[llvm-branch-commits] [llvm] 9abac60 - ADT: Fix reference invalidation in SmallVector::push_back and single-element insert
Duncan P. N. Exon Smith via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Wed Jan 13 19:03:40 PST 2021
Author: Duncan P. N. Exon Smith
Date: 2021-01-13T18:58:24-08:00
New Revision: 9abac60309006db00eca0af406c2e16bef26807c
URL: https://github.com/llvm/llvm-project/commit/9abac60309006db00eca0af406c2e16bef26807c
DIFF: https://github.com/llvm/llvm-project/commit/9abac60309006db00eca0af406c2e16bef26807c.diff
LOG: ADT: Fix reference invalidation in SmallVector::push_back and single-element insert
For small enough, trivially copyable `T`, take the argument by value in
`SmallVector::push_back` and copy it when forwarding to
`SmallVector::insert_one_impl`. Otherwise, when growing, update the
argument appropriately.
Differential Revision: https://reviews.llvm.org/D93779
Added:
Modified:
llvm/include/llvm/ADT/SmallVector.h
llvm/unittests/ADT/SmallVectorTest.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
index 803588143d81..f5293970aa9f 100644
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -220,6 +220,23 @@ class SmallVectorTemplateCommon
}
void assertSafeToEmplace() {}
+ /// Reserve enough space to add one element, and return the updated element
+ /// pointer in case it was a reference to the storage.
+ template <class U>
+ static const T *reserveForAndGetAddressImpl(U *This, const T &Elt) {
+ if (LLVM_LIKELY(This->size() < This->capacity()))
+ return &Elt;
+
+ bool ReferencesStorage = false;
+ int64_t Index = -1;
+ if (LLVM_UNLIKELY(This->isReferenceToStorage(&Elt))) {
+ ReferencesStorage = true;
+ Index = &Elt - This->begin();
+ }
+ This->grow();
+ return ReferencesStorage ? This->begin() + Index : &Elt;
+ }
+
public:
using size_type = size_t;
using
diff erence_type = ptr
diff _t;
@@ -303,7 +320,12 @@ template <typename T, bool = (is_trivially_copy_constructible<T>::value) &&
(is_trivially_move_constructible<T>::value) &&
std::is_trivially_destructible<T>::value>
class SmallVectorTemplateBase : public SmallVectorTemplateCommon<T> {
+ friend class SmallVectorTemplateCommon<T>;
+
protected:
+ static constexpr bool TakesParamByValue = false;
+ using ValueParamT = const T &;
+
SmallVectorTemplateBase(size_t Size) : SmallVectorTemplateCommon<T>(Size) {}
static void destroy_range(T *S, T *E) {
@@ -333,20 +355,28 @@ class SmallVectorTemplateBase : public SmallVectorTemplateCommon<T> {
/// element, or MinSize more elements if specified.
void grow(size_t MinSize = 0);
+ /// Reserve enough space to add one element, and return the updated element
+ /// pointer in case it was a reference to the storage.
+ const T *reserveForAndGetAddress(const T &Elt) {
+ return this->reserveForAndGetAddressImpl(this, Elt);
+ }
+
+ /// Reserve enough space to add one element, and return the updated element
+ /// pointer in case it was a reference to the storage.
+ T *reserveForAndGetAddress(T &Elt) {
+ return const_cast<T *>(this->reserveForAndGetAddressImpl(this, Elt));
+ }
+
public:
void push_back(const T &Elt) {
- this->assertSafeToAdd(&Elt);
- if (LLVM_UNLIKELY(this->size() >= this->capacity()))
- this->grow();
- ::new ((void*) this->end()) T(Elt);
+ const T *EltPtr = reserveForAndGetAddress(Elt);
+ ::new ((void *)this->end()) T(*EltPtr);
this->set_size(this->size() + 1);
}
void push_back(T &&Elt) {
- this->assertSafeToAdd(&Elt);
- if (LLVM_UNLIKELY(this->size() >= this->capacity()))
- this->grow();
- ::new ((void*) this->end()) T(::std::move(Elt));
+ T *EltPtr = reserveForAndGetAddress(Elt);
+ ::new ((void *)this->end()) T(::std::move(*EltPtr));
this->set_size(this->size() + 1);
}
@@ -396,7 +426,18 @@ void SmallVectorTemplateBase<T, TriviallyCopyable>::grow(size_t MinSize) {
/// skipping destruction.
template <typename T>
class SmallVectorTemplateBase<T, true> : public SmallVectorTemplateCommon<T> {
+ friend class SmallVectorTemplateCommon<T>;
+
protected:
+ /// True if it's cheap enough to take parameters by value. Doing so avoids
+ /// overhead related to mitigations for reference invalidation.
+ static constexpr bool TakesParamByValue = sizeof(T) <= 2 * sizeof(void *);
+
+ /// Either const T& or T, depending on whether it's cheap enough to take
+ /// parameters by value.
+ using ValueParamT =
+ typename std::conditional<TakesParamByValue, T, const T &>::type;
+
SmallVectorTemplateBase(size_t Size) : SmallVectorTemplateCommon<T>(Size) {}
// No need to do a destroy loop for POD's.
@@ -437,12 +478,22 @@ class SmallVectorTemplateBase<T, true> : public SmallVectorTemplateCommon<T> {
/// least one more element or MinSize if specified.
void grow(size_t MinSize = 0) { this->grow_pod(MinSize, sizeof(T)); }
+ /// Reserve enough space to add one element, and return the updated element
+ /// pointer in case it was a reference to the storage.
+ const T *reserveForAndGetAddress(const T &Elt) {
+ return this->reserveForAndGetAddressImpl(this, Elt);
+ }
+
+ /// Reserve enough space to add one element, and return the updated element
+ /// pointer in case it was a reference to the storage.
+ T *reserveForAndGetAddress(T &Elt) {
+ return const_cast<T *>(this->reserveForAndGetAddressImpl(this, Elt));
+ }
+
public:
- void push_back(const T &Elt) {
- this->assertSafeToAdd(&Elt);
- if (LLVM_UNLIKELY(this->size() >= this->capacity()))
- this->grow();
- memcpy(reinterpret_cast<void *>(this->end()), &Elt, sizeof(T));
+ void push_back(ValueParamT Elt) {
+ const T *EltPtr = reserveForAndGetAddress(Elt);
+ memcpy(reinterpret_cast<void *>(this->end()), EltPtr, sizeof(T));
this->set_size(this->size() + 1);
}
@@ -462,6 +513,9 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
using size_type = typename SuperClass::size_type;
protected:
+ using SmallVectorTemplateBase<T>::TakesParamByValue;
+ using ValueParamT = typename SuperClass::ValueParamT;
+
// Default ctor - Initialize to empty.
explicit SmallVectorImpl(unsigned N)
: SmallVectorTemplateBase<T>(N) {}
@@ -628,6 +682,12 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
private:
template <class ArgType> iterator insert_one_impl(iterator I, ArgType &&Elt) {
+ // Callers ensure that ArgType is derived from T.
+ static_assert(
+ std::is_same<std::remove_const_t<std::remove_reference_t<ArgType>>,
+ T>::value,
+ "ArgType must be derived from T!");
+
if (I == this->end()) { // Important special case for empty vector.
this->push_back(::std::forward<ArgType>(Elt));
return this->end()-1;
@@ -635,14 +695,11 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
assert(this->isReferenceToStorage(I) && "Insertion iterator is out of bounds.");
- // Check that adding an element won't invalidate Elt.
- this->assertSafeToAdd(&Elt);
-
- if (this->size() >= this->capacity()) {
- size_t EltNo = I-this->begin();
- this->grow();
- I = this->begin()+EltNo;
- }
+ // Grow if necessary.
+ size_t Index = I - this->begin();
+ std::remove_reference_t<ArgType> *EltPtr =
+ this->reserveForAndGetAddress(Elt);
+ I = this->begin() + Index;
::new ((void*) this->end()) T(::std::move(this->back()));
// Push everything else over.
@@ -650,21 +707,48 @@ class SmallVectorImpl : public SmallVectorTemplateBase<T> {
this->set_size(this->size() + 1);
// If we just moved the element we're inserting, be sure to update
- // the reference.
- std::remove_reference_t<ArgType> *EltPtr = &Elt;
- if (this->isReferenceToRange(EltPtr, I, this->end()))
+ // the reference (never happens if TakesParamByValue).
+ static_assert(!TakesParamByValue || std::is_same<ArgType, T>::value,
+ "ArgType must be 'T' when taking by value!");
+ if (!TakesParamByValue && this->isReferenceToRange(EltPtr, I, this->end()))
++EltPtr;
*I = ::std::forward<ArgType>(*EltPtr);
return I;
}
+ template <
+ class ArgType,
+ std::enable_if_t<
+ std::is_same<std::remove_const_t<std::remove_reference_t<ArgType>>,
+ T>::value &&
+ !TakesParamByValue,
+ bool> = false>
+ iterator insert_one_maybe_copy(iterator I, ArgType &&Elt) {
+ return insert_one_impl(I, std::forward<ArgType>(Elt));
+ }
+
+ template <
+ class ArgType,
+ std::enable_if_t<
+ std::is_same<std::remove_const_t<std::remove_reference_t<ArgType>>,
+ T>::value &&
+ TakesParamByValue,
+ bool> = false>
+ iterator insert_one_maybe_copy(iterator I, ArgType &&Elt) {
+ // Copy Elt in order to mitigate reference invalidation without needing to
+ // update the pointer values in insert_one_impl.
+ return insert_one_impl(I, T(Elt));
+ }
+
public:
iterator insert(iterator I, T &&Elt) {
- return insert_one_impl(I, std::move(Elt));
+ return insert_one_maybe_copy(I, std::move(Elt));
}
- iterator insert(iterator I, const T &Elt) { return insert_one_impl(I, Elt); }
+ iterator insert(iterator I, const T &Elt) {
+ return insert_one_maybe_copy(I, Elt);
+ }
iterator insert(iterator I, size_type NumToInsert, const T &Elt) {
// Convert iterator to elt# to avoid invalidating iterator when we reserve()
diff --git a/llvm/unittests/ADT/SmallVectorTest.cpp b/llvm/unittests/ADT/SmallVectorTest.cpp
index d97ab577524f..c880a6b6c543 100644
--- a/llvm/unittests/ADT/SmallVectorTest.cpp
+++ b/llvm/unittests/ADT/SmallVectorTest.cpp
@@ -53,6 +53,7 @@ class Constructable {
Constructable(Constructable && src) : constructed(true) {
value = src.value;
+ src.value = 0;
++numConstructorCalls;
++numMoveConstructorCalls;
}
@@ -74,6 +75,7 @@ class Constructable {
Constructable & operator=(Constructable && src) {
EXPECT_TRUE(constructed);
value = src.value;
+ src.value = 0;
++numAssignmentCalls;
++numMoveAssignmentCalls;
return *this;
@@ -1056,11 +1058,16 @@ class SmallVectorReferenceInvalidationTest : public SmallVectorTestBase {
return N;
}
+ template <class T> static bool isValueType() {
+ return std::is_same<T, typename VectorT::value_type>::value;
+ }
+
void SetUp() override {
SmallVectorTestBase::SetUp();
// Fill up the small size so that insertions move the elements.
- V.append({0, 0, 0});
+ for (int I = 0, E = NumBuiltinElts(V); I != E; ++I)
+ V.emplace_back(I + 1);
}
};
@@ -1074,19 +1081,54 @@ TYPED_TEST_CASE(SmallVectorReferenceInvalidationTest,
SmallVectorReferenceInvalidationTestTypes);
TYPED_TEST(SmallVectorReferenceInvalidationTest, PushBack) {
+ // Note: setup adds [1, 2, ...] to V until it's at capacity in small mode.
auto &V = this->V;
- (void)V;
-#if !defined(NDEBUG) && GTEST_HAS_DEATH_TEST
- EXPECT_DEATH(V.push_back(V.back()), this->AssertionMessage);
-#endif
+ int N = this->NumBuiltinElts(V);
+
+ // Push back a reference to last element when growing from small storage.
+ V.push_back(V.back());
+ EXPECT_EQ(N, V.back());
+
+ // Check that the old value is still there (not moved away).
+ EXPECT_EQ(N, V[V.size() - 2]);
+
+ // Fill storage again.
+ V.back() = V.size();
+ while (V.size() < V.capacity())
+ V.push_back(V.size() + 1);
+
+ // Push back a reference to last element when growing from large storage.
+ V.push_back(V.back());
+ EXPECT_EQ(int(V.size()) - 1, V.back());
}
TYPED_TEST(SmallVectorReferenceInvalidationTest, PushBackMoved) {
+ // Note: setup adds [1, 2, ...] to V until it's at capacity in small mode.
auto &V = this->V;
- (void)V;
-#if !defined(NDEBUG) && GTEST_HAS_DEATH_TEST
- EXPECT_DEATH(V.push_back(std::move(V.back())), this->AssertionMessage);
-#endif
+ int N = this->NumBuiltinElts(V);
+
+ // Push back a reference to last element when growing from small storage.
+ V.push_back(std::move(V.back()));
+ EXPECT_EQ(N, V.back());
+ if (this->template isValueType<Constructable>()) {
+ // Check that the value was moved (not copied).
+ EXPECT_EQ(0, V[V.size() - 2]);
+ }
+
+ // Fill storage again.
+ V.back() = V.size();
+ while (V.size() < V.capacity())
+ V.push_back(V.size() + 1);
+
+ // Push back a reference to last element when growing from large storage.
+ V.push_back(std::move(V.back()));
+
+ // Check the values.
+ EXPECT_EQ(int(V.size()) - 1, V.back());
+ if (this->template isValueType<Constructable>()) {
+ // Check the value got moved out.
+ EXPECT_EQ(0, V[V.size() - 2]);
+ }
}
TYPED_TEST(SmallVectorReferenceInvalidationTest, Resize) {
@@ -1150,20 +1192,53 @@ TYPED_TEST(SmallVectorReferenceInvalidationTest, AssignRange) {
}
TYPED_TEST(SmallVectorReferenceInvalidationTest, Insert) {
+ // Note: setup adds [1, 2, ...] to V until it's at capacity in small mode.
auto &V = this->V;
(void)V;
-#if !defined(NDEBUG) && GTEST_HAS_DEATH_TEST
- EXPECT_DEATH(V.insert(V.begin(), V.back()), this->AssertionMessage);
-#endif
+
+ // Insert a reference to the back (not at end() or else insert delegates to
+ // push_back()), growing out of small mode. Confirm the value was copied out
+ // (moving out Constructable sets it to 0).
+ V.insert(V.begin(), V.back());
+ EXPECT_EQ(int(V.size() - 1), V.front());
+ EXPECT_EQ(int(V.size() - 1), V.back());
+
+ // Fill up the vector again.
+ while (V.size() < V.capacity())
+ V.push_back(V.size() + 1);
+
+ // Grow again from large storage to large storage.
+ V.insert(V.begin(), V.back());
+ EXPECT_EQ(int(V.size() - 1), V.front());
+ EXPECT_EQ(int(V.size() - 1), V.back());
}
TYPED_TEST(SmallVectorReferenceInvalidationTest, InsertMoved) {
+ // Note: setup adds [1, 2, ...] to V until it's at capacity in small mode.
auto &V = this->V;
(void)V;
-#if !defined(NDEBUG) && GTEST_HAS_DEATH_TEST
- EXPECT_DEATH(V.insert(V.begin(), std::move(V.back())),
- this->AssertionMessage);
-#endif
+
+ // Insert a reference to the back (not at end() or else insert delegates to
+ // push_back()), growing out of small mode. Confirm the value was copied out
+ // (moving out Constructable sets it to 0).
+ V.insert(V.begin(), std::move(V.back()));
+ EXPECT_EQ(int(V.size() - 1), V.front());
+ if (this->template isValueType<Constructable>()) {
+ // Check the value got moved out.
+ EXPECT_EQ(0, V.back());
+ }
+
+ // Fill up the vector again.
+ while (V.size() < V.capacity())
+ V.push_back(V.size() + 1);
+
+ // Grow again from large storage to large storage.
+ V.insert(V.begin(), std::move(V.back()));
+ EXPECT_EQ(int(V.size() - 1), V.front());
+ if (this->template isValueType<Constructable>()) {
+ // Check the value got moved out.
+ EXPECT_EQ(0, V.back());
+ }
}
TYPED_TEST(SmallVectorReferenceInvalidationTest, InsertN) {
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