brucem created this revision.
brucem added a subscriber: cfe-commits.
By manipulating a local variable in the loop, when the loop can
be optimized away (due to no non-trivial destructors), this lets
it be fully optimized away and we modify the __end_ separately.
This results in a substantial improvement in the generated code.
Prior to this change, this would be generated (on x86_64):
movq (%rdi), %rdx
movq 8(%rdi), %rcx
cmpq %rdx, %rcx
je LBB2_2
leaq -12(%rcx), %rax
subq %rdx, %rax
movabsq $-6148914691236517205, %rdx ## imm = 0xAAAAAAAAAAAAAAAB
mulq %rdx
shrq $3, %rdx
notq %rdx
leaq (%rdx,%rdx,2), %rax
leaq (%rcx,%rax,4), %rax
movq %rax, 8(%rdi)
And after:
movq (%rdi), %rax
movq %rax, 8(%rdi)
This brings this in line with what other implementations do.
https://reviews.llvm.org/D25241
Files:
include/vector
test/std/containers/sequences/vector/vector.modifiers/clear.pass.cpp
Index: test/std/containers/sequences/vector/vector.modifiers/clear.pass.cpp
===================================================================
--- /dev/null
+++ test/std/containers/sequences/vector/vector.modifiers/clear.pass.cpp
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// void clear();
+
+#include <vector>
+#include <cassert>
+
+#include "min_allocator.h"
+
+int main()
+{
+ {
+ int a[] = {1, 2, 3};
+ std::vector<int> c(a, a+3);
+ c.clear();
+ assert(c.empty());
+ }
+#if TEST_STD_VER >= 11
+ {
+ int a[] = {1, 2, 3};
+ std::vector<int, min_allocator<int>> c(a, a+3);
+ c.clear();
+ assert(c.empty());
+ }
+#endif
+}
Index: include/vector
===================================================================
--- include/vector
+++ include/vector
@@ -413,8 +413,10 @@
void
__vector_base<_Tp, _Allocator>::__destruct_at_end(pointer __new_last) _NOEXCEPT
{
- while (__new_last != __end_)
- __alloc_traits::destroy(__alloc(), _VSTD::__to_raw_pointer(--__end_));
+ pointer __soon_to_be_end = __end_;
+ while (__new_last != __soon_to_be_end)
+ __alloc_traits::destroy(__alloc(),
_VSTD::__to_raw_pointer(--__soon_to_be_end));
+ __end_ = __new_last;
}
template <class _Tp, class _Allocator>
Index: test/std/containers/sequences/vector/vector.modifiers/clear.pass.cpp
===================================================================
--- /dev/null
+++ test/std/containers/sequences/vector/vector.modifiers/clear.pass.cpp
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// void clear();
+
+#include <vector>
+#include <cassert>
+
+#include "min_allocator.h"
+
+int main()
+{
+ {
+ int a[] = {1, 2, 3};
+ std::vector<int> c(a, a+3);
+ c.clear();
+ assert(c.empty());
+ }
+#if TEST_STD_VER >= 11
+ {
+ int a[] = {1, 2, 3};
+ std::vector<int, min_allocator<int>> c(a, a+3);
+ c.clear();
+ assert(c.empty());
+ }
+#endif
+}
Index: include/vector
===================================================================
--- include/vector
+++ include/vector
@@ -413,8 +413,10 @@
void
__vector_base<_Tp, _Allocator>::__destruct_at_end(pointer __new_last) _NOEXCEPT
{
- while (__new_last != __end_)
- __alloc_traits::destroy(__alloc(), _VSTD::__to_raw_pointer(--__end_));
+ pointer __soon_to_be_end = __end_;
+ while (__new_last != __soon_to_be_end)
+ __alloc_traits::destroy(__alloc(), _VSTD::__to_raw_pointer(--__soon_to_be_end));
+ __end_ = __new_last;
}
template <class _Tp, class _Allocator>
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