[PATCH, libstdc++] Implement P0415 More constexpr for std::complex.

[Christophe Lyon]

On Thu, 22 Nov 2018 at 10:20, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 20/11/18 17:58 -0500, Ed Smith-Rowland wrote:
> >On 11/19/18 6:13 AM, Jonathan Wakely wrote:
> >>On 16/11/18 19:39 -0500, Ed Smith-Rowland wrote:
> >>>@@ -322,67 +323,43 @@
> >>>  //@{
> >>>  ///  Return new complex value @a x plus @a y.
> >>>  template<typename _Tp>
> >>>-    inline complex<_Tp>
> >>>+    inline _GLIBCXX20_CONSTEXPR complex<_Tp>
> >>>    operator+(const complex<_Tp>& __x, const complex<_Tp>& __y)
> >>>-    {
> >>>-      complex<_Tp> __r = __x;
> >>>-      __r += __y;
> >>>-      return __r;
> >>>-    }
> >>>+    { return complex<_Tp>(__x.real() + __y.real(), __x.imag() +
> >>>__y.imag()); }
> >>
> >>Is this change (and all the similar ones) really needed?
> >>
> >>Doesn't the fact that all the constructors and member operators of
> >>std::complex mean that the original definition is also valid in a
> >>constexpr function?
> >These changes are rolled back. Sorry.
> >>>@@ -1163,50 +1143,43 @@
> >>>#endif
> >>>
> >>>      template<typename _Tp>
> >>>-        complex&
> >>>+        _GLIBCXX20_CONSTEXPR complex&
> >>>        operator=(const complex<_Tp>&  __z)
> >>>    {
> >>>-      __real__ _M_value = __z.real();
> >>>-      __imag__ _M_value = __z.imag();
> >>>+      _M_value = __z.__rep();
> >>
> >>These changes look OK, but I wonder if we shouldn't ask the compiler
> >>to make it possible to use __real__ and __imag__ in constexpr
> >>functions instead.
> >>
> >>I assume it doesn't, and that's why you made this change. But if it
> >>Just Worked, and the other changes I commented on above are also
> >>unnecessary, then this patch would *mostly* just be adding
> >>_GLIBCXX20_CONSTEXPR which is OK for stage 3 (as it doesn't affect any
> >>dialects except C++2a).
> >
> >Yes, this is the issue.  I agree that constexpr _real__, __imag__would
> >be better.
> >
> >Do you have any idea where this change would be?  I grepped around a
> >little and couldn't figure it out.  if you don't I'll look more.
>
> No idea, sorry.
>
> >Actually, looking at constexpr.c it looks like the old way ought to work...
> >
> >OK, plain assignment works but not the others.  Interesting.
> >
> >>
> >>>@@ -1872,7 +1831,7 @@
> >>>    { return _Tp(); }
> >>>
> >>>  template<typename _Tp>
> >>>-    inline typename __gnu_cxx::__promote<_Tp>::__type
> >>>+    _GLIBCXX_CONSTEXPR inline typename
> >>>__gnu_cxx::__promote<_Tp>::__type
> >>
> >>This should be _GLIBCXX20_CONSTEXPR.
> >Done.
> >>>Index:
> >>>testsuite/26_numerics/complex/comparison_operators/more_constexpr.cc
> >>>===================================================================
> >>>---
> >>>testsuite/26_numerics/complex/comparison_operators/more_constexpr.cc
> >>>(nonexistent)
> >>>+++
> >>>testsuite/26_numerics/complex/comparison_operators/more_constexpr.cc
> >>>(working copy)
> >>>@@ -0,0 +1,51 @@
> >>>+// { dg-do compile { target c++2a } }
> >>
> >>All the tests with { target c++2a} should also have:
> >>
> >>// { dg-options "-std=gnu++2a" }
> >>
> >>Because otherwise they are skipped by default, and only get run when
> >>RUNTESTFLAGS explicitly includes something like
> >>--target_board=unix/-std=gnu++2a
> >>
> >>The dg-options needs to come first, or it doesn't apply before the
> >>check for { target c++2a }.
> >>
> >Thank you, done.
>
> OK for trunk, thanks.
>
> >Updated patch attached.  I'd like to understand why
> >
> >    __real__ _M_value += __z.real();
> >
> >doesn't work though.
>
> Yes, I agree it should. If you don't figure it out please file a bug
> requesting that it works, so somebody else might look into it.
>

Hi,
I have noticed that
FAIL: 26_numerics/complex/requirements/more_constexpr.cc
on arm and aarch64
The error messages:
Excess errors:
/libstdc++-v3/testsuite/26_numerics/complex/requirements/more_constexpr.cc:168:
error: '__float128' was not declared in this scope
/libstdc++-v3/testsuite/26_numerics/complex/requirements/more_constexpr.cc:168:
error: no matching function for call to
'test_operator_members<<expression error>, __float128>()'
/libstdc++-v3/testsuite/26_numerics/complex/requirements/more_constexpr.cc:168:
error: template argument 1 is invalid

Christophe