[PATCH] libstdc++: Ensure active union member is correctly set
Jonathan Wakely
jwakely@redhat.com
Fri Sep 29 15:06:33 GMT 2023
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).
-------------- next part --------------
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());
}
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