This chapter adds a new composite type to Toy, and shows the process of adding a new type to the IR, adding and updating operations to use it, and constant folding operations producing it.
PiperOrigin-RevId: 279107885
Upstream LLVM gained support for #ifndef with https://reviews.llvm.org/D61888
This is changed mechanically via the following command:
find . -name "*.td" -exec sed -i -e ':a' -e 'N' -e '$!ba' -e 's/#ifdef \([A-Z_]*\)\n#else/#ifndef \1/g' {} \;
PiperOrigin-RevId: 277789427
This allows for them to be used on other non-function, or even other function-like, operations. The algorithms are already generic, so this is simply changing the derived pass type. The majority of this change is just ensuring that the nesting of these passes remains the same, as the pass manager won't auto-nest them anymore.
PiperOrigin-RevId: 276573038
This change rewrites Ch-4.md to introduced interfaces in a detailed step-by-step manner, adds examples, and fixes some errors.
PiperOrigin-RevId: 275887017
This part of the tutorial is now covered by a new flow in Toy. This also removes a point of confusion as there is also a proper Linalg dialect.
PiperOrigin-RevId: 275338933
This chapters introduces the notion of a full conversion, and adds support for lowering down to the LLVM dialect, LLVM IR, and thus code generation.
PiperOrigin-RevId: 275337786
This chapter adds a partial lowering of toy operations, all but PrintOp, to a combination of the Affine and Std dialects. This chapter focuses on introducing the conversion framework, the benefits of partial lowering, and how easily dialects may co-exist in the IR.
PiperOrigin-RevId: 275150649
The GenericCallOp needed to have the CallOpInterface to be picked up by the inliner. This also adds a CastOp to perform shape casts that are generated during inlining. The casts generated by the inliner will be folded away after shape inference.
PiperOrigin-RevId: 275150438
This change performs general cleanups of the implementation of ch.4 and fixes some bugs. For example, the operations currently don't inherit from the shape inference interface.
PiperOrigin-RevId: 275089914
This Chapter now introduces and makes use of the Interface concept
in MLIR to demonstrate ShapeInference.
END_PUBLIC
Closestensorflow/mlir#191
PiperOrigin-RevId: 275085151
This change refactors the toyc driver to be much cleaner and easier to extend. It also cleans up a few comments in the combiner.
PiperOrigin-RevId: 274973808
This is using Table-driven Declarative Rewrite Rules (DRR), the previous
version of the tutorial only showed the C++ patterns.
Closestensorflow/mlir#187
PiperOrigin-RevId: 274852321
This effectively rewrites Ch.2 to introduce dialects, operations, and registration instead of deferring to Ch.3. This allows for introducing the best practices up front(using ODS, registering operations, etc.), and limits the opaque API to the chapter document instead of the code.
PiperOrigin-RevId: 274724289
This function-like operation allows one to define functions that have wrapped
LLVM IR function type, in particular variadic functions. The operation was
added in parallel to the existing lowering flow, this commit only switches the
flow to use it.
Using a custom function type makes the LLVM IR dialect type system more
consistent and avoids complex conversion rules for functions that previously
had to use the built-in function type instead of a wrapped LLVM IR dialect type
and perform conversions during the analysis.
PiperOrigin-RevId: 273910855
This makes the name of the conversion pass more consistent with the naming
scheme, since it actually converts from the Loop dialect to the Standard
dialect rather than working with arbitrary control flow operations.
PiperOrigin-RevId: 272612112
This CL finishes the implementation of the lowering part of the [strided memref RFC](https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/MaL8m2nXuio).
Strided memrefs correspond conceptually to the following templated C++ struct:
```
template <typename Elem, size_t Rank>
struct {
Elem *ptr;
int64_t offset;
int64_t sizes[Rank];
int64_t strides[Rank];
};
```
The linearization procedure for address calculation for strided memrefs is the same as for linalg views:
`base_offset + SUM_i index_i * stride_i`.
The following CL will unify Linalg and Standard by removing !linalg.view in favor of strided memrefs.
PiperOrigin-RevId: 272033399
The strided MemRef RFC discusses a normalized descriptor and interaction with library calls (https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/MaL8m2nXuio).
Lowering of nested LLVM structs as value types does not play nicely with externally compiled C/C++ functions due to ABI issues.
Solving the ABI problem generally is a very complex problem and most likely involves taking
a dependence on clang that we do not want atm.
A simple workaround is to pass pointers to memref descriptors at function boundaries, which this CL implement.
PiperOrigin-RevId: 271591708
The RFC for unifying Linalg and Affine compilation passes into an end-to-end flow with a predictable ABI and linkage to external function calls raised the question of why we have variable sized descriptors for memrefs depending on whether they have static or dynamic dimensions (https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/MaL8m2nXuio).
This CL standardizes the ABI on the rank of the memrefs.
The LLVM struct for a memref becomes equivalent to:
```
template <typename Elem, size_t Rank>
struct {
Elem *ptr;
int64_t sizes[Rank];
};
```
PiperOrigin-RevId: 270947276
The helper functions makePositionAttr() and positionAttr() were originally
introduced in the lowering-to-LLVM-dialect pass to construct integer array
attributes that are used for static positions in extract/insertelement.
Constructing an integer array attribute being fairly common, a utility function
Builder::getI64ArrayAttr was later introduced into the Builder API. Drop
makePositionAttr and similar homegrown functions and use that API instead.
PiperOrigin-RevId: 269295836
This change generalizes the structure of the pass manager to allow arbitrary nesting pass managers for other operations, at any level. The only user visible change to existing code is the fact that a PassManager must now provide an MLIRContext on construction. A new class `OpPassManager` has been added that represents a pass manager on a specific operation type. `PassManager` will remain the top-level entry point into the pipeline, with OpPassManagers being nested underneath. OpPassManagers will still be implicitly nested if the operation type on the pass differs from the pass manager. To explicitly build a pipeline, the 'nest' methods on OpPassManager may be used:
// Pass manager for the top-level module.
PassManager pm(ctx);
// Nest a pipeline operating on FuncOp.
OpPassManager &fpm = pm.nest<FuncOp>();
fpm.addPass(...);
// Nest a pipeline under the FuncOp pipeline that operates on spirv::ModuleOp
OpPassManager &spvModulePM = pm.nest<spirv::ModuleOp>();
// Nest a pipeline on FuncOps inside of the spirv::ModuleOp.
OpPassManager &spvFuncPM = spvModulePM.nest<FuncOp>();
To help accomplish this a new general OperationPass is added that operates on opaque Operations. This pass can be inserted in a pass manager of any type to operate on any operation opaquely. An example of this opaque OperationPass is a VerifierPass, that simply runs the verifier opaquely on the current operation.
/// Pass to verify an operation and signal failure if necessary.
class VerifierPass : public OperationPass<VerifierPass> {
void runOnOperation() override {
Operation *op = getOperation();
if (failed(verify(op)))
signalPassFailure();
markAllAnalysesPreserved();
}
};
PiperOrigin-RevId: 266840344
This pass class generalizes the current functionality between FunctionPass and ModulePass, and allows for operating on any operation type. The pass manager currently only supports OpPasses operating on FuncOp and ModuleOp, but this restriction will be relaxed in follow-up changes. A utility class OpPassBase<OpT> allows for generically referring to operation specific passes: e.g. FunctionPassBase == OpPassBase<FuncOp>.
PiperOrigin-RevId: 266442239
This change refactors and cleans up the implementation of the operation walk methods. After this refactoring is that the explicit template parameter for the operation type is no longer needed for the explicit op walks. For example:
op->walk<AffineForOp>([](AffineForOp op) { ... });
is now accomplished via:
op->walk([](AffineForOp op) { ... });
PiperOrigin-RevId: 266209552
- extend canonicalizeMapAndOperands to propagate constant operands into
the map's expressions (and thus drop those operands).
- canonicalizeMapAndOperands previously only dropped duplicate and
unused operands; however, operands that were constants were
retained.
This change makes IR maps/expressions generated by various
utilities/passes even simpler; also makes some of the test checks more
accurate and simpler -- for eg., 0' instead of symbol(%{{.*}}).
Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>
Closestensorflow/mlir#107
COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/107 from bondhugula:canonicalize-maps c889a51486d14fbf7db489f224f881e7e1ff7d72
PiperOrigin-RevId: 266085289
The code and documentation for this chapter of the tutorial have been updated to follow the new flow. The toy 'array' type has been replaced by usages of the MLIR tensor type. The code has also been cleaned up and modernized.
Closestensorflow/mlir#101
PiperOrigin-RevId: 265744086
Change the use of 'array' to 'tensor' to reflect the new flow that the tutorial will follow. Also tidy up some of the documentation, code comments, and fix a few out-dated links.
PiperOrigin-RevId: 265174676
Switch to C++14 standard method as llvm::make_unique has been removed (
https://reviews.llvm.org/D66259). Also mark some targets as c++14 to ease next
integrates.
PiperOrigin-RevId: 263953918
All 'getValue' variants now require that the index is valid, queryable via 'isValidIndex'. 'getSplatValue' now requires that the attribute is a proper splat. This allows for querying these methods on DenseElementAttr with all possible value types; e.g. float, int, APInt, etc. This also allows for removing unnecessary conversions to Attribute that really want the underlying value.
PiperOrigin-RevId: 263437337
Since raw pointers are always passed around for IR construct without
implying any ownership transfer, it can be error prone to have implicit
ownership transferred the same way.
For example this code can seem harmless:
Pass *pass = ....
pm.addPass(pass);
pm.addPass(pass);
pm.run(module);
PiperOrigin-RevId: 263053082
There are currently several different terms used to refer to a parent IR unit in 'get' methods: getParent/getEnclosing/getContaining. This cl standardizes all of these methods to use 'getParent*'.
PiperOrigin-RevId: 262680287
This will allow for reusing the same pattern list, which may be costly to continually reconstruct, on multiple invocations.
PiperOrigin-RevId: 262664599
The entry block is often used recently after insertion. This removes the need to perform an additional lookup in such cases.
PiperOrigin-RevId: 262265671
This CL modifies the LowerLinalgToLoopsPass to use RewritePattern.
This will make it easier to inline Linalg generic functions and regions when emitting to loops in a subsequent CL.
PiperOrigin-RevId: 261894120
Many LLVM transformations benefits from knowing the targets. This enables optimizations,
especially in a JIT context when the target is (generally) well-known.
Closestensorflow/mlir#49
PiperOrigin-RevId: 261840617
This allows for proper forward declaration, as opposed to leaking the internal implementation via a using directive. This also allows for all pattern building to go through 'insert' methods on the OwningRewritePatternList, replacing uses of 'push_back' and 'RewriteListBuilder'.
PiperOrigin-RevId: 261816316
This cl enforces that the conversion of the type signatures for regions, and thus their entry blocks, is handled via ConversionPatterns. A new hook 'applySignatureConversion' is added to the ConversionPatternRewriter to perform the desired conversion on a region. This also means that the handling of rewriting the signature of a FuncOp is moved to a pattern. A default implementation is provided via 'mlir::populateFuncOpTypeConversionPattern'. This removes the hacky implicit 'dynamically legal' status of FuncOp that was present previously, and leaves it up to the user to decide when/how to convert the signature of a function.
PiperOrigin-RevId: 259161999
This specific PatternRewriter will allow for exposing hooks in the future that are only useful for the conversion framework, e.g. type conversions.
PiperOrigin-RevId: 258818122
This cl begins a large refactoring over how signature types are converted in the DialectConversion infrastructure. The signatures of blocks are now converted on-demand when an operation held by that block is being converted. This allows for handling the case where a region is created as part of a pattern, something that wasn't possible previously.
This cl also generalizes the region signature conversion used by FuncOp to work on any region of any operation. This generalization allows for removing the 'apply*Conversion' functions that were specific to FuncOp/ModuleOp. The implementation currently uses a new hook on TypeConverter, 'convertRegionSignature', but this should ideally be removed in favor of using Patterns. That depends on adding support to the PatternRewriter used by ConversionPattern to allow applying signature conversions to regions, which should be coming in a followup.
PiperOrigin-RevId: 258645733
Users generally want several different modes of conversion. This cl refactors DialectConversion to provide two:
* Partial (applyPartialConversion)
- This mode allows for illegal operations to exist in the IR, and does not fail if an operation fails to be legalized.
* Full (applyFullConversion)
- This mode fails if any operation is not properly legalized to the conversion target. This allows for ensuring that the IR after a conversion only contains operations legal for the target.
PiperOrigin-RevId: 258412243
These methods don't compose well with the rest of conversion framework, and create artificial breaks in conversion. Replace these methods with two(populateAffineToStdConversionPatterns and populateLoopToStdConversionPatterns respectively) that populate a list of patterns to perform the same behavior.
PiperOrigin-RevId: 258219277
This field wasn't updated as the insertion point changed, making it potentially dangerous given the multi-level of MLIR(e.g. 'createBlock' would always insert the new block in 'region'). This also allows for building an OpBuilder with just a context.
PiperOrigin-RevId: 257829135
This CL splits the lowering of affine to LLVM into 2 parts:
1. affine -> std
2. std -> LLVM
The conversions mostly consists of splitting concerns between the affine and non-affine worlds from existing conversions.
Short-circuiting of affine `if` conditions was never tested or exercised and is removed in the process, it can be reintroduced later if needed.
LoopParametricTiling.cpp is updated to reflect the newly added ForOp::build.
PiperOrigin-RevId: 257794436
This allows for the attribute to hold symbolic references to other operations than FuncOp. This also allows for removing the dependence on FuncOp from the base Builder.
PiperOrigin-RevId: 257650017
There is already a more general 'getParentOfType' method, and 'getModule' is likely to be misused as functions get placed within different regions than ModuleOp.
PiperOrigin-RevId: 257442243
Change the AsmPrinter to number values breadth-first so that values in adjacent regions can have the same name. This allows for ModuleOp to contain operations that produce results. This also standardizes the special name of region entry arguments to "arg[0-9+]" now that Functions are also operations.
PiperOrigin-RevId: 257225069
Modules can now contain more than just Functions, this just updates the iteration API to reflect that. The 'begin'/'end' methods have also been updated to iterate over opaque Operations.
PiperOrigin-RevId: 257099084
These methods assume that a function is a valid builtin top-level operation, and removing these methods allows for decoupling FuncOp and IR/. Utility "getParentOfType" methods have been added to Operation/OpState to allow for querying the first parent operation of a given type.
PiperOrigin-RevId: 257018913
Address ClangTidy finding:
* std::move of the expression of the trivially-copyable type 'mlir::Module' (aka 'mlir::ModuleOp') has no effect; remove std::move()
PiperOrigin-RevId: 256981849
This is an important step in allowing for the top-level of the IR to be extensible. FuncOp and ModuleOp contain all of the necessary functionality, while using the existing operation infrastructure. As an interim step, many of the usages of Function and Module, including the name, will remain the same. In the future, many of these will be relaxed to allow for many different types of top-level operations to co-exist.
PiperOrigin-RevId: 256427100
As Functions/Modules becomes operations, these methods will conflict with the 'verify' hook already on derived operation types.
PiperOrigin-RevId: 256246112
As with Functions, Module will soon become an operation, which are value-typed. This eases the transition from Module to ModuleOp. A new class, OwningModuleRef is provided to allow for owning a reference to a Module, and will auto-delete the held module on destruction.
PiperOrigin-RevId: 256196193
Move the data members out of Function and into a new impl storage class 'FunctionStorage'. This allows for Function to become value typed, which will greatly simplify the transition of Function to FuncOp(given that FuncOp is also value typed).
PiperOrigin-RevId: 255983022
This functionality is now moved to a new class, ModuleManager. This class allows for inserting functions into a module, and will auto-rename them on insert to ensure a unique name. This now means that users adding new functions to a module must ensure that the function name is unique, as the Module will no longer do it automatically. This also means that Module::getNamedFunction now operates in O(N) instead of the O(c) time it did before. This simplifies the move of Modules to Operations as the ModuleOp will not be able to have this functionality.
PiperOrigin-RevId: 255846088
During conversion, if a type conversion has dangling uses a type conversion must persist after conversion has finished to maintain valid IR. In these cases, we now query the TypeConverter to materialize a conversion for us. This allows for the default case of a full conversion to continue working as expected, but also handle the degenerate cases more robustly.
PiperOrigin-RevId: 255637171
Remove the ability to print an attribute without a type, but allow for attributes to elide the type under certain circumstances. This fixes a bug where attributes within ArrayAttr, and other collection attributes, would never print the type.
PiperOrigin-RevId: 255306974
Now that Locations are attributes, they have direct access to the MLIR context. This allows for simplifying error emission by removing unnecessary context lookups.
PiperOrigin-RevId: 255112791
The current syntax separates the name and value with ':', but ':' is already overloaded by several other things(e.g. trailing types). This makes the syntax difficult to parse in some situtations:
Old:
"foo: 10 : i32"
New:
"foo = 10 : i32"
PiperOrigin-RevId: 255097928
Enable reusing the real mlir-opt main from unit tests and in case where
additional initialization needs to happen before main is invoked (e.g., when
using different command line flag libraries).
PiperOrigin-RevId: 254764575
Now that Locations are Attributes they contain a direct reference to the MLIRContext, i.e. the context can be directly accessed from the given location instead of being explicitly passed in.
PiperOrigin-RevId: 254568329
Conversions from dialect A to dialect B depend on both A and B. Therefore, it
is reasonable for them to live in a separate library that depends on both
DialectA and DialectB library, and does not forces dependees of DialectA or
DialectB to also link in the conversion. Create the directory layout for the
conversions and move the Standard to LLVM dialect conversion as the first
example.
PiperOrigin-RevId: 253312252
* 'get' methods that allow constructing from an ArrayRef of integer or floating point values.
* A 'reshape' method to allow for changing the shape without changing the underlying data.
PiperOrigin-RevId: 252067898
* Add a getCurrentLocation that returns the location directly.
* Add parseOperandList/parseTrailingOperandList overloads without the required operand count.
PiperOrigin-RevId: 251585488
To accomplish this, moving forward users will need to provide a legalization target that defines what operations are legal for the conversion. A target can mark an operation as legal by providing a specific legalization action. The initial actions are:
* Legal
- This action signals that every instance of the given operation is legal,
i.e. any combination of attributes, operands, types, etc. is valid.
* Dynamic
- This action signals that only some instances of a given operation are legal. This
allows for defining fine-tune constraints, like say std.add is only legal when
operating on 32-bit integers.
An example target is shown below:
struct MyTarget : public ConversionTarget {
MyTarget(MLIRContext &ctx) : ConversionTarget(ctx) {
// All operations in the LLVM dialect are legal.
addLegalDialect<LLVMDialect>();
// std.constant op is always legal on this target.
addLegalOp<ConstantOp>();
// std.return op has dynamic legality constraints.
addDynamicallyLegalOp<ReturnOp>();
}
/// Implement the custom legalization handler to handle
/// std.return.
bool isLegal(Operation *op) override {
// Process the dynamic handling for a std.return op (and any others that were
// marked "dynamic").
...
}
};
PiperOrigin-RevId: 251289374
These were just introduced by a previous CL moving MemRef getRank to return int64_t. size_t could be smaller than 64 bits and in equals comparisons, signed vs unsigned doesn't matter. In these cases, we know right now that the particular int64_t is not larger than max size_t (because it currently comes directly from a size() call), the alternative cast plus equals comparison is always safe, so we might as well do it that way and no longer require reasoning deeper into the callstack.
We are already assuming that size() calls fit into int64_t in a number of other cases like the aforementioned getRank() (since exabytes of RAM are rare). If we want to avoid this assumption we will have to come up with a principled way to do it throughout.
--
PiperOrigin-RevId: 250980297
* the 'empty' method should be used to check for emptiness instead of 'size'
* using decl 'CapturableHandle' is unused
* redundant get() call on smart pointer
* using decl 'apply' is unused
* using decl 'ScopeGuard' is unused
--
PiperOrigin-RevId: 250623863
Using ArrayRef introduces issues with the order of evaluation between a constructor and
the arguments of the subsequent calls to the `operator()`.
As a consequence the order of captures is not well-defined can go wrong with certain compilers (e.g. gcc-6.4).
This CL fixes the issue by using lambdas in lieu of ArrayRef.
--
PiperOrigin-RevId: 249114775
Originally, ExecutionEngine was created before MLIR had a proper pass
management infrastructure or an LLVM IR dialect (using the LLVM target
directly). It has been running a bunch of lowering passes to convert the input
IR from Standard+Affine dialects to LLVM IR and, later, to the LLVM IR dialect.
This is no longer necessary and is even undesirable for compilation flows that
perform their own conversion to the LLVM IR dialect. Drop this integration and
make ExecutionEngine accept only the LLVM IR dialect. Users of the
ExecutionEngine can call the relevant passes themselves.
--
PiperOrigin-RevId: 249004676
This means that we can now do something like:
ctx->getRegisteredDialect<LLVMDialect>();
as opposed to:
static_cast<LLVMDialect *>(ctx->getRegisteredDialect("llvm");
--
PiperOrigin-RevId: 247989896
This CL implements the previously unsupported parsing for Range, View and Slice operations.
A pass is introduced to lower to the LLVM.
Tests are moved out of C++ land and into mlir/test/Examples.
This allows better fitting within standard developer workflows.
--
PiperOrigin-RevId: 245796600
This CL starts implementing a Linalg dialect with the objective of supporting
optimizing compilation of loops and library calls for a subset of common linear
algebra operations.
This CL starts by simply adding a linalg.range type and an operation with the
proper roundtripping test.
--
PiperOrigin-RevId: 244189468
other characters within the <>'s now that we can. This will allow quantized
types to use the pretty syntax (among others) after a few changes.
--
PiperOrigin-RevId: 243521268
This allows client to be able to reuse the same logic to setup a module
for the ExecutionEngine without instanciating one. One use case is running
the optimization pipeline but not JIT-ing.
--
PiperOrigin-RevId: 242614380
TensorContractionBase has become too unwieldy with all the CRTP manipulation once less trivial transformations are implemented.
This CL drops CRTP for inheritance and uses the same name comparison trick to figure out what to cast into.
As a byproduct, all the -inl.h files disappear.
To maintain the separation between directories, a LINALG_STEP variable is introduced
--
PiperOrigin-RevId: 242546977
This dialect does not have a global constructor and has to be registered
manually in `main`. Also fix the way it is exercised in the test.
--
PiperOrigin-RevId: 242434886
For some reason, the OSS build on macOS was not happy with the initialization
syntax and was attempting to call a copy constructor. Hotfix it to use a
different syntax pending further investigation.
--
PiperOrigin-RevId: 242432634
making the IR dumps much nicer.
This is part 2/3 of the path to making dialect types more nice. Part 3/3 will
slightly generalize the set of characters allowed in pretty types and make it
more principled.
--
PiperOrigin-RevId: 242249955
* dyn_cast_or_null
- This will first check if the operation is null before trying to 'dyn_cast':
Value *v = ...;
if (auto forOp = dyn_cast_or_null<AffineForOp>(v->getDefiningOp()))
...
* isa_nonnull
- This will first check if the pointer is null before trying to 'isa':
Value *v = ...;
if (isa_nonnull<AffineForOp>(v->getDefiningOp());
...
--
PiperOrigin-RevId: 242171343
Use MLIR's ExecutionEngine to demonstrate how one can implement a simple
JIT-compiler and executor after fully lowering the Linalg dialect to the LLVM
IR dialect, using the direct conversion (not going through standard
loads/stores).
--
PiperOrigin-RevId: 242127690
This CL adds declarative tiling support in the linalg dialect by providing:
1. loop tiling on linalg ops by simply calling into mlir::tile
2. view tiling on linalg ops by:
a. computing the subview between for each tile dimension based on the loop tile size and the mapping of loops to operand ranges.
b. declaring that the tiled form of a tensorcontraction is the same tensorcontraction on subviews, which essentially gives us a recursive form.
Point 2.b is potentially subject to change in the future.
--
PiperOrigin-RevId: 242058658
This CL adds the last bit to convert from linalg.LoadOp and linalg.StoreOp to the affine dialect, as well as a unit test to exercise the conversion.
--
PiperOrigin-RevId: 242045826
Load and Store Linalg operations are converter to their LLVM IR counterparts
preceded by a sequence of operations that recover the effective address of the
accessed element. The address is computed given the subscripts and the view
descriptor as
base_pointer + base_offset + SUM_i subscript_i * stride_i.
Manual testing shows that the resulting LLVM IR for the matrix multiplication
example can be compiled and executed, producing correct results.
--
PiperOrigin-RevId: 241889003
River Riddle