On Fri, 29 Sept 2023 at 17:46, Jonathan Wakely <jwakely....@gmail.com> wrote:
>
> On Fri, 29 Sept 2023 at 17:29, Nathaniel Shead
> <nathanielosh...@gmail.com> wrote:
> >
> > On Fri, Sep 29, 2023 at 04:06:33PM +0100, Jonathan Wakely wrote:
> > > On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwak...@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.
> > >
> >
> > Thanks!
> >
> > > > >
> > > > > 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.
> >
> > I agree, and it also looks clearer to me what is happening.
>
> Good, I'll make this change next week then.
>
>
> >
> > >
> > > > 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).
> >
> > > commit 2668979d3206ff6c039ac0165aae29377a15666c
> > > Author: Jonathan Wakely <jwak...@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();
> > > }
> >
> > What's the difference between __cpp_lib_is_constant_evaluated and
> > __glibcxx_is_constant_evaluated? Should these lines be using the same
> > macro here?
>
> Ah, yeah, they could be the same.
>
> The difference is that (for most feature test macros) the
> __glibcxx_ftm form is defined after including <bits/version.h>, but
> the __cpp_lib_ftm form is only defined only by headers that do:
>
> #define __glibcxx_want_ftm
> #include <bits/version.h>
>
> This means we can ensure that the __cpp_lib_ftm form is only defined
> by headers where we actually want to define it, e.g. in the headers
> that [version.syn] in the standard says should define the macro. So
> for our own internal uses, we should generally rely on the
> __glibcxx_ftm one. Users should rely on __cpp_lib_ftm after including
> the correct header. For example, __glibcxx_atomic_wait is defined
> after including <bits/atomic_wait.h> and so is available for our own
> uses, but __cpp_lib_atomic_wait is only defined after including
> <atomic>, not just <bits/atomic_wait.h>. Or at least, that's the plan
> - I have a pending patch to make everything I just said true :-)
> Currently we "leak" the __cpp_lib_ftm forms into lots of <bits/xxx.h>
> internal headers. That will change next week.
>
> In this specific case, it doesn't matter, because
> __cpp_lib_is_constant_evaluated is defined by <type_traits>, and every
> header includes that. So it doesn't really matter whether our internal
> uses are __cpp_lib_is_constant_evaluated or
> __glibcxx_is_constant_evaluated. But it would be good to be
> consistent.
>
> >
> > > @@ -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
> > > +
> >
> > I wonder if maybe this should still be a call to `_M_init_local_buf()`
> > above, where the `_M_use_local_data()` used to be? That way the logic
> > stays in one place, and I don't imagine the compile time savings of not
> > immediately overwriting the first __len characters would be significant.
>
>
> Yeah, I went back and forth on that, but you're probably right. I'll
> do as you suggested.
Here's what I've pushed to trunk after testing on x86_64-linux.
Thanks again for the fixes.
commit 405a4140fc30bce86b1ec885a98bb17704e0c8c6
Author: Jonathan Wakely <jwak...@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 of 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, input_iterator_tag))
(_M_construct(InIter, InIter, forward_iterator_tag))
(_M_construct(size_type, CharT), reserve()): Likewise.
diff --git a/libstdc++-v3/include/bits/basic_string.h
b/libstdc++-v3/include/bits/basic_string.h
index 4f94cd967cf..0fa32afeb84 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 __cpp_lib_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..f0a44e5e881 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());
@@ -174,11 +174,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
size_type __len = 0;
size_type __capacity = size_type(_S_local_capacity);
- pointer __p = _M_use_local_data();
+ _M_init_local_buf();
while (__beg != __end && __len < __capacity)
{
- __p[__len++] = *__beg;
+ _M_local_buf[__len++] = *__beg;
++__beg;
}
@@ -230,7 +230,7 @@ _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 +263,7 @@ _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 +372,8 @@ _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());
}
