[PATCH] D25241: [libcxx] Improve code generation for vector::clear().

[Bruce Mitchener via cfe-commits]

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>


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