1//===- AsyncRuntimeRefCounting.cpp - Async Runtime Ref Counting -----------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements automatic reference counting for Async runtime
10// operations and types.
11//
12//===----------------------------------------------------------------------===//
13
14#include "mlir/Dialect/Async/Passes.h"
15
16#include "mlir/Analysis/Liveness.h"
17#include "mlir/Dialect/Async/IR/Async.h"
18#include "mlir/Dialect/ControlFlow/IR/ControlFlowOps.h"
19#include "mlir/Dialect/Func/IR/FuncOps.h"
20#include "mlir/IR/ImplicitLocOpBuilder.h"
21#include "mlir/IR/PatternMatch.h"
22#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
23#include "llvm/ADT/SmallSet.h"
24
25namespace mlir {
26#define GEN_PASS_DEF_ASYNCRUNTIMEREFCOUNTINGPASS
27#define GEN_PASS_DEF_ASYNCRUNTIMEPOLICYBASEDREFCOUNTINGPASS
28#include "mlir/Dialect/Async/Passes.h.inc"
29} // namespace mlir
30
31#define DEBUG_TYPE "async-runtime-ref-counting"
32
33using namespace mlir;
34using namespace mlir::async;
35
36//===----------------------------------------------------------------------===//
37// Utility functions shared by reference counting passes.
38//===----------------------------------------------------------------------===//
39
40// Drop the reference count immediately if the value has no uses.
41static LogicalResult dropRefIfNoUses(Value value, unsigned count = 1) {
42 if (!value.getUses().empty())
43 return failure();
44
45 OpBuilder b(value.getContext());
46
47 // Set insertion point after the operation producing a value, or at the
48 // beginning of the block if the value defined by the block argument.
49 if (Operation *op = value.getDefiningOp())
50 b.setInsertionPointAfter(op);
51 else
52 b.setInsertionPointToStart(value.getParentBlock());
53
54 b.create<RuntimeDropRefOp>(value.getLoc(), value, b.getI64IntegerAttr(1));
55 return success();
56}
57
58// Calls `addRefCounting` for every reference counted value defined by the
59// operation `op` (block arguments and values defined in nested regions).
60static LogicalResult walkReferenceCountedValues(
61 Operation *op, llvm::function_ref<LogicalResult(Value)> addRefCounting) {
62 // Check that we do not have high level async operations in the IR because
63 // otherwise reference counting will produce incorrect results after high
64 // level async operations will be lowered to `async.runtime`
65 WalkResult checkNoAsyncWalk = op->walk(callback: [&](Operation *op) -> WalkResult {
66 if (!isa<ExecuteOp, AwaitOp, AwaitAllOp, YieldOp>(op))
67 return WalkResult::advance();
68
69 return op->emitError()
70 << "async operations must be lowered to async runtime operations";
71 });
72
73 if (checkNoAsyncWalk.wasInterrupted())
74 return failure();
75
76 // Add reference counting to block arguments.
77 WalkResult blockWalk = op->walk(callback: [&](Block *block) -> WalkResult {
78 for (BlockArgument arg : block->getArguments())
79 if (isRefCounted(type: arg.getType()))
80 if (failed(Result: addRefCounting(arg)))
81 return WalkResult::interrupt();
82
83 return WalkResult::advance();
84 });
85
86 if (blockWalk.wasInterrupted())
87 return failure();
88
89 // Add reference counting to operation results.
90 WalkResult opWalk = op->walk(callback: [&](Operation *op) -> WalkResult {
91 for (unsigned i = 0; i < op->getNumResults(); ++i)
92 if (isRefCounted(type: op->getResultTypes()[i]))
93 if (failed(Result: addRefCounting(op->getResult(idx: i))))
94 return WalkResult::interrupt();
95
96 return WalkResult::advance();
97 });
98
99 if (opWalk.wasInterrupted())
100 return failure();
101
102 return success();
103}
104
105//===----------------------------------------------------------------------===//
106// Automatic reference counting based on the liveness analysis.
107//===----------------------------------------------------------------------===//
108
109namespace {
110
111class AsyncRuntimeRefCountingPass
112 : public impl::AsyncRuntimeRefCountingPassBase<
113 AsyncRuntimeRefCountingPass> {
114public:
115 AsyncRuntimeRefCountingPass() = default;
116 void runOnOperation() override;
117
118private:
119 /// Adds an automatic reference counting to the `value`.
120 ///
121 /// All values (token, group or value) are semantically created with a
122 /// reference count of +1 and it is the responsibility of the async value user
123 /// to place the `add_ref` and `drop_ref` operations to ensure that the value
124 /// is destroyed after the last use.
125 ///
126 /// The function returns failure if it can't deduce the locations where
127 /// to place the reference counting operations.
128 ///
129 /// Async values "semantically created" when:
130 /// 1. Operation returns async result (e.g. `async.runtime.create`)
131 /// 2. Async value passed in as a block argument (or function argument,
132 /// because function arguments are just entry block arguments)
133 ///
134 /// Passing async value as a function argument (or block argument) does not
135 /// really mean that a new async value is created, it only means that the
136 /// caller of a function transfered ownership of `+1` reference to the callee.
137 /// It is convenient to think that from the callee perspective async value was
138 /// "created" with `+1` reference by the block argument.
139 ///
140 /// Automatic reference counting algorithm outline:
141 ///
142 /// #1 Insert `drop_ref` operations after last use of the `value`.
143 /// #2 Insert `add_ref` operations before functions calls with reference
144 /// counted `value` operand (newly created `+1` reference will be
145 /// transferred to the callee).
146 /// #3 Verify that divergent control flow does not lead to leaked reference
147 /// counted objects.
148 ///
149 /// Async runtime reference counting optimization pass will optimize away
150 /// some of the redundant `add_ref` and `drop_ref` operations inserted by this
151 /// strategy (see `async-runtime-ref-counting-opt`).
152 LogicalResult addAutomaticRefCounting(Value value);
153
154 /// (#1) Adds the `drop_ref` operation after the last use of the `value`
155 /// relying on the liveness analysis.
156 ///
157 /// If the `value` is in the block `liveIn` set and it is not in the block
158 /// `liveOut` set, it means that it "dies" in the block. We find the last
159 /// use of the value in such block and:
160 ///
161 /// 1. If the last user is a `ReturnLike` operation we do nothing, because
162 /// it forwards the ownership to the caller.
163 /// 2. Otherwise we add a `drop_ref` operation immediately after the last
164 /// use.
165 LogicalResult addDropRefAfterLastUse(Value value);
166
167 /// (#2) Adds the `add_ref` operation before the function call taking `value`
168 /// operand to ensure that the value passed to the function entry block
169 /// has a `+1` reference count.
170 LogicalResult addAddRefBeforeFunctionCall(Value value);
171
172 /// (#3) Adds the `drop_ref` operation to account for successor blocks with
173 /// divergent `liveIn` property: `value` is not in the `liveIn` set of all
174 /// successor blocks.
175 ///
176 /// Example:
177 ///
178 /// ^entry:
179 /// %token = async.runtime.create : !async.token
180 /// cf.cond_br %cond, ^bb1, ^bb2
181 /// ^bb1:
182 /// async.runtime.await %token
183 /// async.runtime.drop_ref %token
184 /// cf.br ^bb2
185 /// ^bb2:
186 /// return
187 ///
188 /// In this example ^bb2 does not have `value` in the `liveIn` set, so we have
189 /// to branch into a special "reference counting block" from the ^entry that
190 /// will have a `drop_ref` operation, and then branch into the ^bb2.
191 ///
192 /// After transformation:
193 ///
194 /// ^entry:
195 /// %token = async.runtime.create : !async.token
196 /// cf.cond_br %cond, ^bb1, ^reference_counting
197 /// ^bb1:
198 /// async.runtime.await %token
199 /// async.runtime.drop_ref %token
200 /// cf.br ^bb2
201 /// ^reference_counting:
202 /// async.runtime.drop_ref %token
203 /// cf.br ^bb2
204 /// ^bb2:
205 /// return
206 ///
207 /// An exception to this rule are blocks with `async.coro.suspend` terminator,
208 /// because in Async to LLVM lowering it is guaranteed that the control flow
209 /// will jump into the resume block, and then follow into the cleanup and
210 /// suspend blocks.
211 ///
212 /// Example:
213 ///
214 /// ^entry(%value: !async.value<f32>):
215 /// async.runtime.await_and_resume %value, %hdl : !async.value<f32>
216 /// async.coro.suspend %ret, ^suspend, ^resume, ^cleanup
217 /// ^resume:
218 /// %0 = async.runtime.load %value
219 /// cf.br ^cleanup
220 /// ^cleanup:
221 /// ...
222 /// ^suspend:
223 /// ...
224 ///
225 /// Although cleanup and suspend blocks do not have the `value` in the
226 /// `liveIn` set, it is guaranteed that execution will eventually continue in
227 /// the resume block (we never explicitly destroy coroutines).
228 LogicalResult addDropRefInDivergentLivenessSuccessor(Value value);
229};
230
231} // namespace
232
233LogicalResult AsyncRuntimeRefCountingPass::addDropRefAfterLastUse(Value value) {
234 OpBuilder builder(value.getContext());
235 Location loc = value.getLoc();
236
237 // Use liveness analysis to find the placement of `drop_ref`operation.
238 auto &liveness = getAnalysis<Liveness>();
239
240 // We analyse only the blocks of the region that defines the `value`, and do
241 // not check nested blocks attached to operations.
242 //
243 // By analyzing only the `definingRegion` CFG we potentially loose an
244 // opportunity to drop the reference count earlier and can extend the lifetime
245 // of reference counted value longer then it is really required.
246 //
247 // We also assume that all nested regions finish their execution before the
248 // completion of the owner operation. The only exception to this rule is
249 // `async.execute` operation, and we verify that they are lowered to the
250 // `async.runtime` operations before adding automatic reference counting.
251 Region *definingRegion = value.getParentRegion();
252
253 // Last users of the `value` inside all blocks where the value dies.
254 llvm::SmallSet<Operation *, 4> lastUsers;
255
256 // Find blocks in the `definingRegion` that have users of the `value` (if
257 // there are multiple users in the block, which one will be selected is
258 // undefined). User operation might be not the actual user of the value, but
259 // the operation in the block that has a "real user" in one of the attached
260 // regions.
261 llvm::DenseMap<Block *, Operation *> usersInTheBlocks;
262
263 for (Operation *user : value.getUsers()) {
264 Block *userBlock = user->getBlock();
265 Block *ancestor = definingRegion->findAncestorBlockInRegion(block&: *userBlock);
266 usersInTheBlocks[ancestor] = ancestor->findAncestorOpInBlock(op&: *user);
267 assert(ancestor && "ancestor block must be not null");
268 assert(usersInTheBlocks[ancestor] && "ancestor op must be not null");
269 }
270
271 // Find blocks where the `value` dies: the value is in `liveIn` set and not
272 // in the `liveOut` set. We place `drop_ref` immediately after the last use
273 // of the `value` in such regions (after handling few special cases).
274 //
275 // We do not traverse all the blocks in the `definingRegion`, because the
276 // `value` can be in the live in set only if it has users in the block, or it
277 // is defined in the block.
278 //
279 // Values with zero users (only definition) handled explicitly above.
280 for (auto &blockAndUser : usersInTheBlocks) {
281 Block *block = blockAndUser.getFirst();
282 Operation *userInTheBlock = blockAndUser.getSecond();
283
284 const LivenessBlockInfo *blockLiveness = liveness.getLiveness(block);
285
286 // Value must be in the live input set or defined in the block.
287 assert(blockLiveness->isLiveIn(value) ||
288 blockLiveness->getBlock() == value.getParentBlock());
289
290 // If value is in the live out set, it means it doesn't "die" in the block.
291 if (blockLiveness->isLiveOut(value))
292 continue;
293
294 // At this point we proved that `value` dies in the `block`. Find the last
295 // use of the `value` inside the `block`, this is where it "dies".
296 Operation *lastUser = blockLiveness->getEndOperation(value, startOperation: userInTheBlock);
297 assert(lastUsers.count(lastUser) == 0 && "last users must be unique");
298 lastUsers.insert(Ptr: lastUser);
299 }
300
301 // Process all the last users of the `value` inside each block where the value
302 // dies.
303 for (Operation *lastUser : lastUsers) {
304 // Return like operations forward reference count.
305 if (lastUser->hasTrait<OpTrait::ReturnLike>())
306 continue;
307
308 // We can't currently handle other types of terminators.
309 if (lastUser->hasTrait<OpTrait::IsTerminator>())
310 return lastUser->emitError() << "async reference counting can't handle "
311 "terminators that are not ReturnLike";
312
313 // Add a drop_ref immediately after the last user.
314 builder.setInsertionPointAfter(lastUser);
315 builder.create<RuntimeDropRefOp>(loc, value, builder.getI64IntegerAttr(1));
316 }
317
318 return success();
319}
320
321LogicalResult
322AsyncRuntimeRefCountingPass::addAddRefBeforeFunctionCall(Value value) {
323 OpBuilder builder(value.getContext());
324 Location loc = value.getLoc();
325
326 for (Operation *user : value.getUsers()) {
327 if (!isa<func::CallOp>(user))
328 continue;
329
330 // Add a reference before the function call to pass the value at `+1`
331 // reference to the function entry block.
332 builder.setInsertionPoint(user);
333 builder.create<RuntimeAddRefOp>(loc, value, builder.getI64IntegerAttr(1));
334 }
335
336 return success();
337}
338
339LogicalResult
340AsyncRuntimeRefCountingPass::addDropRefInDivergentLivenessSuccessor(
341 Value value) {
342 using BlockSet = llvm::SmallPtrSet<Block *, 4>;
343
344 OpBuilder builder(value.getContext());
345
346 // If a block has successors with different `liveIn` property of the `value`,
347 // record block successors that do not thave the `value` in the `liveIn` set.
348 llvm::SmallDenseMap<Block *, BlockSet> divergentLivenessBlocks;
349
350 // Use liveness analysis to find the placement of `drop_ref`operation.
351 auto &liveness = getAnalysis<Liveness>();
352
353 // Because we only add `drop_ref` operations to the region that defines the
354 // `value` we can only process CFG for the same region.
355 Region *definingRegion = value.getParentRegion();
356
357 // Collect blocks with successors with mismatching `liveIn` sets.
358 for (Block &block : definingRegion->getBlocks()) {
359 const LivenessBlockInfo *blockLiveness = liveness.getLiveness(&block);
360
361 // Skip the block if value is not in the `liveOut` set.
362 if (!blockLiveness || !blockLiveness->isLiveOut(value))
363 continue;
364
365 BlockSet liveInSuccessors; // `value` is in `liveIn` set
366 BlockSet noLiveInSuccessors; // `value` is not in the `liveIn` set
367
368 // Collect successors that do not have `value` in the `liveIn` set.
369 for (Block *successor : block.getSuccessors()) {
370 const LivenessBlockInfo *succLiveness = liveness.getLiveness(successor);
371 if (succLiveness && succLiveness->isLiveIn(value))
372 liveInSuccessors.insert(Ptr: successor);
373 else
374 noLiveInSuccessors.insert(Ptr: successor);
375 }
376
377 // Block has successors with different `liveIn` property of the `value`.
378 if (!liveInSuccessors.empty() && !noLiveInSuccessors.empty())
379 divergentLivenessBlocks.try_emplace(Key: &block, Args&: noLiveInSuccessors);
380 }
381
382 // Try to insert `dropRef` operations to handle blocks with divergent liveness
383 // in successors blocks.
384 for (auto kv : divergentLivenessBlocks) {
385 Block *block = kv.getFirst();
386 BlockSet &successors = kv.getSecond();
387
388 // Coroutine suspension is a special case terminator for wich we do not
389 // need to create additional reference counting (see details above).
390 Operation *terminator = block->getTerminator();
391 if (isa<CoroSuspendOp>(terminator))
392 continue;
393
394 // We only support successor blocks with empty block argument list.
395 auto hasArgs = [](Block *block) { return !block->getArguments().empty(); };
396 if (llvm::any_of(Range&: successors, P: hasArgs))
397 return terminator->emitOpError()
398 << "successor have different `liveIn` property of the reference "
399 "counted value";
400
401 // Make sure that `dropRef` operation is called when branched into the
402 // successor block without `value` in the `liveIn` set.
403 for (Block *successor : successors) {
404 // If successor has a unique predecessor, it is safe to create `dropRef`
405 // operations directly in the successor block.
406 //
407 // Otherwise we need to create a special block for reference counting
408 // operations, and branch from it to the original successor block.
409 Block *refCountingBlock = nullptr;
410
411 if (successor->getUniquePredecessor() == block) {
412 refCountingBlock = successor;
413 } else {
414 refCountingBlock = &successor->getParent()->emplaceBlock();
415 refCountingBlock->moveBefore(block: successor);
416 OpBuilder builder = OpBuilder::atBlockEnd(block: refCountingBlock);
417 builder.create<cf::BranchOp>(value.getLoc(), successor);
418 }
419
420 OpBuilder builder = OpBuilder::atBlockBegin(block: refCountingBlock);
421 builder.create<RuntimeDropRefOp>(value.getLoc(), value,
422 builder.getI64IntegerAttr(1));
423
424 // No need to update the terminator operation.
425 if (successor == refCountingBlock)
426 continue;
427
428 // Update terminator `successor` block to `refCountingBlock`.
429 for (const auto &pair : llvm::enumerate(First: terminator->getSuccessors()))
430 if (pair.value() == successor)
431 terminator->setSuccessor(block: refCountingBlock, index: pair.index());
432 }
433 }
434
435 return success();
436}
437
438LogicalResult
439AsyncRuntimeRefCountingPass::addAutomaticRefCounting(Value value) {
440 // Short-circuit reference counting for values without uses.
441 if (succeeded(Result: dropRefIfNoUses(value)))
442 return success();
443
444 // Add `drop_ref` operations based on the liveness analysis.
445 if (failed(Result: addDropRefAfterLastUse(value)))
446 return failure();
447
448 // Add `add_ref` operations before function calls.
449 if (failed(Result: addAddRefBeforeFunctionCall(value)))
450 return failure();
451
452 // Add `drop_ref` operations to successors with divergent `value` liveness.
453 if (failed(Result: addDropRefInDivergentLivenessSuccessor(value)))
454 return failure();
455
456 return success();
457}
458
459void AsyncRuntimeRefCountingPass::runOnOperation() {
460 auto functor = [&](Value value) { return addAutomaticRefCounting(value); };
461 if (failed(walkReferenceCountedValues(getOperation(), functor)))
462 signalPassFailure();
463}
464
465//===----------------------------------------------------------------------===//
466// Reference counting based on the user defined policy.
467//===----------------------------------------------------------------------===//
468
469namespace {
470
471class AsyncRuntimePolicyBasedRefCountingPass
472 : public impl::AsyncRuntimePolicyBasedRefCountingPassBase<
473 AsyncRuntimePolicyBasedRefCountingPass> {
474public:
475 AsyncRuntimePolicyBasedRefCountingPass() { initializeDefaultPolicy(); }
476
477 void runOnOperation() override;
478
479private:
480 // Adds a reference counting operations for all uses of the `value` according
481 // to the reference counting policy.
482 LogicalResult addRefCounting(Value value);
483
484 void initializeDefaultPolicy();
485
486 llvm::SmallVector<std::function<FailureOr<int>(OpOperand &)>> policy;
487};
488
489} // namespace
490
491LogicalResult
492AsyncRuntimePolicyBasedRefCountingPass::addRefCounting(Value value) {
493 // Short-circuit reference counting for values without uses.
494 if (succeeded(Result: dropRefIfNoUses(value)))
495 return success();
496
497 OpBuilder b(value.getContext());
498
499 // Consult the user defined policy for every value use.
500 for (OpOperand &operand : value.getUses()) {
501 Location loc = operand.getOwner()->getLoc();
502
503 for (auto &func : policy) {
504 FailureOr<int> refCount = func(operand);
505 if (failed(Result: refCount))
506 return failure();
507
508 int cnt = *refCount;
509
510 // Create `add_ref` operation before the operand owner.
511 if (cnt > 0) {
512 b.setInsertionPoint(operand.getOwner());
513 b.create<RuntimeAddRefOp>(loc, value, b.getI64IntegerAttr(cnt));
514 }
515
516 // Create `drop_ref` operation after the operand owner.
517 if (cnt < 0) {
518 b.setInsertionPointAfter(operand.getOwner());
519 b.create<RuntimeDropRefOp>(loc, value, b.getI64IntegerAttr(-cnt));
520 }
521 }
522 }
523
524 return success();
525}
526
527void AsyncRuntimePolicyBasedRefCountingPass::initializeDefaultPolicy() {
528 policy.push_back(Elt: [](OpOperand &operand) -> FailureOr<int> {
529 Operation *op = operand.getOwner();
530 Type type = operand.get().getType();
531
532 bool isToken = isa<TokenType>(type);
533 bool isGroup = isa<GroupType>(type);
534 bool isValue = isa<ValueType>(type);
535
536 // Drop reference after async token or group error check (coro await).
537 if (isa<RuntimeIsErrorOp>(op))
538 return (isToken || isGroup) ? -1 : 0;
539
540 // Drop reference after async value load.
541 if (isa<RuntimeLoadOp>(op))
542 return isValue ? -1 : 0;
543
544 // Drop reference after async token added to the group.
545 if (isa<RuntimeAddToGroupOp>(op))
546 return isToken ? -1 : 0;
547
548 return 0;
549 });
550}
551
552void AsyncRuntimePolicyBasedRefCountingPass::runOnOperation() {
553 auto functor = [&](Value value) { return addRefCounting(value); };
554 if (failed(walkReferenceCountedValues(getOperation(), functor)))
555 signalPassFailure();
556}
557

source code of mlir/lib/Dialect/Async/Transforms/AsyncRuntimeRefCounting.cpp