Re: [Development] static constexpr in exported classes needs out-of-line definitions
Thanks! Added to
https://wiki.qt.io/Things_To_Look_Out_For_In_Reviews#Variables
On 18.01.24 04:35, Thiago Macieira wrote:
> See http://bugreports.qt.io/browse/QTBUG-121135 for the issue and
> https://codereview.qt-project.org/c/qt/qtbase/+/531251 for the solution.
>
> TL;DR: if you define a static constexpr (which is implicitly inline) member
> variable in a class with export macro, you MUST add the definition of tha
> variable to the .cpp.
>
> Alternatives:
> * move to a class that is not exported
> trick: add an empty base with the variable
> * change to enum (if primitive)
>
> But this happens even for plain int variables. For example:
>
> #include
> #include
> void f(std::vector &v)
> {
> v.push_back(QLocale::FirstTwoDigitYear);
> }
>
> This produced: with GCC 13 at -O2 optimisations:
>
> _Z1fRSt6vectorIiSaIiEE:
> movq8(%rcx), %rdx
> cmpq16(%rcx), %rdx
> je .L29
> movl$1900, (%rdx)
> addq$4, %rdx
> movq%rdx, 8(%rcx)
> ret
> .L29:
> movq__imp__ZN7QLocale17FirstTwoDigitYearE(%rip), %r8
> jmp
> _ZNSt6vectorIiSaIiEE17_M_realloc_insertIJRKiEEEvN9__gnu_cxx17__normal_iteratorIPiS1_EEDpOT_
>
> Interpretation: if the vector has enough pre-allocated space, it simply moves
> 1900 to the end of te array and adjusts the end. But if not, it calls
> std::vector_M_realloc_insert(), which takes the integer by reference, so
> the compiler emits an import from the DLL.
>
> This appears to be a GCC bug/shortcoming. Clang and MSVC apparently
> automatically emit and export the variable for you:
> https://mingw.godbolt.org/z/q4dYdosjh
>
>
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Re: [Development] static constexpr in exported classes needs out-of-line definitions
On Wednesday, 17 January 2024 19:35:17 PST Thiago Macieira wrote: > Alternatives: > * move to a class that is not exported >trick: add an empty base with the variable > * change to enum (if primitive) * change to an static (inline) constexpr function Then there's always a copy, because GCC either inlines or emits a copy for functions. https://mingw.godbolt.org/z/bh95q8PKf MSVC sucks at inlining dllimported functions, but there's no problem because it exported on the other side. -- Thiago Macieira - thiago.macieira (AT) intel.com Cloud Software Architect - Intel DCAI Cloud Engineering smime.p7s Description: S/MIME cryptographic signature -- Development mailing list Development@qt-project.org https://lists.qt-project.org/listinfo/development
Re: [Development] static constexpr in exported classes needs out-of-line definitions
On Wednesday, 17 January 2024 19:35:17 PST Thiago Macieira wrote: > This appears to be a GCC bug/shortcoming. Clang and MSVC apparently > automatically emit and export the variable for you: > https://mingw.godbolt.org/z/q4dYdosjh Reported at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113465 -- Thiago Macieira - thiago.macieira (AT) intel.com Cloud Software Architect - Intel DCAI Cloud Engineering smime.p7s Description: S/MIME cryptographic signature -- Development mailing list Development@qt-project.org https://lists.qt-project.org/listinfo/development
[Development] static constexpr in exported classes needs out-of-line definitions
See http://bugreports.qt.io/browse/QTBUG-121135 for the issue and
https://codereview.qt-project.org/c/qt/qtbase/+/531251 for the solution.
TL;DR: if you define a static constexpr (which is implicitly inline) member
variable in a class with export macro, you MUST add the definition of tha
variable to the .cpp.
Alternatives:
* move to a class that is not exported
trick: add an empty base with the variable
* change to enum (if primitive)
But this happens even for plain int variables. For example:
#include
#include
void f(std::vector &v)
{
v.push_back(QLocale::FirstTwoDigitYear);
}
This produced: with GCC 13 at -O2 optimisations:
_Z1fRSt6vectorIiSaIiEE:
movq8(%rcx), %rdx
cmpq16(%rcx), %rdx
je .L29
movl$1900, (%rdx)
addq$4, %rdx
movq%rdx, 8(%rcx)
ret
.L29:
movq__imp__ZN7QLocale17FirstTwoDigitYearE(%rip), %r8
jmp
_ZNSt6vectorIiSaIiEE17_M_realloc_insertIJRKiEEEvN9__gnu_cxx17__normal_iteratorIPiS1_EEDpOT_
Interpretation: if the vector has enough pre-allocated space, it simply moves
1900 to the end of te array and adjusts the end. But if not, it calls
std::vector_M_realloc_insert(), which takes the integer by reference, so
the compiler emits an import from the DLL.
This appears to be a GCC bug/shortcoming. Clang and MSVC apparently
automatically emit and export the variable for you:
https://mingw.godbolt.org/z/q4dYdosjh
--
Thiago Macieira - thiago.macieira (AT) intel.com
Cloud Software Architect - Intel DCAI Cloud Engineering
smime.p7s
Description: S/MIME cryptographic signature
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