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

I think we should split this discussion:
  TOPIC 1 & 2 & 4: How do implement all use cases and OpenMP 5.X
                   features, including compatibility with other
                   compilers and cross module support.
  TOPIC 3b & 5: Interoperability with clang declare (system vs. user
                 declares)
  TOPIC 3a & 3c: floating point issues?

I inlined comments for Topic 1 below.

I hope that we do not have to discuss topic 2 if we agree neither
attributes nor metadata is necessary, or better, will solve the actual
problem at hand. I don't have strong feeling on topic 4 but I have the
feeling this will become less problematic once we figure out topic 1.

Thanks,
  Johannes


On 05/31, Francesco Petrogalli wrote:
> # 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 still think it will be required to have a standardized one, not
only preferred.

> 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.
For me, this simply means this mangling scheme is not sufficient.

> 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.
I fail to see the problem. We generate X symbols for X different
contexts. Once we get to the point where we vectorize, we determine
which context fits best and choose the corresponding symbol version.

Maybe my view is to naive here, please feel free to correct me.

> 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.
The symbol names should carry all the information we need. If they do
not, we need to improve the mangling scheme such that they do. There is
no attributes/metadata we could use at library boundaries.

> 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
Having an agreed upon mangling for the older feature is not necessarily
important here. We will need more functionality for variants and keeping
the old scheme around with some metadata is not an extensible long-term
solution. So, I would not try to fit variants into the existing
simd-scheme but instead do it the other way around. We define what we
need for variants and implement simd in that scheme.

> 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
> 
-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: not available
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190531/a6311de4/attachment-0001.sig>

Hi Johannes,

Thank you for your feedback! See mi replies below.

Francesco
> On May 31, 2019, at 11:47 AM, Doerfert, Johannes <jdoerfert at
anl.gov> wrote:
> 
> I think we should split this discussion:
>  TOPIC 1 & 2 & 4: How do implement all use cases and OpenMP 5.X
>                   features, including compatibility with other
>                   compilers and cross module support.
Yes, and we have to carefully make this as standard and compatible as possible.

>  TOPIC 3b & 5: Interoperability with clang declare (system vs. user
>                 declares)

I think that Alexey explanation of how the directive are handled internally in
the frontend makes us propound towards the attribute.
>  TOPIC 3a & 3c: floating point issues?
> 
I believe there is no issue there. I have quoted the openMP standard in reply to
Renato:

See https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf,
page 118, lines 23-24:

“The execution of the function or subroutine cannot have any side effects that
would alter its execution for concurrent iterations of a SIMD chunk."
> I inlined comments for Topic 1 below.
> 
> I hope that we do not have to discuss topic 2 if we agree neither
> attributes nor metadata is necessary, or better, will solve the actual
> problem at hand. I don't have strong feeling on topic 4 but I have the
> feeling this will become less problematic once we figure out topic 1.
> 
> Thanks,
>  Johannes
> 
> 
> On 05/31, Francesco Petrogalli wrote:
>> # 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 still think it will be required to have a standardized one, not
> only preferred.
> 
> 
I am all with you in standardizing. x86 and arch64 have their own vector
function ABI, which, although “private”, are to be considered standard.
Opensource and commercial compilers are using them, therefore we have to deal
with this mangling scheme, whether or not OpenMP comes up with a standard
mangling scheme.
>> 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.
> 
> For me, this simply means this mangling scheme is not sufficient.
> 
Can you explain more why you think the mangling scheme is not sufficient? The
mangling scheme is shaped to provide all the information that the OpenMP
directive describes.

The fact that x86 and aarch64 realize such information in different way
(multiple signature/vector extensions) is something that cannot be avoided,
because it is related to architectural aspects that are specific to the vector
extension and transparent to the OpenMP standard.
> 
>> 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.
> 
> I fail to see the problem. We generate X symbols for X different
> contexts. Once we get to the point where we vectorize, we determine
> which context fits best and choose the corresponding symbol version.
> 
Yes, this is exactly what we need to do, under the constrains that  the rules
for  generating "X symbols for X different contexts” are decided by the
Vector Function ABI of the target.

> Maybe my view is to naive here, please feel free to correct me.
> 
> 
>> 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.
> 
> The symbol names should carry all the information we need. If they do
> not, we need to improve the mangling scheme such that they do. There is
> no attributes/metadata we could use at library boundaries.
> 

Hum, I am not sure what you mean by "There is no attributes/metadata we
could use at library boundaries."

In our downstream compiler (Arm compiler for HPC, based on LLVM), we use
`declare simd` to provide vector math functions via custom header file. It works
brilliantly, if not for specific aspects that would be perfectly covered by the
`declare variant`, which might be one of the reason why the OpenMP committee
decided to introduce `declare variant`.

If your concerns is that by adding an attribute that somehow represent something
that is available in an external library is not enough to guarantee that that
symbol is available in the library… not even C code can guarantee that? If the
linker is not pointing to the right library, there is nothing that can prevent
it to fail if the symbol is not present?
> 
>> 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
> 
> Having an agreed upon mangling for the older feature is not necessarily
> important here. We will need more functionality for variants and keeping
> the old scheme around with some metadata is not an extensible long-term
> solution. So, I would not try to fit variants into the existing
> simd-scheme but instead do it the other way around. We define what we
> need for variants and implement simd in that scheme.
> 
I kinda think that having agreed on something is important. It allows to build
other things on top of what have been agreed without breaking compatibility.

On the specific, which are the new functionalities needed for the variants that
would make the current metadata (attributes) for declare simd non extensible?
> 
>> 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
>> 
> 
> -- 
> 
> Johannes Doerfert
> Researcher
> 
> Argonne National Laboratory
> Lemont, IL 60439, USA
> 
> jdoerfert at anl.gov

I think I did misunderstand what you want to do with attributes. This is
my bad. Let me try to explain:

It seems you want the "vector-variants" attributes (which I could not
find with this name in trunk, correct?) to "remember" what vector
versions can be created (wrt. validity), assuming a definition is
available? Correct?
What I was concerned with is the example I sketched somewhere below
which motivates the need for a generalized/standardized name mangling
for OpenMP. I though you wanted to avoid that somehow but if you don't I
misunderstood you. I basically removed the part where the vector
versions have to be created first but I assumed them to be existent (in
the module or somewhere else). That is, I assumed a call to foo and
various symbols available that are specializations of foo. When we then
vectorize foo (or otherwise specialize at some point in the future), you
would scan the module and pick the best match based on the context of
the call.

Now I don't know if I understood your proposal by now but let me ask a
question anyway:

VecClone.cpp:276-278 mentions that the vectorizer is supposed to look at
the vector-variants functions. This works for variants that are created
from definitions in the module but what about #omp declare simd
declarations?


On 05/31, Francesco Petrogalli wrote:
> > On May 31, 2019, at 11:47 AM, Doerfert, Johannes <jdoerfert at
anl.gov> wrote:
> > 
> > I think we should split this discussion:
> >  TOPIC 1 & 2 & 4: How do implement all use cases and OpenMP
5.X
> >                   features, including compatibility with other
> >                   compilers and cross module support.
> 
> Yes, and we have to carefully make this as standard and compatible as
possible.
Agreed.

> >  TOPIC 3b & 5: Interoperability with clang declare (system vs.
user
> >                 declares)
> 
> 
> I think that Alexey explanation of how the directive are handled
> internally in the frontend makes us propound towards the attribute. 
How things are handled right now, especially given that declare variant
is not handled at all, should not limit our design space. If the
argument is that we cannot reasonably implement a solution, that is a
different story.

> >  TOPIC 3a & 3c: floating point issues?
> > 
> 
> I believe there is no issue there. I have quoted the openMP standard in
reply to Renato:
> 
> See
https://www.openmp.org/wp-content/uploads/OpenMP-API-Specification-5.0.pdf, page
118, lines 23-24:
> 
> “The execution of the function or subroutine cannot have any side
> effects that would alter its execution for concurrent iterations of a
> SIMD chunk."
Great.

> > I inlined comments for Topic 1 below.
> > 
> > I hope that we do not have to discuss topic 2 if we agree neither
> > attributes nor metadata is necessary, or better, will solve the actual
> > problem at hand. I don't have strong feeling on topic 4 but I have
the
> > feeling this will become less problematic once we figure out topic 1.
> > 
> > Thanks,
> >  Johannes
> > 
> > 
> > On 05/31, Francesco Petrogalli wrote:
> >> # 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 still think it will be required to have a standardized one, not
> > only preferred.
> > 
> > 
> 
> I am all with you in standardizing. x86 and arch64 have their own
> vector function ABI, which, although “private”, are to be considered
> standard. Opensource and commercial compilers are using them,
> therefore we have to deal with this mangling scheme, whether or not
> OpenMP comes up with a standard mangling scheme.
I don't get the point you are trying to make here. What do you mean by
"we have to deal with"? (I do not suggest to get rid of them.)

 
> >> 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.
> > 
> > For me, this simply means this mangling scheme is not sufficient.
> > 
> 
> Can you explain more why you think the mangling scheme is not
> sufficient? The mangling scheme is shaped to provide all the
> information that the OpenMP directive describes.
I don't know if it is insufficient but I though you hinted towards that.
If we can handle/decode everything we need for declare variants then I
do not object at all. If not, we require respective extension such that
we can. The result should be a superset of the current SIMD encoding and
compatible with the current one.


> The fact that x86 and aarch64 realize such information in different
> way (multiple signature/vector extensions) is something that cannot be
> avoided, because it is related to architectural aspects that are
> specific to the vector extension and transparent to the OpenMP
> standard.
I don't think that is a problem (that's why I "failed to see the
problem" in the comment below). I look at it this way: If #declare simd,
or similar, results in N variants, it should at the end of the day not
be different from declaring these N variants explicitly with the
respective declare variant match clause.

> >> 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.
> > 
> > I fail to see the problem. We generate X symbols for X different
> > contexts. Once we get to the point where we vectorize, we determine
> > which context fits best and choose the corresponding symbol version.
> > 
> 
> Yes, this is exactly what we need to do, under the constrains that
> the rules for  generating "X symbols for X different contexts” are
> decided by the Vector Function ABI of the target.
Sounds good. The vector ABI is used to determine what contexts exists
and what symbols should be created. I would assume the encoding should
be the same as if we specified the versions (/contexts) ourselves via
#declare variant.

> > Maybe my view is to naive here, please feel free to correct me.
> > 
> > 
> >> 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.
> > 
> > The symbol names should carry all the information we need. If they do
> > not, we need to improve the mangling scheme such that they do. There
is
> > no attributes/metadata we could use at library boundaries.
> > 
> Hum, I am not sure what you mean by "There is no attributes/metadata
> we could use at library boundaries."
(This seems to be part of the misunderstanding, I leave my comment here
anyway:)

The simd-related stuff works because it is a uniform mangling scheme
used by all compilers. Take the situation below in which I think we want
to call foo_CTX in the library. If so, we need a name for it.


a.c:  // Compiled by gcc into a library
#omp declare variant (foo) match(CTX)
void foo_CTX(...) {...}

b.c:  // Compiled by clang linked against the library above.
#omp declare variant (foo) match(CTX)
void foo_CTX(...);

void bar(...) {
  #pragma omp CTX
  foo();   // <- What function (symbol) do we call if a.c was compiled
           //    by gcc and b.c with clang?
}
> In our downstream compiler (Arm compiler for HPC, based on LLVM), we
> use `declare simd` to provide vector math functions via custom header
> file. It works brilliantly, if not for specific aspects that would be
> perfectly covered by the `declare variant`, which might be one of the
> reason why the OpenMP committee decided to introduce `declare
> variant`.
But you (assume that you) control the mangling scheme across the entire
infrastructure. Given that the simd mangling is de-facto standardized,
that works.

Side note:
Declare variant, as of 5.0, is not flexible enough for a sensible
inclusion of target specific headers. That will change in 5.1.

> If your concerns is that by adding an attribute that somehow represent
> something that is available in an external library is not enough to
> guarantee that that symbol is available in the library… not even C
> code can guarantee that? If the linker is not pointing to the right
> library, there is nothing that can prevent it to fail if the symbol is
> not present? 
I don't follow the example you describe. I don't want to change anything
in how symbols are looked up or what happens if they are missing.

> >> 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
> > 
> > Having an agreed upon mangling for the older feature is not
necessarily
> > important here. We will need more functionality for variants and
keeping
> > the old scheme around with some metadata is not an extensible
long-term
> > solution. So, I would not try to fit variants into the existing
> > simd-scheme but instead do it the other way around. We define what we
> > need for variants and implement simd in that scheme.
> > 
> 
> I kinda think that having agreed on something is important. It allows
> to build other things on top of what have been agreed without breaking
> compatibility.
>
> On the specific, which are the new functionalities needed for the
> variants that would make the current metadata (attributes) for declare
> simd non extensible?
See first comment.
> >> 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
> >> 
> > 
> > -- 
> > 
> > Johannes Doerfert
> > Researcher
> > 
> > Argonne National Laboratory
> > Lemont, IL 60439, USA
> > 
> > jdoerfert at anl.gov
> 
-- 

Johannes Doerfert
Researcher

Argonne National Laboratory
Lemont, IL 60439, USA

jdoerfert at anl.gov
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 228 bytes
Desc: not available
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190531/47d95918/attachment-0001.sig>