1//===- LocalAliasAnalysis.cpp - Local stateless alias Analysis for MLIR ---===//
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#include "mlir/Analysis/AliasAnalysis/LocalAliasAnalysis.h"
10
11#include "mlir/Analysis/AliasAnalysis.h"
12#include "mlir/IR/Attributes.h"
13#include "mlir/IR/Block.h"
14#include "mlir/IR/Matchers.h"
15#include "mlir/IR/OpDefinition.h"
16#include "mlir/IR/Operation.h"
17#include "mlir/IR/Region.h"
18#include "mlir/IR/Value.h"
19#include "mlir/IR/ValueRange.h"
20#include "mlir/Interfaces/ControlFlowInterfaces.h"
21#include "mlir/Interfaces/FunctionInterfaces.h"
22#include "mlir/Interfaces/SideEffectInterfaces.h"
23#include "mlir/Interfaces/ViewLikeInterface.h"
24#include "mlir/Support/LLVM.h"
25#include "llvm/Support/Casting.h"
26#include <cassert>
27#include <optional>
28#include <utility>
29
30using namespace mlir;
31
32//===----------------------------------------------------------------------===//
33// Underlying Address Computation
34//===----------------------------------------------------------------------===//
35
36/// The maximum depth that will be searched when trying to find an underlying
37/// value.
38static constexpr unsigned maxUnderlyingValueSearchDepth = 10;
39
40/// Given a value, collect all of the underlying values being addressed.
41static void collectUnderlyingAddressValues(Value value, unsigned maxDepth,
42 DenseSet<Value> &visited,
43 SmallVectorImpl<Value> &output);
44
45/// Given a successor (`region`) of a RegionBranchOpInterface, collect all of
46/// the underlying values being addressed by one of the successor inputs. If the
47/// provided `region` is null, as per `RegionBranchOpInterface` this represents
48/// the parent operation.
49static void collectUnderlyingAddressValues(RegionBranchOpInterface branch,
50 Region *region, Value inputValue,
51 unsigned inputIndex,
52 unsigned maxDepth,
53 DenseSet<Value> &visited,
54 SmallVectorImpl<Value> &output) {
55 // Given the index of a region of the branch (`predIndex`), or std::nullopt to
56 // represent the parent operation, try to return the index into the outputs of
57 // this region predecessor that correspond to the input values of `region`. If
58 // an index could not be found, std::nullopt is returned instead.
59 auto getOperandIndexIfPred =
60 [&](RegionBranchPoint pred) -> std::optional<unsigned> {
61 SmallVector<RegionSuccessor, 2> successors;
62 branch.getSuccessorRegions(pred, successors);
63 for (RegionSuccessor &successor : successors) {
64 if (successor.getSuccessor() != region)
65 continue;
66 // Check that the successor inputs map to the given input value.
67 ValueRange inputs = successor.getSuccessorInputs();
68 if (inputs.empty()) {
69 output.push_back(Elt: inputValue);
70 break;
71 }
72 unsigned firstInputIndex, lastInputIndex;
73 if (region) {
74 firstInputIndex = cast<BlockArgument>(Val: inputs[0]).getArgNumber();
75 lastInputIndex = cast<BlockArgument>(Val: inputs.back()).getArgNumber();
76 } else {
77 firstInputIndex = cast<OpResult>(Val: inputs[0]).getResultNumber();
78 lastInputIndex = cast<OpResult>(Val: inputs.back()).getResultNumber();
79 }
80 if (firstInputIndex > inputIndex || lastInputIndex < inputIndex) {
81 output.push_back(Elt: inputValue);
82 break;
83 }
84 return inputIndex - firstInputIndex;
85 }
86 return std::nullopt;
87 };
88
89 // Check branches from the parent operation.
90 auto branchPoint = RegionBranchPoint::parent();
91 if (region)
92 branchPoint = region;
93
94 if (std::optional<unsigned> operandIndex =
95 getOperandIndexIfPred(/*predIndex=*/RegionBranchPoint::parent())) {
96 collectUnderlyingAddressValues(
97 branch.getEntrySuccessorOperands(branchPoint)[*operandIndex], maxDepth,
98 visited, output);
99 }
100 // Check branches from each child region.
101 Operation *op = branch.getOperation();
102 for (Region &region : op->getRegions()) {
103 if (std::optional<unsigned> operandIndex = getOperandIndexIfPred(region)) {
104 for (Block &block : region) {
105 // Try to determine possible region-branch successor operands for the
106 // current region.
107 if (auto term = dyn_cast<RegionBranchTerminatorOpInterface>(
108 block.getTerminator())) {
109 collectUnderlyingAddressValues(
110 term.getSuccessorOperands(branchPoint)[*operandIndex], maxDepth,
111 visited, output);
112 } else if (block.getNumSuccessors()) {
113 // Otherwise, if this terminator may exit the region we can't make
114 // any assumptions about which values get passed.
115 output.push_back(inputValue);
116 return;
117 }
118 }
119 }
120 }
121}
122
123/// Given a result, collect all of the underlying values being addressed.
124static void collectUnderlyingAddressValues(OpResult result, unsigned maxDepth,
125 DenseSet<Value> &visited,
126 SmallVectorImpl<Value> &output) {
127 Operation *op = result.getOwner();
128
129 // If this is a view, unwrap to the source.
130 if (ViewLikeOpInterface view = dyn_cast<ViewLikeOpInterface>(op))
131 return collectUnderlyingAddressValues(view.getViewSource(), maxDepth,
132 visited, output);
133 // Check to see if we can reason about the control flow of this op.
134 if (auto branch = dyn_cast<RegionBranchOpInterface>(op)) {
135 return collectUnderlyingAddressValues(branch, /*region=*/nullptr, result,
136 result.getResultNumber(), maxDepth,
137 visited, output);
138 }
139
140 output.push_back(Elt: result);
141}
142
143/// Given a block argument, collect all of the underlying values being
144/// addressed.
145static void collectUnderlyingAddressValues(BlockArgument arg, unsigned maxDepth,
146 DenseSet<Value> &visited,
147 SmallVectorImpl<Value> &output) {
148 Block *block = arg.getOwner();
149 unsigned argNumber = arg.getArgNumber();
150
151 // Handle the case of a non-entry block.
152 if (!block->isEntryBlock()) {
153 for (auto it = block->pred_begin(), e = block->pred_end(); it != e; ++it) {
154 auto branch = dyn_cast<BranchOpInterface>((*it)->getTerminator());
155 if (!branch) {
156 // We can't analyze the control flow, so bail out early.
157 output.push_back(Elt: arg);
158 return;
159 }
160
161 // Try to get the operand passed for this argument.
162 unsigned index = it.getSuccessorIndex();
163 Value operand = branch.getSuccessorOperands(index)[argNumber];
164 if (!operand) {
165 // We can't analyze the control flow, so bail out early.
166 output.push_back(Elt: arg);
167 return;
168 }
169 collectUnderlyingAddressValues(value: operand, maxDepth, visited, output);
170 }
171 return;
172 }
173
174 // Otherwise, check to see if we can reason about the control flow of this op.
175 Region *region = block->getParent();
176 Operation *op = region->getParentOp();
177 if (auto branch = dyn_cast<RegionBranchOpInterface>(op)) {
178 return collectUnderlyingAddressValues(branch, region, arg, argNumber,
179 maxDepth, visited, output);
180 }
181
182 // We can't reason about the underlying address of this argument.
183 output.push_back(Elt: arg);
184}
185
186/// Given a value, collect all of the underlying values being addressed.
187static void collectUnderlyingAddressValues(Value value, unsigned maxDepth,
188 DenseSet<Value> &visited,
189 SmallVectorImpl<Value> &output) {
190 // Check that we don't infinitely recurse.
191 if (!visited.insert(V: value).second)
192 return;
193 if (maxDepth == 0) {
194 output.push_back(Elt: value);
195 return;
196 }
197 --maxDepth;
198
199 if (BlockArgument arg = dyn_cast<BlockArgument>(Val&: value))
200 return collectUnderlyingAddressValues(arg, maxDepth, visited, output);
201 collectUnderlyingAddressValues(result: cast<OpResult>(Val&: value), maxDepth, visited,
202 output);
203}
204
205/// Given a value, collect all of the underlying values being addressed.
206static void collectUnderlyingAddressValues(Value value,
207 SmallVectorImpl<Value> &output) {
208 DenseSet<Value> visited;
209 collectUnderlyingAddressValues(value, maxDepth: maxUnderlyingValueSearchDepth, visited,
210 output);
211}
212
213//===----------------------------------------------------------------------===//
214// LocalAliasAnalysis: alias
215//===----------------------------------------------------------------------===//
216
217/// Given a value, try to get an allocation effect attached to it. If
218/// successful, `allocEffect` is populated with the effect. If an effect was
219/// found, `allocScopeOp` is also specified if a parent operation of `value`
220/// could be identified that bounds the scope of the allocated value; i.e. if
221/// non-null it specifies the parent operation that the allocation does not
222/// escape. If no scope is found, `allocScopeOp` is set to nullptr.
223static LogicalResult
224getAllocEffectFor(Value value,
225 std::optional<MemoryEffects::EffectInstance> &effect,
226 Operation *&allocScopeOp) {
227 // Try to get a memory effect interface for the parent operation.
228 Operation *op;
229 if (BlockArgument arg = dyn_cast<BlockArgument>(Val&: value))
230 op = arg.getOwner()->getParentOp();
231 else
232 op = cast<OpResult>(Val&: value).getOwner();
233 MemoryEffectOpInterface interface = dyn_cast<MemoryEffectOpInterface>(op);
234 if (!interface)
235 return failure();
236
237 // Try to find an allocation effect on the resource.
238 if (!(effect = interface.getEffectOnValue<MemoryEffects::Allocate>(value)))
239 return failure();
240
241 // If we found an allocation effect, try to find a scope for the allocation.
242 // If the resource of this allocation is automatically scoped, find the parent
243 // operation that bounds the allocation scope.
244 if (llvm::isa<SideEffects::AutomaticAllocationScopeResource>(
245 Val: effect->getResource())) {
246 allocScopeOp = op->getParentWithTrait<OpTrait::AutomaticAllocationScope>();
247 return success();
248 }
249
250 // TODO: Here we could look at the users to see if the resource is either
251 // freed on all paths within the region, or is just not captured by anything.
252 // For now assume allocation scope to the function scope (we don't care if
253 // pointer escape outside function).
254 allocScopeOp = op->getParentOfType<FunctionOpInterface>();
255 return success();
256}
257
258/// Given the two values, return their aliasing behavior.
259AliasResult LocalAliasAnalysis::aliasImpl(Value lhs, Value rhs) {
260 if (lhs == rhs)
261 return AliasResult::MustAlias;
262 Operation *lhsAllocScope = nullptr, *rhsAllocScope = nullptr;
263 std::optional<MemoryEffects::EffectInstance> lhsAlloc, rhsAlloc;
264
265 // Handle the case where lhs is a constant.
266 Attribute lhsAttr, rhsAttr;
267 if (matchPattern(value: lhs, pattern: m_Constant(bind_value: &lhsAttr))) {
268 // TODO: This is overly conservative. Two matching constants don't
269 // necessarily map to the same address. For example, if the two values
270 // correspond to different symbols that both represent a definition.
271 if (matchPattern(value: rhs, pattern: m_Constant(bind_value: &rhsAttr)))
272 return AliasResult::MayAlias;
273
274 // Try to find an alloc effect on rhs. If an effect was found we can't
275 // alias, otherwise we might.
276 return succeeded(Result: getAllocEffectFor(value: rhs, effect&: rhsAlloc, allocScopeOp&: rhsAllocScope))
277 ? AliasResult::NoAlias
278 : AliasResult::MayAlias;
279 }
280 // Handle the case where rhs is a constant.
281 if (matchPattern(value: rhs, pattern: m_Constant(bind_value: &rhsAttr))) {
282 // Try to find an alloc effect on lhs. If an effect was found we can't
283 // alias, otherwise we might.
284 return succeeded(Result: getAllocEffectFor(value: lhs, effect&: lhsAlloc, allocScopeOp&: lhsAllocScope))
285 ? AliasResult::NoAlias
286 : AliasResult::MayAlias;
287 }
288
289 // Otherwise, neither of the values are constant so check to see if either has
290 // an allocation effect.
291 bool lhsHasAlloc = succeeded(Result: getAllocEffectFor(value: lhs, effect&: lhsAlloc, allocScopeOp&: lhsAllocScope));
292 bool rhsHasAlloc = succeeded(Result: getAllocEffectFor(value: rhs, effect&: rhsAlloc, allocScopeOp&: rhsAllocScope));
293 if (lhsHasAlloc == rhsHasAlloc) {
294 // If both values have an allocation effect we know they don't alias, and if
295 // neither have an effect we can't make an assumptions.
296 return lhsHasAlloc ? AliasResult::NoAlias : AliasResult::MayAlias;
297 }
298
299 // When we reach this point we have one value with a known allocation effect,
300 // and one without. Move the one with the effect to the lhs to make the next
301 // checks simpler.
302 if (rhsHasAlloc) {
303 std::swap(a&: lhs, b&: rhs);
304 lhsAlloc = rhsAlloc;
305 lhsAllocScope = rhsAllocScope;
306 }
307
308 // If the effect has a scoped allocation region, check to see if the
309 // non-effect value is defined above that scope.
310 if (lhsAllocScope) {
311 // If the parent operation of rhs is an ancestor of the allocation scope, or
312 // if rhs is an entry block argument of the allocation scope we know the two
313 // values can't alias.
314 Operation *rhsParentOp = rhs.getParentRegion()->getParentOp();
315 if (rhsParentOp->isProperAncestor(other: lhsAllocScope))
316 return AliasResult::NoAlias;
317 if (rhsParentOp == lhsAllocScope) {
318 BlockArgument rhsArg = dyn_cast<BlockArgument>(Val&: rhs);
319 if (rhsArg && rhs.getParentBlock()->isEntryBlock())
320 return AliasResult::NoAlias;
321 }
322 }
323
324 // If we couldn't reason about the relationship between the two values,
325 // conservatively assume they might alias.
326 return AliasResult::MayAlias;
327}
328
329/// Given the two values, return their aliasing behavior.
330AliasResult LocalAliasAnalysis::alias(Value lhs, Value rhs) {
331 if (lhs == rhs)
332 return AliasResult::MustAlias;
333
334 // Get the underlying values being addressed.
335 SmallVector<Value, 8> lhsValues, rhsValues;
336 collectUnderlyingAddressValues(value: lhs, output&: lhsValues);
337 collectUnderlyingAddressValues(value: rhs, output&: rhsValues);
338
339 // If we failed to collect for either of the values somehow, conservatively
340 // assume they may alias.
341 if (lhsValues.empty() || rhsValues.empty())
342 return AliasResult::MayAlias;
343
344 // Check the alias results against each of the underlying values.
345 std::optional<AliasResult> result;
346 for (Value lhsVal : lhsValues) {
347 for (Value rhsVal : rhsValues) {
348 AliasResult nextResult = aliasImpl(lhs: lhsVal, rhs: rhsVal);
349 result = result ? result->merge(other: nextResult) : nextResult;
350 }
351 }
352
353 // We should always have a valid result here.
354 return *result;
355}
356
357//===----------------------------------------------------------------------===//
358// LocalAliasAnalysis: getModRef
359//===----------------------------------------------------------------------===//
360
361ModRefResult LocalAliasAnalysis::getModRef(Operation *op, Value location) {
362 // Check to see if this operation relies on nested side effects.
363 if (op->hasTrait<OpTrait::HasRecursiveMemoryEffects>()) {
364 // TODO: To check recursive operations we need to check all of the nested
365 // operations, which can result in a quadratic number of queries. We should
366 // introduce some caching of some kind to help alleviate this, especially as
367 // this caching could be used in other areas of the codebase (e.g. when
368 // checking `wouldOpBeTriviallyDead`).
369 return ModRefResult::getModAndRef();
370 }
371
372 // Otherwise, check to see if this operation has a memory effect interface.
373 MemoryEffectOpInterface interface = dyn_cast<MemoryEffectOpInterface>(op);
374 if (!interface)
375 return ModRefResult::getModAndRef();
376
377 // Build a ModRefResult by merging the behavior of the effects of this
378 // operation.
379 SmallVector<MemoryEffects::EffectInstance> effects;
380 interface.getEffects(effects);
381
382 ModRefResult result = ModRefResult::getNoModRef();
383 for (const MemoryEffects::EffectInstance &effect : effects) {
384 if (isa<MemoryEffects::Allocate, MemoryEffects::Free>(Val: effect.getEffect()))
385 continue;
386
387 // Check for an alias between the effect and our memory location.
388 // TODO: Add support for checking an alias with a symbol reference.
389 AliasResult aliasResult = AliasResult::MayAlias;
390 if (Value effectValue = effect.getValue())
391 aliasResult = alias(lhs: effectValue, rhs: location);
392
393 // If we don't alias, ignore this effect.
394 if (aliasResult.isNo())
395 continue;
396
397 // Merge in the corresponding mod or ref for this effect.
398 if (isa<MemoryEffects::Read>(Val: effect.getEffect())) {
399 result = result.merge(other: ModRefResult::getRef());
400 } else {
401 assert(isa<MemoryEffects::Write>(effect.getEffect()));
402 result = result.merge(other: ModRefResult::getMod());
403 }
404 if (result.isModAndRef())
405 break;
406 }
407 return result;
408}
409

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source code of mlir/lib/Analysis/AliasAnalysis/LocalAliasAnalysis.cpp