libstdc++: fix memory clobbering in std::vector [PR110879]

Fix ordering to prevent clobbering of class members by a call to deallocate
in _M_realloc_insert and _M_default_append.

Because of recent changes in _M_realloc_insert and _M_default_append,
calls to deallocate were ordered after assignment to class members of
std::vector (in the guard destructor), which is causing said members to
be call-clobbered.  This is preventing further optimization, the
compiler is unable to move memory read out of a hot loop in this case.

This patch reorders the call to before assignments by putting guard in
its own block. Plus a new testsuite for this case.  I'm not very happy
with the new testsuite, but I don't know how to properly test this.

	PR libstdc++/110879

libstdc++-v3/ChangeLog:

	* include/bits/vector.tcc (_M_realloc_insert): End guard
	lifetime just before assignment to class members.
	(_M_default_append): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/pr110879.C: New test.

Signed-off-by: Vladimir Palevich  <palevichva@gmail.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/pr110879.C b/gcc/testsuite/g++.dg/pr110879.C
new file mode 100644
index 0000000..7f0a0a8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr110879.C
@@ -0,0 +1,16 @@
+// { dg-do compile }
+// { dg-options "-O3 -fdump-tree-optimized" }
+
+#include <vector>
+
+std::vector<int> f(std::size_t n) {
+  std::vector<int> res;
+  for (std::size_t i = 0; i < n; ++i) {
+    res.push_back(i);
+  }
+  return res;
+}
+
+// Reads of _M_finish should be optimized out.
+// This regex matches all reads from res variable except for _M_end_of_storage field.
+// { dg-final { scan-tree-dump-not "=\\s*\\S*res_(?!\\S*_M_end_of_storage;)" "optimized" } }