Re: [PATCH] libstdc++: Ensure active union member is correctly set

[Jonathan Wakely <jwakely@redhat.com>]

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On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > Thanks for the comments, here's an updated version of the patch.
> > Bootstrapped and regtested on x86_64-pc-linux-gnu.
>
> Great, I'll get this committed today - thanks!

That's done now.

> >
> > I'll note that there are some existing calls to `_M_use_local_data()`
> > already used only for their side effects without a cast to void, e.g.
> >
> >       /**
> >        *  @brief  Default constructor creates an empty string.
> >        */
> >       _GLIBCXX20_CONSTEXPR
> >       basic_string()
> >       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> >       : _M_dataplus(_M_local_data())
> >       {
> >         _M_use_local_data();
> >         _M_set_length(0);
> >       }
> >
> > I haven't updated these, but should this be changed for consistency?
>
> Yes, good idea. I can do that.

I started to do that, and decided it made more sense to split out the
constexpr loop from _M_use_local_data() into a separate function,
_M_init_local_buf(). Then we can use that for all the places where we
don't care about the return value. That avoids the overhead of using
pointer_traits::pointer_to when we don't need the return value (which
is admittedly only going to be an issue for -O0 code, but I think it's
cleaner this way anyway).

Please see the attached patch and let me know what you think.


> Thanks again for fixing these. I think this might fix some bug reports
> about clang rejecting our std::string in constant expressions, so I'll
> check those.

Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
(so we should backport it to gcc-13 and gcc-12 too).

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commit 2668979d3206ff6c039ac0165aae29377a15666c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Sep 29 12:12:22 2023

    libstdc++: Split std::basic_string::_M_use_local_data into two functions
    
    This splits out the activate-the-union-member-for-constexpr logic from
    _M_use_local_data, so that it can be used separately in cases that don't
    need to use std::pointer_traits<pointer>::pointer_to to obtain the
    return value.
    
    This leaves only three uses of _M_use_local_data() which are all the
    same form:
    
      __s._M_data(_M_use_local_data());
      __s._M_set_length(0);
    
    We could remove _M_use_local_data() and change those three places to use
    a new _M_reset() function that does:
    
      _M_init_local_buf();
      _M_data(_M_local_data());
      _M_set_length(0);
    
    This is left for a future change.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h (_M_init_local_buf()): New
            function.
            (_M_use_local_data()): Use _M_init_local_buf.
            (basic_string(), basic_string(const Alloc&))
            (basic_string(basic_string&&))
            (basic_string(basic_string&&, const Alloc&)): Use
            _M_init_local_buf instead of _M_use_local_data().
            * include/bits/basic_string.tcc (swap(basic_string&))
            (_M_construct(InIter, InIter, forward_iterator_tag))
            (_M_construct(size_type, CharT), reserve()): Likewise.
            (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
            to _M_use_local_data() and initialize the local buffer directly.

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 4f94cd967cf..18a19b8dcbc 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       // Ensure that _M_local_buf is the active member of the union.
       __attribute__((__always_inline__))
       _GLIBCXX14_CONSTEXPR
-      pointer
-      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      void
+      _M_init_local_buf() _GLIBCXX_NOEXCEPT
       {
 #if __cpp_lib_is_constant_evaluated
 	if (std::is_constant_evaluated())
 	  for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
 	    _M_local_buf[__i] = _CharT();
+#endif
+      }
+
+      __attribute__((__always_inline__))
+      _GLIBCXX14_CONSTEXPR
+      pointer
+      _M_use_local_data() _GLIBCXX_NOEXCEPT
+      {
+#if __glibcxx_is_constant_evaluated
+	_M_init_local_buf();
 #endif
 	return _M_local_data();
       }
@@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
       : _M_dataplus(_M_local_data())
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
       : _M_dataplus(_M_local_data(), __a)
       {
-	_M_use_local_data();
+	_M_init_local_buf();
 	_M_set_length(0);
       }
 
@@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	  }
@@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       {
 	if (__str._M_is_local())
 	  {
-	    (void)_M_use_local_data();
+	    _M_init_local_buf();
 	    traits_type::copy(_M_local_buf, __str._M_local_buf,
 			      __str.length() + 1);
 	    _M_length(__str.length());
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 4bc98f2aea7..052eeb9e846 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (__s.length())
 	      {
-		(void)_M_use_local_data();
+		_M_init_local_buf();
 		traits_type::copy(_M_local_buf, __s._M_local_buf,
 				  __s.length() + 1);
 		_M_length(__s.length());
@@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      }
 	    else if (length())
 	      {
-		(void)__s._M_use_local_data();
+		__s._M_init_local_buf();
 		traits_type::copy(__s._M_local_buf, _M_local_buf,
 				  length() + 1);
 		__s._M_length(length());
@@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	else
 	  {
 	    const size_type __tmp_capacity = __s._M_allocated_capacity;
-	    (void)__s._M_use_local_data();
+	    __s._M_init_local_buf();
 	    traits_type::copy(__s._M_local_buf, _M_local_buf,
 			      length() + 1);
 	    _M_data(__s._M_data());
@@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  const size_type __tmp_capacity = _M_allocated_capacity;
 	  if (__s._M_is_local())
 	    {
-	      (void)_M_use_local_data();
+	      _M_init_local_buf();
 	      traits_type::copy(_M_local_buf, __s._M_local_buf,
 				__s.length() + 1);
 	      __s._M_data(_M_data());
@@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	size_type __len = 0;
 	size_type __capacity = size_type(_S_local_capacity);
 
-	pointer __p = _M_use_local_data();
-
 	while (__beg != __end && __len < __capacity)
 	  {
-	    __p[__len++] = *__beg;
+	    _M_local_buf[__len++] = *__beg;
 	    ++__beg;
 	  }
 
+#if __glibcxx_is_constant_evaluated
+	if (std::is_constant_evaluated())
+	  for (size_type __i = __len; __i <= __capacity; ++__i)
+	    _M_local_buf[__i] = _CharT();
+#endif
+
 	struct _Guard
 	{
 	  _GLIBCXX20_CONSTEXPR
@@ -230,7 +234,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    _M_capacity(__dnew);
 	  }
 	else
-	  _M_use_local_data();
+	  _M_init_local_buf();
 
 	// Check for out_of_range and length_error exceptions.
 	struct _Guard
@@ -263,7 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  _M_capacity(__n);
 	}
       else
-	_M_use_local_data();
+	_M_init_local_buf();
 
       if (__n)
 	this->_S_assign(_M_data(), __n, __c);
@@ -372,7 +376,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       if (__length <= size_type(_S_local_capacity))
 	{
-	  this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
+	  _M_init_local_buf();
+	  this->_S_copy(_M_local_buf, _M_data(), __length + 1);
 	  _M_destroy(__capacity);
 	  _M_data(_M_local_data());
 	}