Re: [PATCH] libstdc++: Make __gnu_debug::vector usable in constant expressions [PR109536]

[Patrick Palka <ppalka@redhat.com>]

On Sat, 16 Dec 2023, Jonathan Wakely wrote:

> On Sat, 16 Dec 2023 at 09:14, Jonathan Wakely wrote:
> >
> > On Sat, 16 Dec 2023 at 00:27, Patrick Palka wrote:
> > >
> > > On Wed, 6 Dec 2023, Jonathan Wakely wrote:
> > >
> > > > Any comments on this approach?
> > > >
> > > > -- >8 --
> > > >
> > > > This makes constexpr std::vector (mostly) work in Debug Mode. All safe
> > > > iterator instrumentation and checking is disabled during constant
> > > > evaluation, because it requires mutex locks and calls to non-inline
> > > > functions defined in libstdc++.so. It should be OK to disable the safety
> > > > checks, because most UB should be detected during constant evaluation
> > > > anyway.
> > > >
> > > > We could try to enable the full checking in constexpr, but it would mean
> > > > wrapping all the non-inline functions like _M_attach with an inline
> > > > _M_constexpr_attach that does the iterator housekeeping inline without
> > > > mutex locks when calling for constant evaluation, and calls the
> > > > non-inline function at runtime. That could be done in future if we find
> > > > that we've lost safety or useful checking by disabling the safe
> > > > iterators.
> > > >
> > > > There are a few test failures in C++20 mode, which I'm unable to
> > > > explain. The _Safe_iterator::operator++() member gives errors for using
> > > > non-constexpr functions during constant evaluation, even though those
> > > > functions are guarded by std::is_constant_evaluated() checks. The same
> > > > code works fine for C++23 and up.
> > >
> > > AFAICT these C++20 test failures are really due to the variable
> > > definition of non-literal type
> > >
> > > 381    __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
> > >
> > > which were prohibited in a constexpr function (even if that code was
> > > never executed) until C++23's P2242R3.
> >
> > Ah, I figured it was a core change but I couldn't recall which one. Thanks.

Yeah the diagnostic blaming the non-constexpr call to _M_incrementable
was outright wrong and misleading, I filed PR113041 about that.

> >
> > > We can use an immediately invoked lambda to work around this:
> > >
> > > 381    [this] {
> > > 382      __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
> > > 383      ++base();
> > > 384    }();
> > > 385    return *this;
> >
> > We'd need some #if as this code has to work for C++98. But that's doable.
> 
> The attached patch seems simpler, I'm testing it now.

Would the operator+=, operator-= and the copy assign operator= also need
adjustment?

We could somewhat cleanly encapsulate the lambda workaround with a pair
of macros surrounding the problematic variable, something like:

diff --git a/libstdc++-v3/include/debug/safe_iterator.h b/libstdc++-v3/include/debug/safe_iterator.h
index 26f008982f8..df3b4d33f04 100644
--- a/libstdc++-v3/include/debug/safe_iterator.h
+++ b/libstdc++-v3/include/debug/safe_iterator.h
@@ -360,6 +360,14 @@ namespace __gnu_debug
 	return base().operator->();
       }
 
+#if __cplusplus >= 202002L && __cpp_constexpr < 202110L
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN [&] { do
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END while(false); }()
+#else
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN do
+# define _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END while(false)
+#endif
+
       // ------ Input iterator requirements ------
       /**
        *  @brief Iterator preincrement
@@ -378,8 +386,10 @@ namespace __gnu_debug
 	_GLIBCXX_DEBUG_VERIFY(this->_M_incrementable(),
 			      _M_message(__msg_bad_inc)
 			      ._M_iterator(*this, "this"));
-	__gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
-	++base();
+	_GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_BEGIN {
+	  __gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
+	  ++base();
+	} _GLIBCXX20_CONSTEXPR_NON_LITERAL_SCOPE_END;
 	return *this;
       }