Add a doc/tutorial on traversing the IR

[stephenneuendorffer]

Love it. Thanks for adding this!

mlir/docs/Tutorials/UnderstandingTheIRStructure.md

5

"illustrates". Sentence structure is awkward.

11

period instead of semicolon

26–27

is itself wrapping -> itself wraps. I've been wondering if a picture would help to show the structure, rather than just text?

30

will inspect -> inspects

55

"anything else" -> "anything other"

68

has/holds

80

Complete sentence?

160

is exposing -> exposes

164–165

duplicated type could be auto?

171

is exposing -> exposes

178

"is exposing" again

181

maybe emphasize that this is a deep traversal, while getOps is a shallow traversal?

184

Recursively traverse...

192

LinalgOp operations?

201–202

Maybe introduce other WalkResult options here?

225

is referencing -> references

A picture would also help here, I think.

227

inspects...and prints.

246

results -> result Values?

253

enumerate is cool! Maybe provide a link to it here?

274

Code for this pass....

[mehdi_amini]

Address comments

mlir/docs/Tutorials/UnderstandingTheIRStructure.md

164–165

I don't think auto would help readability here.

181

I was trying to express it with "recursively nested"

201–202

There is only interrupt() / advance() ?

225

I agree a picture would be nice!

253

I don't think I can link from a code sample in a markdown file?

[mehdi_amini]

Add two pictures to illustrate the structur

mlir/docs/Tutorials/UnderstandingTheIRStructure.md

+# Understanding the IR Structure
+
+The MLIR Language Reference describes the
+[High Level Structure](../LangRef/#high-level-structure), this document
+illustrate this structure through examples and illustrate some mechanisms to
+manipulate it in C++.
+
+We will implement a [pass](../PassManagement/#operation-pass) that traverses any
+MLIR input and prints the entity inside the IR. A pass (or in general almost any
+piece of IR) is always rooted with an operation. Most of the time the top-level
+operation is a `ModuleOp` ; the MLIR `PassManager` is actually limited to
+operation on a top-level `ModuleOp`. As such a pass starts with an operation,
+and so will our traversal:
+
+```
+  void runOnOperation() override {
+    Operation *op = getOperation();
+    resetIndent();
+    printOperation(op);
+  }
+```
+
+## Traversing the IR Nesting
+
+The IR is recursively nested, an `Operation` can have one or multiple nested
+`Region`s, each of which is actually a list of `Blocks`, each of which is itself
+wrapping a list of `Operation`s. Our traversal will follow this structure with
+three methods: `printOperation()`, `printRegion()`, and `printBlock()`.
+
+The first method will inspect the properties of an operation, before iterating
+on the nested regions and print them individually:
+
+```c++
+  void printOperation(Operation *op) {
+    // Print the operation itself and some of its properties
+    printIndent() << "visiting op: '" << op->getName() << "' with "
+                  << op->getNumOperands() << " operands and "
+                  << op->getNumResults() << " results\n";
+    // Print the operation attributes
+    if (!op->getAttrs().empty()) {
+      printIndent() << op->getAttrs().size() << " attributes:\n";
+      for (NamedAttribute attr : op->getAttrs())
+        printIndent() << " - '" << attr.first << "' : '" << attr.second
+                      << "'\n";
+    }
+
+    // Recurse into each of the regions attached to the operation.
+    printIndent() << " " << op->getNumRegions() << " nested regions:\n";
+    auto indent = pushIndent();
+    for (Region &region : op->getRegions())
+      printRegion(region);
+  }
+```
+
+A `Region` does not hold anything else than a list of `Block`s:
+
+```c++
+  void printRegion(Region &region) {
+    // A region does not hold anything by itself other than a list of blocks.
+    printIndent() << "Region with " << region.getBlocks().size()
+                  << " blocks:\n";
+    auto indent = pushIndent();
+    for (Block &block : region.getBlocks())
+      printBlock(block);
+  }
+```
+
+Finally, a `Block` has a list of arguments, and hold a list of `Operation`s:
+
+```c++
+  void printBlock(Block &block) {
+    // Print the block intrinsics properties (basically: argument list)
+    printIndent()
+        << "Block with " << block.getNumArguments() << " arguments, "
+        << block.getNumSuccessors()
+        << " successors, and "
+        // Note, this `.size()` is traversing a linked-list and is O(n).
+        << block.getOperations().size() << " operations\n";
+
+    // Block main role is to hold a list of Operations: let's recurse.
+    auto indent = pushIndent();
+    for (Operation &op : block.getOperations())
+      printOperation(&op);
+  }
+```
+
+The code for the pass is available
+[here in the repo](https://github.com/llvm/llvm-project/blob/master/mlir/test/lib/IR/TestPrintNesting.cpp)
+and can be exercised with `mlir-opt -test-print-nesting`.
+
+### Example
+
+The Pass introduced in the previous section can be applied on the following IR
+with `mlir-opt -test-print-nesting -allow-unregistered-dialect
+llvm-project/mlir/test/IR/print-ir-nesting.mlir`:
+
+```mlir
+"module"() ( {
+  %0:4 = "dialect.op1"() {"attribute name" = 42 : i32} : () -> (i1, i16, i32, i64)
+  "dialect.op2"() ( {
+    "dialect.innerop1"(%0#0, %0#1) : (i1, i16) -> ()
+  },  {
+    "dialect.innerop2"() : () -> ()
+    "dialect.innerop3"(%0#0, %0#2, %0#3)[^bb1, ^bb2] : (i1, i32, i64) -> ()
+  ^bb1(%1: i32):  // pred: ^bb0
+    "dialect.innerop4"() : () -> ()
+    "dialect.innerop5"() : () -> ()
+  ^bb2(%2: i64):  // pred: ^bb0
+    "dialect.innerop6"() : () -> ()
+    "dialect.innerop7"() : () -> ()
+  }) {"other attribute" = 42 : i64} : () -> ()
+  "module_terminator"() : () -> ()
+}) : () -> ()
+```
+
+And will yield the following output:
+
+```
+visiting op: 'module' with 0 operands and 0 results
+ 1 nested regions:
+  Region with 1 blocks:
+    Block with 0 arguments, 0 successors, and 3 operations
+      visiting op: 'dialect.op1' with 0 operands and 4 results
+      1 attributes:
+       - 'attribute name' : '42 : i32'
+       0 nested regions:
+      visiting op: 'dialect.op2' with 0 operands and 0 results
+       2 nested regions:
+        Region with 1 blocks:
+          Block with 0 arguments, 0 successors, and 1 operations
+            visiting op: 'dialect.innerop1' with 2 operands and 0 results
+             0 nested regions:
+        Region with 3 blocks:
+          Block with 0 arguments, 2 successors, and 2 operations
+            visiting op: 'dialect.innerop2' with 0 operands and 0 results
+             0 nested regions:
+            visiting op: 'dialect.innerop3' with 3 operands and 0 results
+             0 nested regions:
+          Block with 1 arguments, 0 successors, and 2 operations
+            visiting op: 'dialect.innerop4' with 0 operands and 0 results
+             0 nested regions:
+            visiting op: 'dialect.innerop5' with 0 operands and 0 results
+             0 nested regions:
+          Block with 1 arguments, 0 successors, and 2 operations
+            visiting op: 'dialect.innerop6' with 0 operands and 0 results
+             0 nested regions:
+            visiting op: 'dialect.innerop7' with 0 operands and 0 results
+             0 nested regions:
+      visiting op: 'module_terminator' with 0 operands and 0 results
+       0 nested regions:
+```
+
+## Other IR Traversal Methods.
+
+In many cases, unwrapping the recursive structure of the IR is cumbersome and
+you may be interested in using other helpers.
+
+### Filtered iterator: `getOps<OpTy>()`
+
+For example the `Block` class is exposing a convenient templated method
+`getOps<OpTy>()` that provided a filtered iterator. Here is an example:
+
+```c++
+  auto varOps = entryBlock.getOps<spirv::GlobalVariableOp>();
+  for (spirv::GlobalVariableOp gvOp : varOps) {
+     // process each GlobalVariable Operation in the block.
+     ...
+  }
+```
+
+Similarly, the `Region` class is exposing the same `getOps` method that will
+iterate on all the blocks in the region.
+
+### Walkers
+
+The `getOps<OpTy>()` is useful to iterate on some Operations inside a single
+block (or a single region), however it is frequently interesting to traverse the
+IR in a nested fashion. To this end MLIR is exposing the `walk()` helper on
+`Operation`, `Block`, and `Region`. This helper takes a single argument: a
+callback method that will be invoked for every operation recursively nested
+under the provided entity.
+
+```c++
+  // Traverse recusively all the regions and blocks nested inside the function
+  // and apply the callback on every single operation in post-order.
+  getFunction().walk([&](mlir::Operation *op) {
+    // process Operation `op`.
+  });
+```
+
+The provided callback can be specialized to filter on a particular type of
+Operation, for example the following will apply the callback only on `LinalgOp`
+nested inside the function:
+
+```c++
+  getFunction.walk([](LinalgOp linalgOp) {
+    // process LinalgOp `linalgOp`.
+  });
+```
+
+Finally, the callback can optionally stop the walk by returning a
+`WalkResult::interrupt()` value. For example the following walk will find all
+`AllocOp` nested inside the function and interrupt the traversal if one of them
+does not satisfy a criteria:
+
+```c++
+  WalkResult result = getFunction().walk([&](AllocOp allocOp) {
+    if (!isValid(allocOp))
+      return WalkResult::interrupt();
+    return WalkResult::advance();
+  });
+  if (result.wasInterrupted())
+    // One alloc wasn't matching.
+    ...
+```
+
+## Traversing the def-use chains
+
+Another relationship in the IR is the one that links a `Value` with its users.
+As defined in the
+[language reference](https://mlir.llvm.org/docs/LangRef/#high-level-structure),
+each Value is either a `BlockArgument` or the result of exactly one `Operation`
+(an `Operation` can have multiple results, each of them is a separate `Value`).
+The users of a `Value` are `Operation`s, through their arguments: each
+`Operation` argument is referencing a single `Value`.
+
+Here is a code sample that inspects the operands of an `Operation` and print
+some information about them:
+
+```c++
+  // Print information about the producer of each of the operands.
+  for (Value operand : op->getOperands()) {
+    if (Operation *producer = operand.getDefiningOp()) {
+      llvm::outs() << "  - Operand produced by operation '"
+                   << producer->getName() << "'\n";
+    } else {
+      // If there is no defining op, the Value is necessarily a Block
+      // argument.
+      auto blockArg = operand.cast<BlockArgument>();
+      llvm::outs() << "  - Operand produced by Block argument, number "
+                   << blockArg.getArgNumber() << "\n";
+    }
+  }
+```
+
+Similarly, the following code sample iterates through the results produced by an
+`Operation` and for each result will iterate the users of these results and
+print informations about them:
+
+```c++
+  // Print information about the user of each of the result.
+  llvm::outs() << "Has " << op->getNumResults() << " results:\n";
+  for (auto indexedResult : llvm::enumerate(op->getResults())) {
+    Value result = indexedResult.value();
+    llvm::outs() << "  - Result " << indexedResult.index();
+    if (result.use_empty()) {
+      llvm::outs() << " has no uses\n";
+      continue;
+    }
+    if (result.hasOneUse()) {
+      llvm::outs() << " has a single use: ";
+    } else {
+      llvm::outs() << " has "
+                   << std::distance(result.getUses().begin(),
+                                    result.getUses().end())
+                   << " uses:\n";
+    }
+    for (Operation *userOp : result.getUsers()) {
+      llvm::outs() << "    - " << userOp->getName() << "\n";
+    }
+  }
+```
+
+The illustrating code for the pass is available
+[here in the repo](https://github.com/llvm/llvm-project/blob/master/mlir/test/lib/IR/TestPrintDefUse.cpp)
+and can be exercised with `mlir-opt -test-print-defuse`.

mlir/test/IR/print-ir-defuse.mlir

+// RUN: mlir-opt -test-print-defuse  -allow-unregistered-dialect %s | FileCheck %s
+
+// CHECK: Visiting op 'dialect.op1' with 0 operands:
+// CHECK: Has 4 results:
+// CHECK:   - Result 0 has a single use:     - dialect.op2
+// CHECK:   - Result 1 has no uses
+// CHECK:   - Result 2 has 2 uses:
+// CHECK:     - dialect.innerop1
+// CHECK:     - dialect.op2
+// CHECK:   - Result 3 has no uses
+// CHECK: Visiting op 'dialect.op2' with 2 operands:
+// CHECK:   - Operand produced by operation 'dialect.op1'
+// CHECK:   - Operand produced by operation 'dialect.op1'
+// CHECK: Has 0 results:
+// CHECK: Visiting op 'dialect.innerop1' with 2 operands:
+// CHECK:   - Operand produced by Block argument, number 0
+// CHECK:   - Operand produced by operation 'dialect.op1'
+// CHECK: Has 0 results:
+// CHECK: Visiting op 'dialect.op3' with 0 operands:
+// CHECK: Has 0 results:
+// CHECK: Visiting op 'module_terminator' with 0 operands:
+// CHECK: Has 0 results:
+// CHECK: Visiting op 'module' with 0 operands:
+// CHECK: Has 0 results:
+
+%results:4 = "dialect.op1"() : () -> (i1, i16, i32, i64)
+"dialect.op2"(%results#0, %results#2) : (i1, i32) -> ()
+"dialect.op3"() ({
+  ^bb0(%arg0 : i1):
+    "dialect.innerop1"(%arg0, %results#2) : (i1, i32) -> ()
+}) : () -> ()

mlir/test/IR/print-ir-nesting.mlir

+// RUN: mlir-opt -test-print-nesting  -allow-unregistered-dialect %s | FileCheck %s
+
+// CHECK: visiting op: 'module' with 0 operands and 0 results
+// CHECK:  1 nested regions:
+// CHECK:   Region with 1 blocks:
+// CHECK:     Block with 0 arguments, 0 successors, and 3 operations
+module {
+
+
+// CHECK:       visiting op: 'dialect.op1' with 0 operands and 4 results
+// CHECK:       1 attributes:
+// CHECK:        - 'attribute name' : '42 : i32'
+// CHECK:        0 nested regions:
+  %results:4 = "dialect.op1"() { "attribute name" = 42 : i32 } : () -> (i1, i16, i32, i64)
+
+
+// CHECK:       visiting op: 'dialect.op2' with 0 operands and 0 results
+// CHECK:        2 nested regions:
+  "dialect.op2"() ({
+
+// CHECK:         Region with 1 blocks:
+// CHECK:           Block with 0 arguments, 0 successors, and 1 operations
+// CHECK:             visiting op: 'dialect.innerop1' with 2 operands and 0 results
+// CHECK:              0 nested regions:
+    "dialect.innerop1"(%results#0, %results#1) : (i1, i16) -> ()
+
+// CHECK:         Region with 3 blocks:
+  },{
+
+// CHECK:           Block with 0 arguments, 2 successors, and 2 operations
+// CHECK:             visiting op: 'dialect.innerop2' with 0 operands and 0 results
+// CHECK:              0 nested regions:
+    "dialect.innerop2"() : () -> ()
+// CHECK:             visiting op: 'dialect.innerop3' with 3 operands and 0 results
+// CHECK:              0 nested regions:
+    "dialect.innerop3"(%results#0, %results#2, %results#3)[^bb1, ^bb2] : (i1, i32, i64) -> ()
+// CHECK:           Block with 1 arguments, 0 successors, and 2 operations
+  ^bb1(%arg1 : i32):
+// CHECK:             visiting op: 'dialect.innerop4' with 0 operands and 0 results
+// CHECK:              0 nested regions:
+    "dialect.innerop4"() : () -> ()
+// CHECK:             visiting op: 'dialect.innerop5' with 0 operands and 0 results
+// CHECK:              0 nested regions:
+    "dialect.innerop5"() : () -> ()
+// CHECK:           Block with 1 arguments, 0 successors, and 2 operations
+  ^bb2(%arg2 : i64):
+// CHECK:             visiting op: 'dialect.innerop6' with 0 operands and 0 results
+// CHECK:              0 nested regions:
+    "dialect.innerop6"() : () -> ()
+// CHECK:             visiting op: 'dialect.innerop7' with 0 operands and 0 results
+// CHECK:              0 nested regions:
+    "dialect.innerop7"() : () -> ()
+  }) : () -> ()
+
+// CHECK:       visiting op: 'module_terminator' with 0 operands and 0 results
+
+} // module

mlir/test/lib/IR/CMakeLists.txt

 # Exclude tests from libMLIR.so
 add_mlir_library(MLIRTestIR
   TestFunc.cpp
   TestInterfaces.cpp
   TestMatchers.cpp
+  TestPrintDefUse.cpp
+  TestPrintNesting.cpp
   TestSideEffects.cpp
   TestSymbolUses.cpp
   TestTypes.cpp
 
   EXCLUDE_FROM_LIBMLIR
 
   LINK_LIBS PUBLIC
   MLIRPass
   MLIRTestDialect
   )
 include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../Dialect/Test)
 include_directories(${CMAKE_CURRENT_BINARY_DIR}/../Dialect/Test)

mlir/test/lib/IR/TestPrintDefUse.cpp

+//===- TestPrintDefUse.cpp - Passes to illustrate the IR def-use chains ---===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/StandardOps/IR/Ops.h"
+#include "mlir/IR/Function.h"
+#include "mlir/Pass/Pass.h"
+
+using namespace mlir;
+
+namespace {
+/// This pass illustrates the IR def-use chains through printing.
+struct TestPrintDefUsePass
+    : public PassWrapper<TestPrintDefUsePass, OperationPass<>> {
+  void runOnOperation() override {
+    // Recursively traverse the IR nested under the current operation and print
+    // every single operation and their operands and users.
+    getOperation()->walk([](Operation *op) {
+      llvm::outs() << "Visiting op '" << op->getName() << "' with "
+                   << op->getNumOperands() << " operands:\n";
+
+      // Print information about the producer of each of the operands.
+      for (Value operand : op->getOperands()) {
+        if (Operation *producer = operand.getDefiningOp()) {
+          llvm::outs() << "  - Operand produced by operation '"
+                       << producer->getName() << "'\n";
+        } else {
+          // If there is no defining op, the Value is necessarily a Block
+          // argument.
+          auto blockArg = operand.cast<BlockArgument>();
+          llvm::outs() << "  - Operand produced by Block argument, number "
+                       << blockArg.getArgNumber() << "\n";
+        }
+      }
+
+      // Print information about the user of each of the result.
+      llvm::outs() << "Has " << op->getNumResults() << " results:\n";
+      for (auto indexedResult : llvm::enumerate(op->getResults())) {
+        Value result = indexedResult.value();
+        llvm::outs() << "  - Result " << indexedResult.index();
+        if (result.use_empty()) {
+          llvm::outs() << " has no uses\n";
+          continue;
+        }
+        if (result.hasOneUse()) {
+          llvm::outs() << " has a single use: ";
+        } else {
+          llvm::outs() << " has "
+                       << std::distance(result.getUses().begin(),
+                                        result.getUses().end())
+                       << " uses:\n";
+        }
+        for (Operation *userOp : result.getUsers()) {
+          llvm::outs() << "    - " << userOp->getName() << "\n";
+        }
+      }
+    });
+  }
+};
+} // end anonymous namespace
+
+namespace mlir {
+void registerTestPrintDefUsePass() {
+  PassRegistration<TestPrintDefUsePass>("test-print-defuse",
+                                        "Test various printing.");
+}
+} // namespace mlir

mlir/test/lib/IR/TestPrintNesting.cpp

+//===- TestPrintNesting.cpp - Passes to illustrate the IR nesting ---------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/StandardOps/IR/Ops.h"
+#include "mlir/IR/Function.h"
+#include "mlir/Pass/Pass.h"
+
+using namespace mlir;
+
+namespace {
+/// This pass illustrates the IR nesting through printing.
+struct TestPrintNestingPass
+    : public PassWrapper<TestPrintNestingPass, OperationPass<>> {
+  // Entry point for the pass.
+  void runOnOperation() override {
+    Operation *op = getOperation();
+    resetIndent();
+    printOperation(op);
+  }
+
+  /// The three methods below are mutually recursive and follow the nesting of
+  /// the IR: operation->region->block->operation->...
+
+  void printOperation(Operation *op) {
+    // Print the operation itself and some of its properties
+    printIndent() << "visiting op: '" << op->getName() << "' with "
+                  << op->getNumOperands() << " operands and "
+                  << op->getNumResults() << " results\n";
+    // Print the operation attributes
+    if (!op->getAttrs().empty()) {
+      printIndent() << op->getAttrs().size() << " attributes:\n";
+      for (NamedAttribute attr : op->getAttrs())
+        printIndent() << " - '" << attr.first << "' : '" << attr.second
+                      << "'\n";
+    }
+
+    // Recurse into each of the regions attached to the operation.
+    printIndent() << " " << op->getNumRegions() << " nested regions:\n";
+    auto indent = pushIndent();
+    for (Region &region : op->getRegions())
+      printRegion(region);
+  }
+
+  void printRegion(Region &region) {
+    // A region does not hold anything by itself other than a list of blocks.
+    printIndent() << "Region with " << region.getBlocks().size()
+                  << " blocks:\n";
+    auto indent = pushIndent();
+    for (Block &block : region.getBlocks())
+      printBlock(block);
+  }
+
+  void printBlock(Block &block) {
+    // Print the block intrinsics properties (basically: argument list)
+    printIndent()
+        << "Block with " << block.getNumArguments() << " arguments, "
+        << block.getNumSuccessors()
+        << " successors, and "
+        // Note, this `.size()` is traversing a linked-list and is O(n).
+        << block.getOperations().size() << " operations\n";
+
+    // Block main role is to hold a list of Operations: let's recurse.
+    auto indent = pushIndent();
+    for (Operation &op : block.getOperations())
+      printOperation(&op);
+  }
+
+  /// Manages the indentation as we traverse the IR nesting.
+  int indent;
+  struct IdentRAII {
+    int &indent;
+    IdentRAII(int &indent) : indent(indent) {}
+    ~IdentRAII() { --indent; }
+  };
+  void resetIndent() { indent = 0; }
+  IdentRAII pushIndent() { return IdentRAII(++indent); }
+
+  llvm::raw_ostream &printIndent() {
+    for (int i = 0; i < indent; ++i)
+      llvm::outs() << "  ";
+    return llvm::outs();
+  }
+};
+} // end anonymous namespace
+
+namespace mlir {
+void registerTestPrintNestingPass() {
+  PassRegistration<TestPrintNestingPass>("test-print-nesting",
+                                         "Test various printing.");
+}
+} // namespace mlir

mlir/tools/mlir-opt/mlir-opt.cpp

 void registerTestLoopMappingPass();
 void registerTestLoopPermutationPass();
 void registerTestLoopUnrollingPass();
 void registerTestMatchers();
 void registerTestMemRefDependenceCheck();
 void registerTestMemRefStrideCalculation();
 void registerTestOpaqueLoc();
 void registerTestPreparationPassWithAllowedMemrefResults();
+void registerTestPrintDefUsePass();
+void registerTestPrintNestingPass();
 void registerTestRecursiveTypesPass();
 void registerTestReducer();
 void registerTestSpirvEntryPointABIPass();
 void registerTestSCFUtilsPass();
 void registerTestVectorConversions();
 void registerVectorizerTestPass();
 } // namespace mlir
 
#endif
   registerTestLoopFusion();
   registerTestLoopMappingPass();
   registerTestLoopUnrollingPass();
   registerTestMatchers();
   registerTestMemRefDependenceCheck();
   registerTestMemRefStrideCalculation();
   registerTestOpaqueLoc();
   registerTestPreparationPassWithAllowedMemrefResults();
+  registerTestPrintDefUsePass();
+  registerTestPrintNestingPass();
   registerTestRecursiveTypesPass();
   registerTestReducer();
   registerTestGpuParallelLoopMappingPass();
   registerTestSpirvEntryPointABIPass();
   registerTestSCFUtilsPass();
   registerTestVectorConversions();
   registerVectorizerTestPass();
 }