The documentation has become a bit stale with age, and doesn't include great documentation for some newer concepts. This revision tidies up a majority of it, with some more cleanup to come in the future. The documentation for the declarative specification is also moved from OpDefinitions.md to Interfaces.md, which is a much more logical place for it to live.
Differential Revision: https://reviews.llvm.org/D92895
This is part of a larger refactoring the better congregates the builtin structures under the BuiltinDialect. This also removes the problematic "standard" naming that clashes with the "standard" dialect, which is not defined within IR/. A temporary forward is placed in StandardTypes.h to allow time for downstream users to replaced references.
Differential Revision: https://reviews.llvm.org/D92435
Many pages have had their titles renamed over time,
causing broken links to spread throughout the documentation.
Reviewed By: ftynse
Differential Revision: https://reviews.llvm.org/D92093
* Makes `pip install pybind11` do the right thing with no further config.
* Since we now require a version of pybind11 greater than many LTS OS installs (>=2.6), a more convenient way to get a recent version is preferable.
* Also adds the version spec to find_package so it will skip older versions that may be lying around.
* Tested the full matrix of old system install, no system install, pip install and no pip install.
Differential Revision: https://reviews.llvm.org/D91903
This commit does the renaming mentioned in the title in order to bring
'spv' dialect closer to the MLIR naming conventions.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D91792
This commit does the renaming mentioned in the title in order to bring
'spv' dialect closer to the MLIR naming conventions.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D91797
This commit does the renaming mentioned in the title in order to bring
'spv' dialect closer to the MLIR naming conventions.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D91715
- Add `mlirElementsAttrGetType` C API.
- Add `def_buffer` binding to PyDenseElementsAttribute.
- Implement the protocol to access the buffer.
Differential Revision: https://reviews.llvm.org/D91021
This commit does the renaming mentioned in the title in order to bring
`spv` dialect closer to the MLIR naming conventions.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D91609
The tokens are already handled by the lexer. This revision exposes them
through the parser interface.
This revision also adds missing functions for question mark parsing and
completes the list of valid punctuation tokens in the documentation.
Differential Revision: https://reviews.llvm.org/D90907
Added documentation about the bufferization features.
Furthermore, the usage of pre- and post-processing is described.
This also includes information about optimization functionalities.
Differential Revision: https://reviews.llvm.org/D90675
This patch adds an `ElementwiseMappable` trait as discussed in the RFC
here:
https://llvm.discourse.group/t/rfc-std-elementwise-ops-on-tensors/2113/23
This trait can power a number of transformations and analyses.
A subsequent patch adds a convert-elementwise-to-linalg pass exhibits
how this trait allows writing generic transformations.
See https://reviews.llvm.org/D90354 for that patch.
This trait slightly changes some verifier messages, but the diagnostics
are usually about as good. I fiddled with the ordering of the trait in
the .td file trait lists to minimize the changes here.
Differential Revision: https://reviews.llvm.org/D90731
- Change syntax for FuncOp to be `func <visibility>? @name` instead of printing the
visibility in the attribute dictionary.
- Since printFunctionLikeOp() and parseFunctionLikeOp() are also used by other
operations, make the "inline visibility" an opt-in feature.
- Updated unit test to use and check the new syntax.
Differential Revision: https://reviews.llvm.org/D90859
Previously, they were only defined for `FuncOp`.
To support this, `FunctionLike` needs a way to get an updated type
from the concrete operation. This adds a new hook for that purpose,
called `getTypeWithoutArgsAndResults`.
For now, `FunctionLike` continues to assume the type is
`FunctionType`, and concrete operations that use another type can hide
the `getType`, `setType`, and `getTypeWithoutArgsAndResults` methods.
Reviewed By: rriddle
Differential Revision: https://reviews.llvm.org/D90363
* Finishes support for Context, InsertionPoint and Location to be carried by the thread using context managers.
* Introduces type casters and utilities so that DefaultPyMlirContext and DefaultPyLocation in method signatures does the right thing (allows explicit or gets from the thread context).
* Extend the rules for the thread context stack to handle nesting, appropriately inheriting and clearing depending on whether the context is the same.
* Refactors all method signatures to follow the new convention on trailing parameters for defaulting parameters (loc, ip, context). When the objects are carried in the thread context, this allows most explicit uses of these values to be elided.
* Removes the style guide section on putting accessors to construct global objects on the PyMlirContext: this style fails to make good use of the new facility since it is often the only thing remaining needing an MlirContext.
* Moves Module parse/creation from mlir.ir.Context to static methods on mlir.ir.Module.
* Moves Context.create_operation to a static Operation.create method.
* Moves Type parsing from mlir.ir.Context to static methods on mlir.ir.Type.
* Moves Attribute parsing from mlir.ir.Context to static methods on mlir.ir.Attribute.
* Move Location factory methods from mlir.ir.Context to static methods on mlir.ir.Location.
* Refactors the std dialect fake "ODS" generated code to take advantage of the new scheme.
Differential Revision: https://reviews.llvm.org/D90547
Often times the legality of inlining can change depending on if the callable is going to be inlined in-place, or cloned. For example, some operations are not allowed to be duplicated and can only be inlined if the original callable will cease to exist afterwards. The new `wouldBeCloned` flag allows for dialects to hook into this when determining legality.
Differential Revision: https://reviews.llvm.org/D90360
In certain situations it isn't legal to inline a call operation, but this isn't something that is possible(at least not easily) to prevent with the current hooks. This revision adds a new hook so that dialects with call operations that shouldn't be inlined can prevent it.
Differential Revision: https://reviews.llvm.org/D90359
A recent commit introduced a new syntax for specifying builder arguments in
ODS, which is better amenable to automated processing, and deprecated the old
form. Transition all dialects as well as Linalg ODS generator to use the new
syntax.
Add a deprecation notice to ODS generator.
Reviewed By: rriddle, jpienaar
Differential Revision: https://reviews.llvm.org/D90038
The Pattern class was originally intended to be used for solely matching operations, but that use never materialized. All of the pattern infrastructure uses RewritePattern, and the infrastructure for pure matching(Matchers.h) is implemented inline. This means that this class isn't a useful abstraction at the moment, so this revision refactors it to solely encapsulate the "metadata" of a pattern. The metadata includes the various state describing a pattern; benefit, root operation, etc. The API on PatternApplicator is updated to now operate on `Pattern`s as nothing special from `RewritePattern` is necessary.
This refactoring is also necessary for the upcoming use of PDL patterns alongside C++ rewrite patterns.
Differential Revision: https://reviews.llvm.org/D86258
The documentation claims that an op with the trait FunctionLike has a
single region containing the blocks that corresponding to the body of
the function. It then goes on to say that the absence of a region
corresponds to an external function when, in fact, this is represented
by a single empty region. This patch changes the wording in the
documentation to match the implementation.
Signed-off-by: Frej Drejhammar <frej.drejhammar@gmail.com>
Co-authored-by: Frej Drejhammar <frej.drejhammar@gmail.com>
Co-authored-by: Klas Segeljakt <klasseg@kth.se>
Reviewed By: ftynse
Differential Revision: https://reviews.llvm.org/D89868
Historically, custom builder specification in OpBuilder has been accepting the
formal parameter list for the builder method as a raw string containing C++.
While this worked well to connect the signature and the body, this became
problematic when ODS needs to manipulate the parameter list, e.g. to inject
OpBuilder or to trim default values when generating the definition. This has
also become inconsistent with other method declarations, in particular in
interface definitions.
Introduce the possibility to define OpBuilder formal parameters using a
TableGen dag similarly to other methods. Additionally, introduce a mechanism to
declare parameters with default values using an additional class. This
mechanism can be reused in other methods. The string-based builder signature
declaration is deprecated and will be removed after a transition period.
Reviewed By: jpienaar
Differential Revision: https://reviews.llvm.org/D89470
The initial goal of this interface is to fix the current problems with verifying symbol user operations, but can extend beyond that in the future. The current problems with the verification of symbol uses are:
* Extremely inefficient:
Most current symbol users perform the symbol lookup using the slow O(N) string compare methods, which can lead to extremely long verification times in large modules.
* Invalid/break the constraints of verification pass
If the symbol reference is not-flat(and even if it is flat in some cases) a verifier for an operation is not permitted to touch the referenced operation because it may be in the process of being mutated by a different thread within the pass manager.
The new SymbolUserOpInterface exposes a method `verifySymbolUses` that will be invoked from the parent symbol table to allow for verifying the constraints of any referenced symbols. This method is passed a `SymbolTableCollection` to allow for O(1) lookups of any necessary symbol operation.
Differential Revision: https://reviews.llvm.org/D89512
* It reads as more of a TODO for the future and has been long obsoleted by later work.
* One of the authors of the referenced paper called this out as "weird stuff from two years ago" when reviewing the more recent TOSA RFC.
Differential Revision: https://reviews.llvm.org/D89329
This is the same diff as https://reviews.llvm.org/D88809/ except side effect
free check is removed for involution and a FIXME is added until the dependency
is resolved for shared builds. The old diff has more details on possible fixes.
Reviewed By: rriddle, andyly
Differential Revision: https://reviews.llvm.org/D89333
This revision introduces support for buffer allocation for any named linalg op.
To avoid template instantiating many ops, a new ConversionPattern is created to capture the LinalgOp interface.
Some APIs are updated to remain consistent with MLIR style:
`OwningRewritePatternList * -> OwningRewritePatternList &`
`BufferAssignmentTypeConverter * -> BufferAssignmentTypeConverter &`
Differential revision: https://reviews.llvm.org/D89226
This reverts commit 1ceaffd95a.
The build is broken with -DBUILD_SHARED_LIBS=ON ; seems like a possible
layering issue to investigate:
tools/mlir/lib/IR/CMakeFiles/obj.MLIRIR.dir/Operation.cpp.o: In function `mlir::MemoryEffectOpInterface::hasNoEffect(mlir::Operation*)':
Operation.cpp:(.text._ZN4mlir23MemoryEffectOpInterface11hasNoEffectEPNS_9OperationE[_ZN4mlir23MemoryEffectOpInterface11hasNoEffectEPNS_9OperationE]+0x9c): undefined reference to `mlir::MemoryEffectOpInterface::getEffects(llvm::SmallVectorImpl<mlir::SideEffects::EffectInstance<mlir::MemoryEffects::Effect> >&)'
This change allows folds to be done on a newly introduced involution trait rather than having to manually rewrite this optimization for every instance of an involution
Reviewed By: rriddle, andyly, stephenneuendorffer
Differential Revision: https://reviews.llvm.org/D88809
While affine maps are part of the builtin memref type, there is very
limited support for manipulating them in the standard dialect. Add
transpose to the set of ops to complement the existing view/subview ops.
This is a metadata transformation that encodes the transpose into the
strides of a memref.
I'm planning to use this when lowering operations on strided memrefs,
using the transpose to remove the stride without adding a dependency on
linalg dialect.
Differential Revision: https://reviews.llvm.org/D88651
The documentation for the NormalizeMemRefs pass and the associated MemRefsNormalizable
traits was confusing and not on the website. This update clarifies the language
around the difference between a MemRef Type, an operation that accesses the value of
MemRef Type, and better documents the limitations of the current implementation.
This patch also includes some basic debugging information for the pass so people
might have a chance of figuring out why it doesn't work on their code.
Differential Revision: https://reviews.llvm.org/D88532
- Eliminate incorrect |
- Eliminate memspace0 as the memory spaces currently are integer literals and memory
space 0 is not explicitly printed.
Differential Revision: https://reviews.llvm.org/D88171
* Fixes a rather egregious bug with respect to the inability to return arbitrary objects from py::init (was causing aliasing of multiple py::object -> native instance).
* Makes Modules and Operations referencable types so that they can be reliably depended on.
* Uniques python operation instances within a context. Opens the door for further accounting.
* Next I will retrofit region and block to be dependent on the operation, and I will attempt to model the API to avoid detached regions/blocks, which will simplify things a lot (in that world, only operations can be detached).
* Added quite a bit of test coverage to check for leaks and reference issues.
* Supercedes: https://reviews.llvm.org/D87213
Differential Revision: https://reviews.llvm.org/D87958
The OpBuilder is required to start with OpBuilder and OperationState, so remove
the need for the user to specify it. To make it simpler to update callers,
retain the legacy behavior for now and skip injecting OpBuilder/OperationState
when params start with OpBuilder.
Related to bug 47442.
Differential Revision: https://reviews.llvm.org/D88050
Vendor/device information are not resource limits. Moving to
target environment directly for better organization.
Reviewed By: mravishankar
Differential Revision: https://reviews.llvm.org/D87911
This revision allows representing a reduction at the level of linalg on tensors for named ops. When a structured op has a reduction and returns tensor(s), new conventions are added and documented.
As an illustration, the syntax for a `linalg.matmul` writing into a buffer is:
```
linalg.matmul ins(%a, %b : memref<?x?xf32>, tensor<?x?xf32>)
outs(%c : memref<?x?xf32>)
```
, whereas the syntax for a `linalg.matmul` returning a new tensor is:
```
%d = linalg.matmul ins(%a, %b : tensor<?x?xf32>, memref<?x?xf32>)
init(%c : memref<?x?xf32>)
-> tensor<?x?xf32>
```
Other parts of linalg will be extended accordingly to allow mixed buffer/tensor semantics in the presence of reductions.
Numerous MLIR functions return instances of `StringRef` to refer to a
non-owning fragment of a string (usually owned by the context). This is a
relatively simple class that is defined in LLVM. Provide a simple wrapper in
the MLIR C API that contains the pointer and length of the string fragment and
use it for Standard attribute functions that return StringRef instead of the
previous, callback-based mechanism.
Reviewed By: stellaraccident
Differential Revision: https://reviews.llvm.org/D87677
This adds some initial support for regions and does not support formatting the specific arguments of a region. For now this can be achieved by using a custom directive that formats the arguments and then parses the region.
Differential Revision: https://reviews.llvm.org/D86760
This revision adds support for custom directives to the declarative assembly format. This allows for users to use C++ for printing and parsing subsections of an otherwise declaratively specified format. The custom directive is structured as follows:
```
custom-directive ::= `custom` `<` UserDirective `>` `(` Params `)`
```
`user-directive` is used as a suffix when this directive is used during printing and parsing. When parsing, `parseUserDirective` will be invoked. When printing, `printUserDirective` will be invoked. The first parameter to these methods must be a reference to either the OpAsmParser, or OpAsmPrinter. The type of rest of the parameters is dependent on the `Params` specified in the assembly format.
Differential Revision: https://reviews.llvm.org/D84719
When dealing with dialects that will results in function calls to
external libraries, it is important to be able to handle maps as some
dialects may require mapped data. Before this patch, the detection of
whether normalization can apply or not, operations are compared to an
explicit list of operations (`alloc`, `dealloc`, `return`) or to the
presence of specific operation interfaces (`AffineReadOpInterface`,
`AffineWriteOpInterface`, `AffineDMAStartOp`, or `AffineDMAWaitOp`).
This patch add a trait, `MemRefsNormalizable` to determine if an
operation can have its `memrefs` normalized.
This trait can be used in turn by dialects to assert that such
operations are compatible with normalization of `memrefs` with
nontrivial memory layout specification. An example is given in the
literal tests.
Differential Revision: https://reviews.llvm.org/D86236
This patch updates the type conversion section of the documentation.
It includes the modelling of array strides and the mapping of the
naturally padded structs.
Reviewed By: mravishankar
Differential Revision: https://reviews.llvm.org/D86674
This patch updates the SPIR-V to LLVM conversion manual.
Particularly, the following sections are added:
- `spv.EntryPoint`/`spv.ExecutionMode` handling
- Mapping for `spv.AccessChain`
- Change in allowed storage classes for `spv.globalVariable`
- Change of the runner section name
Reviewed By: mravishankar
Differential Revision: https://reviews.llvm.org/D86288
Provide C API for MLIR standard attributes. Since standard attributes live
under lib/IR in core MLIR, place the C APIs in the IR library as well (standard
ops will go in a separate library).
Affine map and integer set attributes are only exposed as placeholder types
with IsA support due to the lack of C APIs for the corresponding types.
Integer and floating point attribute APIs expecting APInt and APFloat are not
exposed pending decision on how to support APInt and APFloat.
Reviewed By: stellaraccident
Differential Revision: https://reviews.llvm.org/D86143
The documentation needs a refresh now that "kinds" are no longer a concept. This revision also adds mentions to a few other new concepts, e.g. traits and interfaces.
Differential Revision: https://reviews.llvm.org/D86182
This greatly simplifies a large portion of the underlying infrastructure, allows for lookups of singleton classes to be much more efficient and always thread-safe(no locking). As a result of this, the dialect symbol registry has been removed as it is no longer necessary.
For users broken by this change, an alert was sent out(https://llvm.discourse.group/t/removing-kinds-from-attributes-and-types) that helps prevent a majority of the breakage surface area. All that should be necessary, if the advice in that alert was followed, is removing the kind passed to the ::get methods.
Differential Revision: https://reviews.llvm.org/D86121
Provide C API for MLIR standard types. Since standard types live under lib/IR
in core MLIR, place the C APIs in the IR library as well (standard ops will go
into a separate library). This also defines a placeholder for affine maps that
are necessary to construct a memref, but are not yet exposed to the C API.
Reviewed By: stellaraccident
Differential Revision: https://reviews.llvm.org/D86094
We have been asking for this systematically, mention it in the documentation.
Reviewed By: mehdi_amini
Differential Revision: https://reviews.llvm.org/D85902
This infrastructure has evolved a lot over the course of MLIRs lifetime, and has never truly been documented outside of rationale or proposals. This revision aims to document the infrastructure and user facing API, with the rationale specific portions moved to the Rationale folder and updated.
Differential Revision: https://reviews.llvm.org/D85260
This revision updates the documentation for dialect conversion, as many concepts have changed/evolved over time.
Differential Revision: https://reviews.llvm.org/D85167
Provide printing functions for most IR objects in C API (except Region that
does not have a `print` function, and Module that is expected to be printed as
Operation instead). The printing is based on a callback that is called with
chunks of the string representation and forwarded user-defined data.
Reviewed By: stellaraccident, Jing, mehdi_amini
Differential Revision: https://reviews.llvm.org/D85748
Using intptr_t is a consensus for MLIR C API, but the change was missing
from 75f239e975 (that was using unsigned initially) due to a
misrebase.
Reviewed By: stellaraccident, mehdi_amini
Differential Revision: https://reviews.llvm.org/D85751
This revision refactors the default definition of the attribute and type `classof` methods to use the TypeID of the concrete class instead of invoking the `kindof` method. The TypeID is already used as part of uniquing, and this allows for removing the need for users to define any of the type casting utilities themselves.
Differential Revision: https://reviews.llvm.org/D85356
Due to the original type system implementation, LLVMDialect in MLIR contains an
LLVMContext in which the relevant objects (types, metadata) are created. When
an MLIR module using the LLVM dialect (and related intrinsic-based dialects
NVVM, ROCDL, AVX512) is converted to LLVM IR, it could only live in the
LLVMContext owned by the dialect. The type system no longer relies on the
LLVMContext, so this limitation can be removed. Instead, translation functions
now take a reference to an LLVMContext in which the LLVM IR module should be
constructed. The caller of the translation functions is responsible for
ensuring the same LLVMContext is not used concurrently as the translation no
longer uses a dialect-wide context lock.
As an additional bonus, this change removes the need to recreate the LLVM IR
module in a different LLVMContext through printing and parsing back, decreasing
the compilation overhead in JIT and GPU-kernel-to-blob passes.
Reviewed By: rriddle, mehdi_amini
Differential Revision: https://reviews.llvm.org/D85443
Introduce an initial version of C API for MLIR core IR components: Value, Type,
Attribute, Operation, Region, Block, Location. These APIs allow for both
inspection and creation of the IR in the generic form and intended for wrapping
in high-level library- and language-specific constructs. At this point, there
is no stability guarantee provided for the API.
Reviewed By: stellaraccident, lattner
Differential Revision: https://reviews.llvm.org/D83310
Updated the documentation with new MLIR LLVM types for
vectors, pointers, arrays and structs. Also, changed remaining
tabs to spaces.
Reviewed By: ftynse
Differential Revision: https://reviews.llvm.org/D85277
Updated the documentation for SPIR-V to LLVM conversion, particularly:
- Added a section on control flow
- Added a section on memory ops
- Added a section on GLSL ops
Also, moved `spv.FunctionCall` to control flow section. Added a new section
that will be used to describe the modelling of runtime-related ops.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D84734
Unit attributes are given meaning by their existence, and thus have no meaningful value beyond "is it present". As such, in the format of an operation unit attributes are generally used to guard the printing of other elements and aren't generally printed themselves; as the presence of the group when parsing means that the unit attribute should be added. This revision adds support to the declarative format for eliding unit attributes in situations where they anchor an optional group, but aren't the first element.
For example,
```
let assemblyFormat = "(`is_optional` $unit_attr^)? attr-dict";
```
would print `foo.op is_optional` when $unit_attr is present, instead of the current `foo.op is_optional unit`.
Differential Revision: https://reviews.llvm.org/D84577
The current output is a bit clunky and requires including files+macros everywhere, or manually wrapping the file inclusion in a registration function. This revision refactors the pass backend to automatically generate `registerFooPass`/`registerFooPasses` functions that wrap the pass registration. `gen-pass-decls` now takes a `-name` input that specifies a tag name for the group of passes that are being generated. For each pass, the generator now produces a `registerFooPass` where `Foo` is the name of the definition specified in tablegen. It also generates a `registerGroupPasses`, where `Group` is the tag provided via the `-name` input parameter, that registers all of the passes present.
Differential Revision: https://reviews.llvm.org/D84983
This diff fixes some obsolete examples in the Linalg dialect documentation: https://mlir.llvm.org/docs/Dialects/Linalg/
These examples are used to explain the basic properties of the Linalg dialect, which are not automatically generated from TableGen and are using out-of-date MLIR/Linalg syntax.
This diff extends each example by adding essential attributes and changing its syntax to make it processible by `mlir-opt`. There is also a command attached to each example that says how the example can be processed.
Reviewed By: ftynse
Differential Revision: https://reviews.llvm.org/D84229
Introduce support for mutable storage in the StorageUniquer infrastructure.
This makes MLIR have key-value storage instead of just uniqued key storage. A
storage instance now contains a unique immutable key and a mutable value, both
stored in the arena allocator that belongs to the context. This is a
preconditio for supporting recursive types that require delayed initialization,
in particular LLVM structure types. The functionality is exercised in the test
pass with trivial self-recursive type. So far, recursive types can only be
printed in parsed in a closed type system. Removing this restriction is left
for future work.
Differential Revision: https://reviews.llvm.org/D84171
Missing line breaks in the example under `Codegen of Unranked Memref` section.
Reviewed By: mehdi_amini
Differential Revision: https://reviews.llvm.org/D84484
SPIR-V lowering does not use `MemrefDescriptor`s when lowering memref
types. This adds rationale for the choice made.
Differential Revision: https://reviews.llvm.org/D84184
This patch adds documentation for SPIR-V to LLVM conversion. It describes
the approaches taken and what is currently supported by this conversion
framework.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D83322
Some dialects have semantics which is not well represented by common
SSA structures with dominance constraints. This patch allows
operations to declare the 'kind' of their contained regions.
Currently, two kinds are allowed: "SSACFG" and "Graph". The only
difference between them at the moment is that SSACFG regions are
required to have dominance, while Graph regions are not required to
have dominance. The intention is that this Interface would be
generated by ODS for existing operations, although this has not yet
been implemented. Presumably, if someone were interested in code
generation, we might also have a "CFG" dialect, which defines control
flow, but does not require SSA.
The new behavior is mostly identical to the previous behavior, since
registered operations without a RegionKindInterface are assumed to
contain SSACFG regions. However, the behavior has changed for
unregistered operations. Previously, these were checked for
dominance, however the new behavior allows dominance violations, in
order to allow the processing of unregistered dialects with Graph
regions. One implication of this is that regions in unregistered
operations with more than one op are no longer CSE'd (since it
requires dominance info).
I've also reorganized the LangRef documentation to remove assertions
about "sequential execution", "SSA Values", and "Dominance". Instead,
the core IR is simply "ordered" (i.e. totally ordered) and consists of
"Values". I've also clarified some things about how control flow
passes between blocks in an SSACFG region. Control Flow must enter a
region at the entry block and follow terminator operation successors
or be returned to the containing op. Graph regions do not define a
notion of control flow.
see discussion here:
https://llvm.discourse.group/t/rfc-allowing-dialects-to-relax-the-ssa-dominance-condition/833/53
Differential Revision: https://reviews.llvm.org/D80358
This adds a `parseOptionalAttribute` method to the OpAsmParser that allows for parsing optional attributes, in a similar fashion to how optional types are parsed. This also enables the use of attribute values as the first element of an assembly format optional group.
Differential Revision: https://reviews.llvm.org/D83712
River Riddle