# MLIR

Multi-Level IR Compiler Framework

# Understanding the IR Structure

The MLIR Language Reference describes the High Level Structure, this document illustrates this structure through examples, and introduces at the same time the C++ APIs involved in manipulating it.

We will implement a 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 Regions, each of which is actually a list of Blocks, each of which itself wraps a list of Operations. Our traversal will follow this structure with three methods: printOperation(), printRegion(), and printBlock().

The first method inspects the properties of an operation, before iterating on the nested regions and print them individually:

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 other than a list of Blocks:

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 holds a list of Operations:

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";

// A block main role is to hold a list of Operations: let's recurse into
// printing each operation.
auto indent = pushIndent();
for (Operation &op : block.getOperations())
printOperation(&op);
}

The code for the pass is available here in the repo 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:

"builtin.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} : () -> ()
}) : () -> ()

And will yield the following output:

visiting op: 'builtin.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:
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 exposes a convenient templated method getOps<OpTy>() that provided a filtered iterator. Here is an example:

auto varOps = entryBlock.getOps<spirv::GlobalVariableOp>();
for (spirv::GlobalVariableOp gvOp : varOps) {
// process each GlobalVariable Operation in the block.
...
}

Similarly, the Region class exposes 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 immediately listed 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 exposes 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.

// Recursively traverse 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 operations nested inside the function:

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:

WalkResult result = getFunction().walk([&](AllocOp allocOp) {
if (!isValid(allocOp))
return WalkResult::interrupt();
});
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, 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 Operations, through their arguments: each Operation argument references a single Value.

Here is a code sample that inspects the operands of an Operation and prints some information about them:

// 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 result Values produced by an Operation and for each result will iterate the users of these results and print informations about them:

// 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 this pass is available here in the repo and can be exercised with mlir-opt -test-print-defuse.

The chaining of Values and their uses can be viewed as following:

The uses of a Value (OpOperand or BlockOperand) are also chained in a doubly linked-list, which is particularly useful when replacing all uses of a Value with a new one (“RAUW”):