[Development] 64-bit QFlags support

[Thiago Macieira]

On Thursday 8 August 2024 11:22:39 GMT-7 Thiago Macieira wrote:
> My solution is to have moc parse a new declaration Q_DECLARE_FLAGS64, which
> indicates the flag is 64-bit, and then generate different data. I have not
> implemented Q_FLAGS64 or Q_ENUM64 yet.

An alternative solution I am looking into right now is to move the integer 
array generation to C++ constexpr functions, like we did for the string array. 
In those functions, we can know that the enumeration is 64-bit wide and then 
generate the extra fields without having to have moc understand that they are 
64-bit. In theory this will work, but in practice we need to test the multiple 
compilers extensively to know if we won't overflow limitations in constexpr-
time execution or just plain compiler bugs (MSVC fails with [1] for example 
and generates bad code). The moc output would look something like:

static constexpr std::array qt_meta_data_CLASSUserClassENDCLASS =
    QtMocHelpers::uintData()
        .addClassInfo(1, 3)
        .addClassInfo(4, 5)
        .addMethod(6, 2, 0x06, { QMetaType::Void, 0x80000000 | 17,    8 })
        .addMethod(7, 2, 0x06, { QMetaType::Void, 0x80000000 | 9,    8 })
        .addMethod(8, 2, 0x0a, { QMetaType::Void })
        .addEnum(10, 10, 0x00, { 11, 12 }, { UserClass::V0, UserClass::V1 })
        .addEnum(13, 14, 0x01, { 15, 16 }, { UserClass::F0, UserClass::F1 })
        .generate(/* revision */ 12, /* flags */ 0);

There's no guarantee this will work, and especially that it will for C++17, 
though I wouldn't mind declaring you must use C++20 to access 64-bit 
enumerators.

If this works, we get the best of both worlds: all 64-bit enumerations and 
flags automatically get their high part stored without impacting the existing 
32-bit ones, and moc doesn't need to know that they are 64-bit.

Long-term, we should generate the char, integer and metatype arrays in a 
single call, so it can be used with C++ reflection. I have a hard time figuring 
out how that can be accomplished even without it.

[1] https://codereview.qt-project.org/c/qt/qtbase/+/537384

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Principal Engineer - Intel DCAI Platform & System Engineering
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