[libcxx] Improve code generation for vector::clear().

waywardmonkeys · waywardmonkeys · commit 50910bf842ca · 2017-03-23T14:39:23.000Z
Summary: 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. Subscribers: cfe-commits Differential Revision: https://reviews.llvm.org/D25241 llvm-svn: 298601
diff --git a/libcxx/include/vector b/libcxx/include/vector
@@ -413,8 +413,10 @@ inline _LIBCPP_INLINE_VISIBILITY
 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>
diff --git a/libcxx/test/std/containers/sequences/vector/vector.modifiers/clear.pass.cpp b/libcxx/test/std/containers/sequences/vector/vector.modifiers/clear.pass.cpp
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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"
+#include "asan_testing.h"
+
+int main()
+{
+    {
+    int a[] = {1, 2, 3};
+    std::vector<int> c(a, a+3);
+    c.clear();
+    assert(c.empty());
+    LIBCPP_ASSERT(c.__invariants());
+    LIBCPP_ASSERT(is_contiguous_container_asan_correct(c));
+    }
+#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());
+    LIBCPP_ASSERT(c.__invariants());
+    LIBCPP_ASSERT(is_contiguous_container_asan_correct(c));
+    }
+#endif
+}

Original file line number	Diff line number	Diff line change
`@@ -413,8 +413,10 @@ inline _LIBCPP_INLINE_VISIBILITY`
`413`	`413`	`void`
`414`	`414`	`__vector_base<_Tp, _Allocator>::__destruct_at_end(pointer __new_last) _NOEXCEPT`
`415`	`415`	`{`
`416`		`- while (__new_last != __end_)`
`417`		`- __alloc_traits::destroy(__alloc(), _VSTD::__to_raw_pointer(--__end_));`
	`416`	`+ pointer __soon_to_be_end = __end_;`
	`417`	`+ while (__new_last != __soon_to_be_end)`
	`418`	`+ __alloc_traits::destroy(__alloc(), _VSTD::__to_raw_pointer(--__soon_to_be_end));`
	`419`	`+ __end_ = __new_last;`
`418`	`420`	`}`
`419`	`421`
`420`	`422`	`template <class _Tp, class _Allocator>`