To help along cases where begin and end are passed on to another function
and then uses std::distance on them. In the case of the testcases added,
std::lexicographical_compare_three_way (for C++20). We now able to opimize
`vector < known empty vector` to a constant where before we would need to load
begin() and end() and do a comparison of them.
Note for C++20+ at -O1 we still don't optimize it to a constant but that is
due to not doing copy propagation for aggregates.
v2 change: Use __OPTIMIZE__ as requested.
Bootstrapped and tested on x86_64-linux-gnu.
PR libstdc++/111499
libstdc++-v3/ChangeLog:
* include/bits/stl_vector.h (vector::begin(), vector::end(),
vector::cbegin(), vector::cend()): Add __builtin_unreachable call to
announce that _M_finish is greater than or equal to _M_start.
gcc/testsuite/ChangeLog:
* g++.dg/tree-ssa/pr111499-1.C: New test.
* g++.dg/tree-ssa/pr111499.C: New test.
Signed-off-by: Andrew Pinski <quic_apin...@quicinc.com>
---
gcc/testsuite/g++.dg/tree-ssa/pr111499-1.C | 16 ++++++++
gcc/testsuite/g++.dg/tree-ssa/pr111499.C | 15 +++++++
libstdc++-v3/include/bits/stl_vector.h | 48 +++++++++++++++++++---
3 files changed, 73 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/tree-ssa/pr111499-1.C
create mode 100644 gcc/testsuite/g++.dg/tree-ssa/pr111499.C
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr111499-1.C
b/gcc/testsuite/g++.dg/tree-ssa/pr111499-1.C
new file mode 100644
index 00000000000..737f6673301
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr111499-1.C
@@ -0,0 +1,16 @@
+// { dg-do compile }
+// { dg-options "-O1 -fdump-tree-optimized -Wextra -Wall" }
+#include <vector>
+
+bool f(const std::vector<unsigned char> &v)
+{
+ std::vector<unsigned char> w;
+ return v < w;
+}
+// This should compile to empty function; check that no size of
+// vector is determined and there is no allocation
+// For C++20+, at -O1 there is a missed optimization dealing with copy
propagation for aggregates
+// { dg-final { scan-tree-dump-not "_M_start" "optimized" { xfail c++20 } } }
+// { dg-final { scan-tree-dump-not "delete" "optimized" } }
+// { dg-final { scan-tree-dump-not " = MEM" "optimized" } }
+// { dg-final { scan-tree-dump "return 0" "optimized" { xfail c++20 } } }
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr111499.C
b/gcc/testsuite/g++.dg/tree-ssa/pr111499.C
new file mode 100644
index 00000000000..faa38299d4d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr111499.C
@@ -0,0 +1,15 @@
+// { dg-do compile }
+// { dg-options "-O2 -fdump-tree-optimized -Wextra -Wall" }
+#include <vector>
+
+bool f(const std::vector<unsigned char> &v)
+{
+ std::vector<unsigned char> w;
+ return v < w;
+}
+// This should compile to empty function; check that no size of
+// vector is determined and there is no allocation
+// { dg-final { scan-tree-dump-not "_M_start" "optimized" } }
+// { dg-final { scan-tree-dump-not "delete" "optimized" } }
+// { dg-final { scan-tree-dump-not " = MEM" "optimized" } }
+// { dg-final { scan-tree-dump "return 0" "optimized" } }
diff --git a/libstdc++-v3/include/bits/stl_vector.h
b/libstdc++-v3/include/bits/stl_vector.h
index 458adc987da..a4b1d1bda79 100644
--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -996,7 +996,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
iterator
begin() _GLIBCXX_NOEXCEPT
- { return iterator(this->_M_impl._M_start); }
+ {
+#ifdef __OPTIMIZE__
+ if (this->_M_impl._M_start > this->_M_impl._M_finish)
+ __builtin_unreachable();
+#endif
+ return iterator(this->_M_impl._M_start);
+ }
/**
* Returns a read-only (constant) iterator that points to the
@@ -1006,7 +1012,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
const_iterator
begin() const _GLIBCXX_NOEXCEPT
- { return const_iterator(this->_M_impl._M_start); }
+ {
+#ifdef __OPTIMIZE__
+ if (this->_M_impl._M_start > this->_M_impl._M_finish)
+ __builtin_unreachable();
+#endif
+ return const_iterator(this->_M_impl._M_start);
+ }
/**
* Returns a read/write iterator that points one past the last
@@ -1016,7 +1028,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
iterator
end() _GLIBCXX_NOEXCEPT
- { return iterator(this->_M_impl._M_finish); }
+ {
+#ifdef __OPTIMIZE__
+ if (this->_M_impl._M_start > this->_M_impl._M_finish)
+ __builtin_unreachable();
+#endif
+ return iterator(this->_M_impl._M_finish);
+ }
/**
* Returns a read-only (constant) iterator that points one past
@@ -1026,7 +1044,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
_GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
const_iterator
end() const _GLIBCXX_NOEXCEPT
- { return const_iterator(this->_M_impl._M_finish); }
+ {
+#ifdef __OPTIMIZE__
+ if (this->_M_impl._M_start > this->_M_impl._M_finish)
+ __builtin_unreachable();
+#endif
+ return const_iterator(this->_M_impl._M_finish);
+ }
/**
* Returns a read/write reverse iterator that points to the
@@ -1077,7 +1101,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
[[__nodiscard__]] _GLIBCXX20_CONSTEXPR
const_iterator
cbegin() const noexcept
- { return const_iterator(this->_M_impl._M_start); }
+ {
+#ifdef __OPTIMIZE__
+ if (this->_M_impl._M_start > this->_M_impl._M_finish)
+ __builtin_unreachable();
+#endif
+ return const_iterator(this->_M_impl._M_start);
+ }
/**
* Returns a read-only (constant) iterator that points one past
@@ -1087,7 +1117,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
[[__nodiscard__]] _GLIBCXX20_CONSTEXPR
const_iterator
cend() const noexcept
- { return const_iterator(this->_M_impl._M_finish); }
+ {
+#ifdef __OPTIMIZE__
+ if (this->_M_impl._M_start > this->_M_impl._M_finish)
+ __builtin_unreachable();
+#endif
+ return const_iterator(this->_M_impl._M_finish);
+ }
/**
* Returns a read-only (constant) reverse iterator that points
--
2.43.0
