[PATCH] PR libstdc++/112682 More efficient std::basic_string move

[PATCH] PR libstdc++/112682 More efficient std::basic_string move

[Antony Polukhin]

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A few places in bits/basic_string.h use `traits_type::copy` to copy
`__str.length() + 1` bytes.

Despite the knowledge that `__str.length()` is not greater than 15 the
compiler emits (and sometimes inlines) a `memcpy` call. That results
in a quite big set of instructions https://godbolt.org/z/j35MMfxzq

Replacing `__str.length() + 1` with `_S_local_capacity + 1` explicitly
forces the compiler to copy the whole `__str._M_local_buf`. As a
result the assembly becomes almost 5 times shorter and without any
function calls or multiple conditional jumps
https://godbolt.org/z/bfq8bxra9

This patch always copies `_S_local_capacity + 1` if working with
`std::char_traits<char>`.

PR libstdc++/112682:
        * include/bits/basic_string.h: Optimize string moves.


P.S.: still not sure that this optimization is not an UB or fine for
libstdc++. However, the assembly looks much better with it.

-- 
Best regards,
Antony Polukhin

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diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 1b8ebca7dad..7a5e348280c 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -188,6 +188,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       : basic_string(__svw._M_sv.data(), __svw._M_sv.size(), __a) { }
 #endif
 
+      _GLIBCXX17_CONSTEXPR
+      static bool
+      _S_permit_copying_indeterminate() noexcept
+      {
+	// Copying compile-time known _S_local_capacity + 1 bytes is much more
+	// efficient than copying runtime known __str.length() + 1. This
+	// function returns true, if such initialization is permitted even if
+	// the right side has indeterminate values.
+	//
+	// [dcl.init] permits initializing with indeterminate value of unsigned
+	// narrow character type.
+	//
+	// Library users should not specialize char_traits<char> so this is
+	// not observable for user.
+	return is_same<traits_type, char_traits<char> >::value;
+	  }
+
       // Use empty-base optimization: http://www.cantrip.org/emptyopt.html
       struct _Alloc_hider : allocator_type // TODO check __is_final
       {
@@ -672,8 +689,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    traits_type::copy(_M_local_buf, __str._M_local_buf,
-			      __str.length() + 1);
+	    size_type __copy_count = _S_local_capacity + 1;
+	    if _GLIBCXX17_CONSTEXPR (!_S_permit_copying_indeterminate())
+	      __copy_count = __str.length() + 1;
+	    traits_type::copy(_M_local_buf, __str._M_local_buf, __copy_count);
 	  }
 	else
 	  {
@@ -711,8 +730,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    traits_type::copy(_M_local_buf, __str._M_local_buf,
-			      __str.length() + 1);
+	    size_type __copy_count = _S_local_capacity + 1;
+	    if _GLIBCXX17_CONSTEXPR (!_S_permit_copying_indeterminate())
+	      __copy_count = __str.length() + 1;
+	    traits_type::copy(_M_local_buf, __str._M_local_buf, __copy_count);
 	    _M_length(__str.length());
 	    __str._M_set_length(0);
 	  }