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

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

1 files changed, 40 insertions, 0 deletions

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
new file mode 100644
index 00000000000..5f053eb8565
--- /dev/null
+++ 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
+}