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[mlir][bufferization] Simplify helper potentiallyAliasesMemref #78690

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Merged
merged 1 commit into from
Jan 19, 2024
Merged

[mlir][bufferization] Simplify helper potentiallyAliasesMemref #78690

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26 changes: 11 additions & 15 deletions mlir/lib/Dialect/Bufferization/Transforms/BufferDeallocationSimplification.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -77,18 +77,12 @@ static bool distinctAllocAndBlockArgument(Value v1, Value v2) {
return areDistinct(v1Base, v2Base) || areDistinct(v2Base, v1Base);
}

/// Checks if `memref` may or must alias a MemRef in `otherList`. It is often a
/// requirement of optimization patterns that there cannot be any aliasing
/// memref in order to perform the desired simplification. The `allowSelfAlias`
/// argument indicates whether `memref` may be present in `otherList` which
/// makes this helper function applicable to situations where we already know
/// that `memref` is in the list but also when we don't want it in the list.
/// Checks if `memref` may potentially alias a MemRef in `otherList`. It is
/// often a requirement of optimization patterns that there cannot be any
/// aliasing memref in order to perform the desired simplification.
static bool potentiallyAliasesMemref(AliasAnalysis &analysis,
ValueRange otherList, Value memref,
bool allowSelfAlias) {
ValueRange otherList, Value memref) {
for (auto other : otherList) {
if (allowSelfAlias && other == memref)
continue;
if (distinctAllocAndBlockArgument(other, memref))
continue;
if (!analysis.alias(other, memref).isNo())
Expand Down Expand Up @@ -243,7 +237,7 @@ struct RemoveRetainedMemrefsGuaranteedToNotAlias

for (auto retainedMemref : deallocOp.getRetained()) {
if (potentiallyAliasesMemref(aliasAnalysis, deallocOp.getMemrefs(),
retainedMemref, false)) {
retainedMemref)) {
newRetainedMemrefs.push_back(retainedMemref);
replacements.push_back({});
continue;
Expand Down Expand Up @@ -314,11 +308,13 @@ struct SplitDeallocWhenNotAliasingAnyOther

SmallVector<Value> remainingMemrefs, remainingConditions;
SmallVector<SmallVector<Value>> updatedConditions;
for (auto [memref, cond] :
llvm::zip(deallocOp.getMemrefs(), deallocOp.getConditions())) {
for (int64_t i = 0, e = deallocOp.getMemrefs().size(); i < e; ++i) {
Value memref = deallocOp.getMemrefs()[i];
Value cond = deallocOp.getConditions()[i];
SmallVector<Value> otherMemrefs(deallocOp.getMemrefs());
otherMemrefs.erase(otherMemrefs.begin() + i);
Comment on lines +314 to +315
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This probably even fixes a bug when the same memref is contained in deallocOp.getMemrefs() multiple times. The old implementation would allow splitting which would lead to a double-free if I understand correctly without actually testing it or looking into it in detail. Do we have a test-case for that?

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I tried to write a test for that. We have another pattern that removes duplicate memrefs (DeallocRemoveDuplicateDeallocMemrefs). I think whether the bug is triggered or not depends on the order in which the patterns are applied. I was not able to trigger the double dealloc in practice. (I'll add the test case back anyway, just to be sure.)

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Makes sense. It's a bit annoying that MLIR doesn't have the utility to easily test individual patterns :/

// Check if `memref` can split off into a separate bufferization.dealloc.
if (potentiallyAliasesMemref(aliasAnalysis, deallocOp.getMemrefs(),
memref, true)) {
if (potentiallyAliasesMemref(aliasAnalysis, otherMemrefs, memref)) {
// `memref` alias with other memrefs, do not split off.
remainingMemrefs.push_back(memref);
remainingConditions.push_back(cond);
Expand Down
11 changes: 11 additions & 0 deletions mlir/test/Dialect/Bufferization/Transforms/buffer-deallocation-simplification.mlir
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Expand Up @@ -160,3 +160,14 @@ func.func @alloc_and_bbarg(%arg0: memref<5xf32>, %arg1: index, %arg2: index, %ar
// CHECK: bufferization.dealloc (%[[view]] : memref<f32>)
// CHECK-NOT: retain
// CHECK: scf.yield %[[alloc]], %[[true]]

// -----

func.func @duplicate_memref(%arg0: memref<5xf32>, %arg1: memref<6xf32>, %c: i1) -> i1 {
%0 = bufferization.dealloc (%arg0, %arg0 : memref<5xf32>, memref<5xf32>) if (%c, %c) retain (%arg1 : memref<6xf32>)
return %0 : i1
}

// CHECK-LABEL: func @duplicate_memref(
// CHECK: %[[r:.*]] = bufferization.dealloc (%{{.*}} : memref<5xf32>) if (%{{.*}}) retain (%{{.*}} : memref<6xf32>)
// CHECK: return %[[r]]