libstdc++: Teach optimizer that empty COW strings are empty [PR107087]

The compiler doesn't know about the invariant that the _S_empty_rep()
object is immutable and so _M_length and _M_refcount are always zero.
This means that we get warnings about writing possibly-non-zero length
strings into buffers that can't hold them. If we teach the compiler that
the empty rep is always zero length, it knows it can be copied into any
buffer.

For Stage 1 we might want to also consider adding this to capacity():

	if (_S_empty_rep()._M_capacity != 0)
	  __builtin_unreachable();

And this to _Rep::_M_is_leaked() and _Rep::_M_is_shared():

	  if (_S_empty_rep()._M_refcount != 0)
	    __builtin_unreachable();

libstdc++-v3/ChangeLog:

	PR tree-optimization/107087
	* include/bits/cow_string.h (basic_string::size()): Add
	optimizer hint that _S_empty_rep()._M_length is always zero.
	(basic_string::length()): Call size().

1 files changed, 9 insertions, 2 deletions

diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index 1ee84e6..b602436 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -907,17 +907,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     public:
       // Capacity:
+
       ///  Returns the number of characters in the string, not including any
       ///  null-termination.
       size_type
       size() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_length; }
+      {
+#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0 && __OPTIMIZE__
+	if (_S_empty_rep()._M_length != 0)
+	  __builtin_unreachable();
+#endif
+	return _M_rep()->_M_length;
+      }
 
       ///  Returns the number of characters in the string, not including any
       ///  null-termination.
       size_type
       length() const _GLIBCXX_NOEXCEPT
-      { return _M_rep()->_M_length; }
+      { return size(); }
 
       ///  Returns the size() of the largest possible %string.
       size_type