notinbranch
>>>>> double foo(double) {…};
>>>>> 
>>>>> On x86, this generates at least 4 symbols (one for SSE, one
for AVX, one for AVX2, and one for AVX512: https://godbolt.org/z/TLYXPi)
>>>>> 
>>>>> On aarch64, the same declaration generates a unique symbol,
as specified in the Vector Function ABI.
>>>>> 
>>>>> This means that the attribute (or metadata) that carries
the information on the available vector version needs to deal also with things
that are not usually visible at IR level, but that might still need to be
provided to be able to decide which particular instruction set/ vector extension
needs to be targeted.
>>>>> 
>>>>> I used an example based on `declare simd` instead of
`declare variant` because the attribute/metadata needed for `declare variant` is
a modification of the one needed for `declare simd`, which has already been
agreed in a previous RFC proposed by Intel [1], and for which Intel has already
provided an implementation [2]. The changes proposed in this RFC are fully
compatible with the work that is being don for the VecClone pass in [2].
>>>>> 
>>>>> [1]
http://lists.llvm.org/pipermail/cfe-dev/2016-March/047732.html
>>>>> [2] VecCLone pass: https://reviews.llvm.org/D22792
>>>>> 
>>>>> The good news is that as far as AArch64 and x86 are
concerned, the only thing that will differ in the mangled name is the
“<isa>” token. As far as I can tell, the mangling scheme of the rest of
the vector name is the same, therefore a lot of infrastructure in terms of
mangling and demangling can be reused. In fact, the `mangleVectorParameters`
function in
https://clang.llvm.org/doxygen/CGOpenMPRuntime_8cpp_source.html#l09918 could
already be shared among x86 and aarch64.
>>>>> 
>>>>> TOPIC 2: metadata vs attribute
>>>>> 
>>>>> From a functionality point of view, I don’t care whether we
use metadata or attributes. The VecClone pass mentioned in TOPIC 1 uses the
following:
>>>>> 
>>>>> attributes #0 = { nounwind uwtable
“vector-variants"="_ZGVbM4vv_vec_sum,_ZGVbN4vv_vec_sum,_ZGVcM8vv_vec_sum,_ZGVcN8vv_vec_sum,_ZGVdM8vv_vec_sum,_ZGVdN8vv_vec_sum,_ZGVeM16vv_vec_sum,_ZGVeN16”}
>>>>> 
>>>>> This is an attribute (I though it was metadata?), I am
happy to reword the RFC using the right terminology (sorry for messing this up).
>>>>> 
>>>>> Also, @Renato expressed concern that metadata might be
dropped by optimization passes - would using attributes prevent that?
>>>>> 
>>>>> TOPIC 3: "There is no way to notify the backend how
conformant the SIMD versions are.”
>>>>> 
>>>>> @Shawn, I am afraid I don’t understand what you mean by
“conformant” here. Can you elaborate with an example?
>>>>> 
>>>>> TOPIC 3: interaction of the `omp declare variant` with
`clang declare variant`
>>>>> 
>>>>> I believe this is described in the `Option behavior, and
interaction with OpenMP`. The option `-fclang-declare-variant` is there to make
the OpenMP based one orthogonal. Of course, we might decide to make
-fclang-declare-variant on/off by default, and have default behavior when
interacting with -fopenmp-simd. For the sake of compatibility with other
compilers, we might need to require -fno-clang-declare-variant when targeting
-fopenmp-[simd].
>>>>> 
>>>>> TOPIC 3: "there are no special arguments / flags /
status regs that are used / changed in the vector version that the compiler will
have to "just know”
>>>>> 
>>>>> I believe that this concern is raised by the problem of
handling FP exceptions? If that’s the case, the compiler is not allowed to do
any assumption on the vector function about that, and treat it with the same
knowledge of any other function, depending on the visibility it has in the
compilation unit. @Renato, does this answer your question?
>>>>> 
>>>>> TOPIC 4: attribute in function declaration vs attribute
function call site
>>>>> 
>>>>> We discussed this in the previous version of the proposal.
Having it in the call sites guarantees that incompatible vector version are used
when merging modules compiled for different targets. I don’t have a use case for
this, if I remember correctly this was asked by @Hideki Saito. Hideki, any
comment on this?
>>>>> 
>>>>> TOPIC 5: overriding system header (the discussion on
#pragma omp/clang/system variants initiated by @Hal Finkel).
>>>>> 
>>>>> I though that the split among #pragma clang declare variant
and #pragma omp declare variant was already providing the orthogonality between
system header and user header. Meaning that a user should always prefer the omp
version (for portability to other compilers) instead of the #pragma clang one,
which would be relegated to system headers and headers provided by the compiler.
Am I missing something? If so, I am happy to add a “system” version of the
directive, as it would be quite easy to do given most of the parsing
infrastructure will be shared.
>>>>> 
>>>>> 
>>>>>> On May 30, 2019, at 12:53 PM, Philip Reames
<listmail at philipreames.com> wrote:
>>>>>> 
>>>>>> 
>>>>>>>>> On 5/30/19 9:05 AM, Doerfert, Johannes
wrote:
>>>>>>>>>> On 05/29, Finkel, Hal J. via cfe-dev
wrote:
>>>>>>>>>>> On 5/29/19 1:52 PM, Philip Reames
wrote:
>>>>>>>>>>> On 5/28/19 7:55 PM, Finkel, Hal J.
wrote:
>>>>>>>>>>> On 5/28/19 3:31 PM, Philip Reames
via cfe-dev wrote:
>>>>>>>>>>> I generally like the idea of having
support in IR for vectorization of
>>>>>>>>>>> custom functions.  I have several
use cases which would benefit from this.
>>>>>>>>>>> 
>>>>>>>>>>> I'd suggest a couple of
reframings to the IR representation though.
>>>>>>>>>>> 
>>>>>>>>>>> First, this should probably be
specified as metadata/attribute on a
>>>>>>>>>>> function declaration.  Allowing the
callsite variant is fine, but it
>>>>>>>>>>> should primarily be a property of
the called function, not of the call
>>>>>>>>>>> site.  Being able to specify it
once per declaration is much cleaner.
>>>>>>>>>> I agree. We should support this both on
the function declaration and on
>>>>>>>>>> the call sites.
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>>>> Second, I really don't like the
mangling use here.  We need a better way
>>>>>>>>>>> to specify the properties of the
function then it's mangled name.  One
>>>>>>>>>>> thought to explore is to directly
use the Value of the function
>>>>>>>>>>> declaration (since this is metadata
and we can do that), and then tie
>>>>>>>>>>> the properties to the function
declaration in some way?  Sorry, I don't
>>>>>>>>>>> really have a specific suggestion
here.
>>>>>>>>>> Is the problem the mangling or the fact
that the mangling is
>>>>>>>>>> ABI/target-specific? One option is to
use LLVM's mangling scheme (the
>>>>>>>>>> one we use for intrinsics) and then
provide some backend infrastructure
>>>>>>>>>> to translate later.
>>>>>>>>> Well, both honestly.  But mangling with a
non-target specific scheme is
>>>>>>>>> a lot better, so I might be okay with that.
Good idea.
>>>>>>>> 
>>>>>>>> I liked your idea of directly encoding the
signature in the metadata,
>>>>>>>> but I think that we want to continue to use
attributes, and not
>>>>>>>> metadata, and the options for attributes seem
more limited - unless we
>>>>>>>> allow attributes to take metadata arguments -
maybe that's an
>>>>>>>> enhancement worth considering.
>>>>>>> I recently talked to people in the OpenMP language
committee meeting
>>>>>>> about this and, thinking forward to the actual
implementation/use of the
>>>>>>> OpenMP 5.x declare variant feature, I'd say:
>>>>>>> 
>>>>>>> - We will need a mangling scheme if we want to
allow variants on
>>>>>>> declarations that are defined elsewhere.
>>>>>>> - We will need a (OpenMP) standardized mangling
scheme if we want
>>>>>>> interoperability between compilers.
>>>>>>> 
>>>>>>> I assume we want both so I think we will need both.
>>>>>> If I'm reading this correctly, this describes a
need for the frontend to
>>>>>> have a mangling scheme.  Nothing in here would seem to
prevent the
>>>>>> frontend for generating a declaration for a mangled
external symbol and
>>>>>> then referencing that declaration.  Am I missing
something?
>>>>>>> 
>>>>>>> That said, I think this should allow us to avoid
attributes/metadata
>>>>>>> which seems to me like a good thing right now.
>>>>>>> 
>>>>>>> Cheers,
>>>>>>> Johannes
>>>>>>> 
>>>>>>> 
>>>>>>>>>>>> On 5/28/19 12:44 PM, Francesco
Petrogalli via llvm-dev wrote:
>>>>>>>>>>>> Dear all,
>>>>>>>>>>>> 
>>>>>>>>>>>> This RFC is a proposal to
provide auto-vectorization functionality for user provided vector functions.
>>>>>>>>>>>> 
>>>>>>>>>>>> The proposal is a modification
of an RFC that I have sent out a couple of months ago, with the title `[RFC]
Re-implementing -fveclib with OpenMP` (see
http://lists.llvm.org/pipermail/llvm-dev/2018-December/128426.html). The
previous RFC is to be considered abandoned.
>>>>>>>>>>>> 
>>>>>>>>>>>> The original RFC was proposing
to re-implement the `-fveclib` command line option. This proposal avoids that,
and limits its scope to the mechanics of providing vector function in user code
that the compiler can pick up for auto-vectorization. This narrower scope limits
the impact of changes that are needed in both clang and LLVM.
>>>>>>>>>>>> 
>>>>>>>>>>>> Please let me know what you
think.
>>>>>>>>>>>> 
>>>>>>>>>>>> Kind regards,
>>>>>>>>>>>> 
>>>>>>>>>>>> Francesco
>>>>>>>>>>>> 
>>>>>>>>>>>> 
>>>>>>>>>>>>
================================================================================>>>>>>>>>>>>
>>>>>>>>>>>> Introduction
>>>>>>>>>>>>
===========>>>>>>>>>>>>
>>>>>>>>>>>> This RFC encompasses the
proposal of informing the vectorizer about the
>>>>>>>>>>>> availability of vector
functions provided by the user. The mechanism is
>>>>>>>>>>>> based on the use of the
directive `declare variant` introduced in OpenMP
>>>>>>>>>>>> 5.0 [^1].
>>>>>>>>>>>> 
>>>>>>>>>>>> The mechanism proposed has the
following properties:
>>>>>>>>>>>> 
>>>>>>>>>>>> 1.  Decouples the compiler
front-end that knows about the availability
>>>>>>>>>>>> of vectorized routines, from
the back-end that knows how to make use
>>>>>>>>>>>> of them.
>>>>>>>>>>>> 2.  Enable support for a
developer's own vector libraries without
>>>>>>>>>>>> requiring changes to the
compiler.
>>>>>>>>>>>> 3.  Enables other frontends
(e.g. f18) to add scalar-to-vector function
>>>>>>>>>>>> mappings as relevant for their
own runtime libraries, etc.
>>>>>>>>>>>> 
>>>>>>>>>>>> The implemetation consists of
two separate sets of changes.
>>>>>>>>>>>> 
>>>>>>>>>>>> The first set is a set o
changes in `llvm`, and consists of:
>>>>>>>>>>>> 
>>>>>>>>>>>> 1.  [Changes in LLVM
IR](#llvmIR) to provide information about the
>>>>>>>>>>>> availability of user-defined
vector functions via metadata attached
>>>>>>>>>>>> to an `llvm::CallInst`.
>>>>>>>>>>>> 2.  [An
infrastructure](#infrastructure) that can be queried to retrive
>>>>>>>>>>>> information about the available
vector functions associated to a
>>>>>>>>>>>> `llvm::CallInst`.
>>>>>>>>>>>> 3.  [Changes in the
LoopVectorizer](#LV) to use the API to query the
>>>>>>>>>>>> metadata.
>>>>>>>>>>>> 
>>>>>>>>>>>> The second set consists of the
changes [changes in clang](#clang) that
>>>>>>>>>>>> are needed too to recognize the
`#pragma clang declare variant`
>>>>>>>>>>>> directive.
>>>>>>>>>>>> 
>>>>>>>>>>>> Proposed changes
>>>>>>>>>>>>
===============>>>>>>>>>>>>
>>>>>>>>>>>> We propose an implementation
that uses `#pragma clang declare variant`
>>>>>>>>>>>> to inform the backend
components about the availability of vector
>>>>>>>>>>>> version of scalar functions
found in IR. The mechanism relies in storing
>>>>>>>>>>>> such information in IR
metadata, and therefore makes the
>>>>>>>>>>>> auto-vectorization of function
calls a mid-end (`opt`) process that is
>>>>>>>>>>>> independent on the front-end
that generated such IR metadata.
>>>>>>>>>>>> 
>>>>>>>>>>>> This implementation provides a
generic mechanism that the users of the
>>>>>>>>>>>> LLVM compiler will be able to
use for interfacing their own vector
>>>>>>>>>>>> routines for generic code.
>>>>>>>>>>>> 
>>>>>>>>>>>> The implementation can also
expose vectorization-specific descriptors --
>>>>>>>>>>>> for example, like the `linear`
and `uniform` clauses of the OpenMP
>>>>>>>>>>>> `declare simd` directive --
that could be used to finely tune the
>>>>>>>>>>>> automatic vectorization of some
functions (think for example the
>>>>>>>>>>>> vectorization of `double
sincos(double , double *, double *)`, where
>>>>>>>>>>>> `linear` can be used to give
extra information about the memory layout
>>>>>>>>>>>> of the 2 pointers parameters in
the vector version).
>>>>>>>>>>>> 
>>>>>>>>>>>> The directive `#pragma clang
declare variant` follows the syntax of the
>>>>>>>>>>>> `#pragma omp declare variant`
directive of OpenMP.
>>>>>>>>>>>> 
>>>>>>>>>>>> We define the new directive in
the `clang` namespace instead of using
>>>>>>>>>>>> the `omp` one of OpenMP to
allow the compiler to perform
>>>>>>>>>>>> auto-vectorization outside of
an OpenMP SIMD context.
>>>>>>>>>>>> 
>>>>>>>>>>>> The mechanism is base on OpenMP
to provide a uniform user experience
>>>>>>>>>>>> across the two mechanism, and
to maximise the number of shared
>>>>>>>>>>>> components of the
infrastructure needed in the compiler frontend to
>>>>>>>>>>>> enable the feature.
>>>>>>>>>>>> 
>>>>>>>>>>>> Changes in LLVM IR {#llvmIR}
>>>>>>>>>>>> ------------------
>>>>>>>>>>>> 
>>>>>>>>>>>> The IR is enriched with
metadata that details the availability of vector
>>>>>>>>>>>> versions of an associated
scalar function. This metadata is attached to
>>>>>>>>>>>> the call site of the scalar
function.
>>>>>>>>>>>> 
>>>>>>>>>>>> The metadata takes the form of
an attribute containing a comma separated
>>>>>>>>>>>> list of vector function
mappings. Each entry has a unique name that
>>>>>>>>>>>> follows the Vector Function
ABI[^2] and real name that is used when
>>>>>>>>>>>> generating calls to this vector
function.
>>>>>>>>>>>> 
>>>>>>>>>>>> vfunc_name1(real_name1),
vfunc_name2(real_name2)
>>>>>>>>>>>> 
>>>>>>>>>>>> The Vector Function ABI name
describes the signature of the vector
>>>>>>>>>>>> function so that properties
like vectorisation factor can be queried
>>>>>>>>>>>> during compilation.
>>>>>>>>>>>> 
>>>>>>>>>>>> The `(real name)` token is
optional and assumed to match the Vector
>>>>>>>>>>>> Function ABI name when omitted.
>>>>>>>>>>>> 
>>>>>>>>>>>> For example, the availability
of a 2-lane double precision `sin`
>>>>>>>>>>>> function via SVML when
targeting AVX on x86 is provided by the following
>>>>>>>>>>>> IR.
>>>>>>>>>>>> 
>>>>>>>>>>>> // ...
>>>>>>>>>>>> ... = call double @sin(double)
#0
>>>>>>>>>>>> // ...
>>>>>>>>>>>> 
>>>>>>>>>>>> #0 = { vector-variant =
{"_ZGVcN2v_sin(__svml_sin2),
>>>>>>>>>>>>                          
_ZGVdN4v_sin(__svml_sin4),
>>>>>>>>>>>>                          
..."} }
>>>>>>>>>>>> 
>>>>>>>>>>>> The string
`"_ZGVcN2v_sin(__svml_sin2)"` in this vector-variant
>>>>>>>>>>>> attribute provides information
on the shape of the vector function via
>>>>>>>>>>>> the string `_ZGVcN2v_sin`,
mangled according to the Vector Function ABI
>>>>>>>>>>>> for Intel, and remaps the
standard Vector Function ABI name to the
>>>>>>>>>>>> non-standard name
`__svml_sin2`.
>>>>>>>>>>>> 
>>>>>>>>>>>> This metadata is compatible
with the proposal "Proposal for function
>>>>>>>>>>>> vectorization and loop
vectorization with function calls",[^3] that uses
>>>>>>>>>>>> Vector Function ABI mangled
names to inform the vectorizer about the
>>>>>>>>>>>> availability of vector
functions. The proposal extends the original by
>>>>>>>>>>>> allowing the explicit mapping
of the Vector Function ABI mangled name to
>>>>>>>>>>>> a non-standard name, which
allows the use of existing vector libraries.
>>>>>>>>>>>> 
>>>>>>>>>>>> The `vector-variant` attribute
needs to be attached on a per-call basis
>>>>>>>>>>>> to avoid conflicts when merging
modules with different vector variants.
>>>>>>>>>>>> 
>>>>>>>>>>>> The query infrastructure: SVFS
{#infrastructure}
>>>>>>>>>>>> ------------------------------
>>>>>>>>>>>> 
>>>>>>>>>>>> The Search Vector Function
System (SVFS) is constructed from an
>>>>>>>>>>>> `llvm::Module` instance so it
can create function definitions. The SVFS
>>>>>>>>>>>> exposes an API with two
methods.
>>>>>>>>>>>> 
>>>>>>>>>>>> ###
`SVFS::isFunctionVectorizable`
>>>>>>>>>>>> 
>>>>>>>>>>>> This method queries the
avilability of a vectorized version of a
>>>>>>>>>>>> function. The signature of the
method is as follows.
>>>>>>>>>>>> 
>>>>>>>>>>>> bool
isFunctionVectorizable(llvm::CallInst * Call, ParTypeMap Params);
>>>>>>>>>>>> 
>>>>>>>>>>>> The method determine the
availability of vector version of the function
>>>>>>>>>>>> invoked by the `Call` parameter
by looking at the `vector-variant`
>>>>>>>>>>>> metadata.
>>>>>>>>>>>> 
>>>>>>>>>>>> The `Params` argument is a map
that associates the position of a
>>>>>>>>>>>> parameter in the `CallInst` to
its `ParameterType` descriptor. The
>>>>>>>>>>>> `ParameterType` descriptor
holds information about the shape of the
>>>>>>>>>>>> correspondend parameter in the
signature of the vector function. This
>>>>>>>>>>>> `ParamaterType` is used to
query the SVMS about the availability of
>>>>>>>>>>>> vector version that have
`linear`, `uniform` or `align` parameters (in
>>>>>>>>>>>> the sense of OpenMP 4.0 and
onwards).
>>>>>>>>>>>> 
>>>>>>>>>>>> The method
`isFunctionVectorizable`, when invoked with an empty
>>>>>>>>>>>> `ParTypeMap`, is equivalent to
the `TargetLibraryInfo` method
>>>>>>>>>>>>
`isFunctionVectorizable(StrinRef Name)`.
>>>>>>>>>>>> 
>>>>>>>>>>>> ###
`SVFS::getVectorizedFunction`
>>>>>>>>>>>> 
>>>>>>>>>>>> This method returns the vector
function declaration that correspond to
>>>>>>>>>>>> the needs of the vectorization
technique that is being run.
>>>>>>>>>>>> 
>>>>>>>>>>>> The signature of the function
is as follows.
>>>>>>>>>>>> 
>>>>>>>>>>>> std::pair<llvm::FunctionType
*, std::string> getVectorizedFunction(
>>>>>>>>>>>>   llvm::CallInst * Call,
unsigned VF, bool IsMasked, ParTypeSet Params);
>>>>>>>>>>>> 
>>>>>>>>>>>> The `Call` parameter is the
call instance that is being vectorized, the
>>>>>>>>>>>> `VF` parameter represent the
vectorization factor (how many lanes), the
>>>>>>>>>>>> `IsMasked` parameter decides
whether or not the signature of the vector
>>>>>>>>>>>> function is required to have a
mask parameter, the `Params` parameter
>>>>>>>>>>>> describes the shape of the
vector function as in the
>>>>>>>>>>>> `isFunctionVectorizable`
method.
>>>>>>>>>>>> 
>>>>>>>>>>>> The methods uses the
`vector-variant` metadata and returns the function
>>>>>>>>>>>> signature and the name of the
function based on the input parameters.
>>>>>>>>>>>> 
>>>>>>>>>>>> The SVFS can add new function
definitions, in the same module as the
>>>>>>>>>>>> `Call`, to provide vector
functions that are not present within the
>>>>>>>>>>>> vector-variant metadata. For
example, if a library provides a vector
>>>>>>>>>>>> version of a function with a
vectorization factor of 2, but the
>>>>>>>>>>>> vectorizer is requesting a
vectorization factor of 4, the SVFS is
>>>>>>>>>>>> allowed to create a definition
that calls the 2-lane version twice. This
>>>>>>>>>>>> capability applies similarly
for providing masked and unmasked versions
>>>>>>>>>>>> when the request does not match
what is available in the library.
>>>>>>>>>>>> 
>>>>>>>>>>>> This method is equivalent to
the TLI method
>>>>>>>>>>>> `StringRef
getVectorizedFunction(StringRef F, unsigned VF) const;`.
>>>>>>>>>>>> 
>>>>>>>>>>>> Notice that to fully support
OpenMP vectorization we need to think about
>>>>>>>>>>>> a fuzzy matching mechanism that
is able to select a candidate in the
>>>>>>>>>>>> calling context. However, this
proposal is intended for scalar-to-vector
>>>>>>>>>>>> mappings of math-like functions
that are most likely to associate a
>>>>>>>>>>>> unique vector candidate in most
contexts. Therefore, extending this
>>>>>>>>>>>> behavior to a generic one is an
aspect of the implementation that will
>>>>>>>>>>>> be treated in a separate RFC
about the vectorization pass.
>>>>>>>>>>>> 
>>>>>>>>>>>> ### Scalable vectorization
>>>>>>>>>>>> 
>>>>>>>>>>>> Both methods of the SVFS API
will be extended with a boolean parameter
>>>>>>>>>>>> to specify whether scalable
signatures are needed by the user of the
>>>>>>>>>>>> SVFS.
>>>>>>>>>>>> 
>>>>>>>>>>>> Changes in clang {#clang}
>>>>>>>>>>>> ----------------
>>>>>>>>>>>> 
>>>>>>>>>>>> We use clang to generate the
metadata described above.
>>>>>>>>>>>> 
>>>>>>>>>>>> In the compilation unit, the
vector function definition or declaration
>>>>>>>>>>>> must be visible and associated
to the scalar version via the
>>>>>>>>>>>> `#pragma clang declare variant`
according to the rule defined by the
>>>>>>>>>>>> correspondent `#pragma omp
declare variant` defined in OpenMP 5.0, as in
>>>>>>>>>>>> the following example.
>>>>>>>>>>>> 
>>>>>>>>>>>> #pragma clang declare
variant(vector_sinf) \
>>>>>>>>>>>>
match(construct=simd(simdlen(4),notinbranch), device={isa("simd")})
>>>>>>>>>>>> extern float sinf(float);
>>>>>>>>>>>> 
>>>>>>>>>>>> float32x4_t
vector_sinf(float32x4_t x);
>>>>>>>>>>>> 
>>>>>>>>>>>> The `construct` set in the
directive, together with the `device` set, is
>>>>>>>>>>>> used to generate the vector
mangled name to be used in the
>>>>>>>>>>>> `vector-variant` attribute, for
example `_ZGVnN2v_sin`, when targeting
>>>>>>>>>>>> AArch64 Advanced SIMD code
generation. The rule for mangling the name of
>>>>>>>>>>>> the scalar function in the
vector name are defined in the the Vector
>>>>>>>>>>>> Function ABI specification of
the target.
>>>>>>>>>>>> 
>>>>>>>>>>>> The part of the vector-variant
attribute that redirects the call to
>>>>>>>>>>>> `vector_sinf` is derived from
the `variant-id` specified in the
>>>>>>>>>>>> `variant` clause.
>>>>>>>>>>>> 
>>>>>>>>>>>> Summary
>>>>>>>>>>>>
======>>>>>>>>>>>>
>>>>>>>>>>>> New `clang` directive in clang
>>>>>>>>>>>> ------------------------------
>>>>>>>>>>>> 
>>>>>>>>>>>> `#pragma omp declare variant`,
same as `#pragma omp declare variant`
>>>>>>>>>>>> restricted to the `simd`
context selector, from OpenMP 5.0+.
>>>>>>>>>>>> 
>>>>>>>>>>>> Option behavior, and
interaction with OpenMP
>>>>>>>>>>>>
--------------------------------------------
>>>>>>>>>>>> 
>>>>>>>>>>>> The behavior described below
makes sure that
>>>>>>>>>>>> `#pragma cland declare variant`
function vectorization and OpenMP
>>>>>>>>>>>> function vectorization are
orthogonal.
>>>>>>>>>>>> 
>>>>>>>>>>>> `-fclang-declare-variant`
>>>>>>>>>>>> 
>>>>>>>>>>>> :   The `#pragma clang declare
variant` directives are parsed and used
>>>>>>>>>>>> to populate the
`vector-variant` attribute.
>>>>>>>>>>>> 
>>>>>>>>>>>> `-fopenmp[-simd]`
>>>>>>>>>>>> 
>>>>>>>>>>>> :   The `#pragma omp declare
variant` directives are parsed and used to
>>>>>>>>>>>> populate the `vector-variant`
attribute.
>>>>>>>>>>>> 
>>>>>>>>>>>> `-fopenmp[-simd]`and
`-fno-clang-declare-variant`
>>>>>>>>>>>> 
>>>>>>>>>>>> :   The directive `#pragma omp
declare variant` is used to populate the
>>>>>>>>>>>> `vector-variant` attribute in
IR. The directive
>>>>>>>>>>>> `#pragma   clang declare
variant` are ignored.
>>>>>>>>>>>> 
>>>>>>>>>>>> [^1]:
<https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf>
>>>>>>>>>>>> 
>>>>>>>>>>>> [^2]: Vector Function ABI for
x86:
>>>>>>>>>>>>
<https://software.intel.com/en-us/articles/vector-simd-function-abi>.
>>>>>>>>>>>> Vector Function ABI for
AArch64:
>>>>>>>>>>>>
https://developer.arm.com/products/software-development-tools/hpc/arm-compiler-for-hpc/vector-function-abi
>>>>>>>>>>>> 
>>>>>>>>>>>> [^3]:
<http://lists.llvm.org/pipermail/cfe-dev/2016-March/047732.html>
>>>>>>>>>>>> 
>>>>>>>>>>>>
_______________________________________________
>>>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>>
_______________________________________________
>>>>>>>>>>> cfe-dev mailing list
>>>>>>>>>>> cfe-dev at lists.llvm.org
>>>>>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>>>> -- 
>>>>>>>> Hal Finkel
>>>>>>>> Lead, Compiler Technology and Programming
Languages
>>>>>>>> Leadership Computing Facility
>>>>>>>> Argonne National Laboratory
>>>>>>>> 
>>>>>>>> _______________________________________________
>>>>>>>> cfe-dev mailing list
>>>>>>>> cfe-dev at lists.llvm.org
>>>>>>>>
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>> 
>>>> _______________________________________________
>>>> cfe-dev mailing list
>>>> cfe-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>> 
>