[libcxx-commits] [libcxx] [libc++] `std::ranges::advance`: avoid unneeded bounds checks when advancing iterator (PR #84126)
Jan Kokemüller via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Mar 29 02:54:55 PDT 2024
https://github.com/jiixyj updated https://github.com/llvm/llvm-project/pull/84126
>From dc039616cee1453a38729a4f2f6adc03fd93549c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller at gmail.com>
Date: Wed, 6 Mar 2024 07:49:25 +0100
Subject: [PATCH 1/6] skip unneeded checks against '__bound_sentinel' when
advancing iterator
---
libcxx/include/__iterator/advance.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/libcxx/include/__iterator/advance.h b/libcxx/include/__iterator/advance.h
index 7959bdeae32643..296db1aaab6526 100644
--- a/libcxx/include/__iterator/advance.h
+++ b/libcxx/include/__iterator/advance.h
@@ -170,14 +170,14 @@ struct __fn {
} else {
// Otherwise, if `n` is non-negative, while `bool(i != bound_sentinel)` is true, increments `i` but at
// most `n` times.
- while (__i != __bound_sentinel && __n > 0) {
+ while (__n > 0 && __i != __bound_sentinel) {
++__i;
--__n;
}
// Otherwise, while `bool(i != bound_sentinel)` is true, decrements `i` but at most `-n` times.
if constexpr (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>) {
- while (__i != __bound_sentinel && __n < 0) {
+ while (__n < 0 && __i != __bound_sentinel) {
--__i;
++__n;
}
>From b23e7fe87d6eaa65cf684749e2eb1ceee51176d0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller at gmail.com>
Date: Sat, 9 Mar 2024 11:22:59 +0100
Subject: [PATCH 2/6] add tests
---
.../iterator_count_sentinel.pass.cpp | 22 ++++++++++++++++++
libcxx/test/support/test_iterators.h | 23 +++++++++++++++++--
2 files changed, 43 insertions(+), 2 deletions(-)
diff --git a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
index a1c15640182162..72584e60d9f89f 100644
--- a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
@@ -42,6 +42,13 @@ constexpr void check_forward(int* first, int* last, std::iter_difference_t<It> n
// regardless of the iterator category.
assert(it.stride_count() == M);
assert(it.stride_displacement() == M);
+ if (n == 0) {
+ assert(it.equals_count() == 0);
+ } else {
+ assert(it.equals_count() > 0);
+ assert(it.equals_count() == M || it.equals_count() == M + 1);
+ assert(it.equals_count() <= n);
+ }
}
}
@@ -95,6 +102,7 @@ constexpr void check_backward(int* first, int* last, std::iter_difference_t<It>
(void)std::ranges::advance(it, n, sent);
assert(it.stride_count() <= 1);
assert(it.stride_displacement() <= 1);
+ assert(it.equals_count() == 0);
}
}
@@ -213,6 +221,20 @@ constexpr bool test() {
assert(i == iota_iterator{INT_MIN+1});
}
+ // Check that we don't do an unneeded bounds check when decrementing a
+ // `bidirectional_iterator` that doesn't model `sized_sentinel_for`.
+ {
+ static_assert(std::bidirectional_iterator<bidirectional_iterator<iota_iterator>>);
+ static_assert(!std::sized_sentinel_for<bidirectional_iterator<iota_iterator>,
+ bidirectional_iterator<iota_iterator>>);
+
+ auto it = stride_counting_iterator(bidirectional_iterator(iota_iterator{+1}));
+ auto sent = stride_counting_iterator(bidirectional_iterator(iota_iterator{-2}));
+ assert(std::ranges::advance(it, -3, sent) == 0);
+ assert(base(base(it)) == iota_iterator{-2});
+ assert(it.equals_count() == 3);
+ }
+
return true;
}
diff --git a/libcxx/test/support/test_iterators.h b/libcxx/test/support/test_iterators.h
index c92ce375348ff4..e551ab5a62d08b 100644
--- a/libcxx/test/support/test_iterators.h
+++ b/libcxx/test/support/test_iterators.h
@@ -725,11 +725,14 @@ struct common_input_iterator {
# endif // TEST_STD_VER >= 20
// Iterator adaptor that counts the number of times the iterator has had a successor/predecessor
-// operation called. Has two recorders:
+// operation or an equality comparison operation called. Has three recorders:
// * `stride_count`, which records the total number of calls to an op++, op--, op+=, or op-=.
// * `stride_displacement`, which records the displacement of the calls. This means that both
// op++/op+= will increase the displacement counter by 1, and op--/op-= will decrease the
// displacement counter by 1.
+// * `equals_count`, which records the total number of calls to an op== or op!=. If compared
+// against a sentinel object, that sentinel object must call the `record_equality_comparison`
+// function so that the comparison is counted correctly.
template <class It>
class stride_counting_iterator {
public:
@@ -754,6 +757,8 @@ class stride_counting_iterator {
constexpr difference_type stride_displacement() const { return stride_displacement_; }
+ constexpr difference_type equals_count() const { return equals_count_; }
+
constexpr decltype(auto) operator*() const { return *It(base_); }
constexpr decltype(auto) operator[](difference_type n) const { return It(base_)[n]; }
@@ -838,9 +843,15 @@ class stride_counting_iterator {
return base(x) - base(y);
}
+ constexpr void record_equality_comparison() const
+ {
+ ++equals_count_;
+ }
+
constexpr bool operator==(stride_counting_iterator const& other) const
requires std::sentinel_for<It, It>
{
+ record_equality_comparison();
return It(base_) == It(other.base_);
}
@@ -875,6 +886,7 @@ class stride_counting_iterator {
decltype(base(std::declval<It>())) base_;
difference_type stride_count_ = 0;
difference_type stride_displacement_ = 0;
+ mutable difference_type equals_count_ = 0;
};
template <class It>
stride_counting_iterator(It) -> stride_counting_iterator<It>;
@@ -887,7 +899,14 @@ class sentinel_wrapper {
public:
explicit sentinel_wrapper() = default;
constexpr explicit sentinel_wrapper(const It& it) : base_(base(it)) {}
- constexpr bool operator==(const It& other) const { return base_ == base(other); }
+ constexpr bool operator==(const It& other) const {
+ // If supported, record statistics about the equality operator call
+ // inside `other`.
+ if constexpr (requires { other.record_equality_comparison(); }) {
+ other.record_equality_comparison();
+ }
+ return base_ == base(other);
+ }
friend constexpr It base(const sentinel_wrapper& s) { return It(s.base_); }
private:
decltype(base(std::declval<It>())) base_;
>From 4051b3f569e6f70cb55755537123cdd67ddcd2c4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller at gmail.com>
Date: Sat, 9 Mar 2024 11:27:49 +0100
Subject: [PATCH 3/6] apply clang-format
---
.../iterator_count_sentinel.pass.cpp | 6 +++---
libcxx/test/support/test_iterators.h | 9 +++------
2 files changed, 6 insertions(+), 9 deletions(-)
diff --git a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
index 72584e60d9f89f..5dffb5267c391b 100644
--- a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
@@ -225,10 +225,10 @@ constexpr bool test() {
// `bidirectional_iterator` that doesn't model `sized_sentinel_for`.
{
static_assert(std::bidirectional_iterator<bidirectional_iterator<iota_iterator>>);
- static_assert(!std::sized_sentinel_for<bidirectional_iterator<iota_iterator>,
- bidirectional_iterator<iota_iterator>>);
+ static_assert(
+ !std::sized_sentinel_for<bidirectional_iterator<iota_iterator>, bidirectional_iterator<iota_iterator>>);
- auto it = stride_counting_iterator(bidirectional_iterator(iota_iterator{+1}));
+ auto it = stride_counting_iterator(bidirectional_iterator(iota_iterator{+1}));
auto sent = stride_counting_iterator(bidirectional_iterator(iota_iterator{-2}));
assert(std::ranges::advance(it, -3, sent) == 0);
assert(base(base(it)) == iota_iterator{-2});
diff --git a/libcxx/test/support/test_iterators.h b/libcxx/test/support/test_iterators.h
index e551ab5a62d08b..7ffb74990fa4dd 100644
--- a/libcxx/test/support/test_iterators.h
+++ b/libcxx/test/support/test_iterators.h
@@ -843,16 +843,13 @@ class stride_counting_iterator {
return base(x) - base(y);
}
- constexpr void record_equality_comparison() const
- {
- ++equals_count_;
- }
+ constexpr void record_equality_comparison() const { ++equals_count_; }
constexpr bool operator==(stride_counting_iterator const& other) const
requires std::sentinel_for<It, It>
{
- record_equality_comparison();
- return It(base_) == It(other.base_);
+ record_equality_comparison();
+ return It(base_) == It(other.base_);
}
friend constexpr bool operator<(stride_counting_iterator const& x, stride_counting_iterator const& y)
>From c41370a113a718f11256ca57d9dd62d4835e46f7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller at gmail.com>
Date: Tue, 19 Mar 2024 19:32:55 +0100
Subject: [PATCH 4/6] remove unneeded check
---
.../range.iter.ops.advance/iterator_count_sentinel.pass.cpp | 1 -
1 file changed, 1 deletion(-)
diff --git a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
index 5dffb5267c391b..843dcc39d944ca 100644
--- a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
@@ -45,7 +45,6 @@ constexpr void check_forward(int* first, int* last, std::iter_difference_t<It> n
if (n == 0) {
assert(it.equals_count() == 0);
} else {
- assert(it.equals_count() > 0);
assert(it.equals_count() == M || it.equals_count() == M + 1);
assert(it.equals_count() <= n);
}
>From a4da00177fadef0acdea54aabe8a5c2c8e558187 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller at gmail.com>
Date: Fri, 29 Mar 2024 09:58:02 +0100
Subject: [PATCH 5/6] test for the extra bounds check iff 'n > M'
---
.../iterator_count_sentinel.pass.cpp | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
index 843dcc39d944ca..44be579001be13 100644
--- a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
@@ -45,7 +45,13 @@ constexpr void check_forward(int* first, int* last, std::iter_difference_t<It> n
if (n == 0) {
assert(it.equals_count() == 0);
} else {
- assert(it.equals_count() == M || it.equals_count() == M + 1);
+ if (n > M) {
+ // We "hit" the bound, so there is one extra equality check.
+ assert(it.equals_count() == M + 1);
+ } else {
+ assert(it.equals_count() == M);
+ }
+ // In any case, there must not be more than `n` bounds checks.
assert(it.equals_count() <= n);
}
}
>From 7f8ba1f01885567de8e4e6c82f3ca341ee698914 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20Kokem=C3=BCller?= <jan.kokemueller at gmail.com>
Date: Fri, 29 Mar 2024 10:54:06 +0100
Subject: [PATCH 6/6] adapt 'check_backward' test so that it can test
bidirectional iterators as well
---
.../iterator_count_sentinel.pass.cpp | 59 ++++++++++++-------
1 file changed, 37 insertions(+), 22 deletions(-)
diff --git a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
index 44be579001be13..c3f378d3af1a30 100644
--- a/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
+++ b/libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.advance/iterator_count_sentinel.pass.cpp
@@ -88,7 +88,11 @@ constexpr void check_forward_sized_sentinel(int* first, int* last, std::iter_dif
template <typename It>
constexpr void check_backward(int* first, int* last, std::iter_difference_t<It> n, int* expected) {
- static_assert(std::random_access_iterator<It>, "This test doesn't support non random access iterators");
+ // Check preconditions for `advance` when called with negative `n`:
+ // <https://eel.is/c++draft/iterators#range.iter.op.advance-5>
+ assert(n < 0);
+ static_assert(std::bidirectional_iterator<It>);
+
using Difference = std::iter_difference_t<It>;
Difference const M = (expected - last); // expected travel distance (which is negative)
@@ -104,10 +108,34 @@ constexpr void check_backward(int* first, int* last, std::iter_difference_t<It>
{
auto it = stride_counting_iterator(It(last));
auto sent = stride_counting_iterator(It(first));
+ static_assert(std::bidirectional_iterator<stride_counting_iterator<It>>);
+
(void)std::ranges::advance(it, n, sent);
- assert(it.stride_count() <= 1);
- assert(it.stride_displacement() <= 1);
- assert(it.equals_count() == 0);
+
+ if constexpr (std::sized_sentinel_for<It, It>) {
+ if (expected == first) {
+ // In this case, the algorithm can just do `it = std::move(sent);`
+ // instead of doing iterator arithmetic:
+ // <https://eel.is/c++draft/iterators#range.iter.op.advance-4.1>
+ assert(it.stride_count() == 0);
+ assert(it.stride_displacement() == 0);
+ } else {
+ assert(it.stride_count() == 1);
+ assert(it.stride_displacement() == 1);
+ }
+ assert(it.equals_count() == 0);
+ } else {
+ assert(it.stride_count() == -M);
+ assert(it.stride_displacement() == M);
+ if (-n > -M) {
+ // We "hit" the bound, so there is one extra equality check.
+ assert(it.equals_count() == -M + 1);
+ } else {
+ assert(it.equals_count() == -M);
+ }
+ // In any case, there must not be more than `-n` bounds checks.
+ assert(it.equals_count() <= -n);
+ }
}
}
@@ -201,11 +229,12 @@ constexpr bool test() {
check_forward_sized_sentinel<int*>( range, range+size, n, expected);
}
- {
- // Note that we can only test ranges::advance with a negative n for iterators that
- // are sized sentinels for themselves, because ranges::advance is UB otherwise.
- // In particular, that excludes bidirectional_iterators since those are not sized sentinels.
+ // Exclude the `n == 0` case for the backwards checks.
+ // Input and forward iterators are not tested as the backwards case does
+ // not apply for them.
+ if (n > 0) {
int* expected = n > size ? range : range + size - n;
+ check_backward<bidirectional_iterator<int*>>(range, range+size, -n, expected);
check_backward<random_access_iterator<int*>>(range, range+size, -n, expected);
check_backward<contiguous_iterator<int*>>( range, range+size, -n, expected);
check_backward<int*>( range, range+size, -n, expected);
@@ -226,20 +255,6 @@ constexpr bool test() {
assert(i == iota_iterator{INT_MIN+1});
}
- // Check that we don't do an unneeded bounds check when decrementing a
- // `bidirectional_iterator` that doesn't model `sized_sentinel_for`.
- {
- static_assert(std::bidirectional_iterator<bidirectional_iterator<iota_iterator>>);
- static_assert(
- !std::sized_sentinel_for<bidirectional_iterator<iota_iterator>, bidirectional_iterator<iota_iterator>>);
-
- auto it = stride_counting_iterator(bidirectional_iterator(iota_iterator{+1}));
- auto sent = stride_counting_iterator(bidirectional_iterator(iota_iterator{-2}));
- assert(std::ranges::advance(it, -3, sent) == 0);
- assert(base(base(it)) == iota_iterator{-2});
- assert(it.equals_count() == 3);
- }
-
return true;
}
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