[libstdc++-v3] Add __builtion_unreachable to vector::size(), vector::capacity()

[Jan Hubicka]

Hi,
This patch makes it clear that vector sizes and capacities are not
negative.  With recent change to ipa-fnsummary this should not affect
inlining and improves codegen of some vector manipulation functions.

I tested clang build.  Looking for throw_bad calls there are only 3
called considerably often (bad_allloc, bad_array_new_length and
function_callv).
The patch seems to reduce bad_alloc and bad_array_new_length calls
considerably:

bad_alloc 380->147
bad_array_new_length 832->128

Bootstrapped/regtested x86_64-linux and approved by Jonathan.

libstdc++-v3/ChangeLog:

        PR tree-optimization/109442
        * include/bits/stl_vector.h: (vector::size(),
        vector::capacity()): Add __builtin_unreachable call to announce
        that size and capacity are non-negative.

gcc/testsuite/ChangeLog:

        PR tree-optimization/109442
        * g++.dg/tree-ssa/pr109442.C: New test.

diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr109442.C 
b/gcc/testsuite/g++.dg/tree-ssa/pr109442.C
new file mode 100644
index 00000000000..ec40c470c8d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr109442.C
@@ -0,0 +1,12 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-O1 -fdump-tree-optimized" }
+#include <vector>
+#define T int
+T vat1(std::vector<T> v1) {
+    auto v = v1;
+    return 10;
+}
+// This should compile to empty function; check that no size of
+// vector is determined and there is no allocation
+// { dg-final { scan-tree-dump-not "_M_start" "optimized" } }
+// { dg-final { scan-tree-dump-not "delete" "optimized" } }
diff --git a/libstdc++-v3/include/bits/stl_vector.h 
b/libstdc++-v3/include/bits/stl_vector.h
index df48ba3377f..3ece16b2fac 100644
--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -1114,7 +1114,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
       size_type
       size() const _GLIBCXX_NOEXCEPT
-      { return size_type(this->_M_impl._M_finish - this->_M_impl._M_start); }
+      {
+       ptrdiff_t __dif = this->_M_impl._M_finish - this->_M_impl._M_start;
+       if (__dif < 0)
+          __builtin_unreachable ();
+       return size_type(__dif);
+      }

       /**  Returns the size() of the largest possible %vector.  */
       _GLIBCXX_NODISCARD _GLIBCXX20_CONSTEXPR
@@ -1201,8 +1206,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       size_type
       capacity() const _GLIBCXX_NOEXCEPT
       {
-       return size_type(this->_M_impl._M_end_of_storage
-                          - this->_M_impl._M_start);
+       ptrdiff_t __dif = this->_M_impl._M_end_of_storage
+                         - this->_M_impl._M_start;
+       if (__dif < 0)
+          __builtin_unreachable ();
+       return size_type(__dif);
       }

       /**