Hi,
after playing hard with this task I was wrong.
I reimplemented my structure in few different versions and final ASM
code is the same in all cases.
Everything is OK and works great.
Best Regards
Zygmunt
2017-10-02 23:57 GMT+02:00 Zygmunt Ptak <zygmuntptak@xxxxxxxxx>:
> Hi Jens,
>
> This helps just a little.
> I figure out that I can use something like:
>
> template<typename RegType, intptr_t ptr>
> class Register {
> RegType & reg() { return *reinterpret_cast<RegType*>(ptr); }
> };
>
> and then, here is important:
> I have to use this:
>
> const intptr_t reg_value_b = reinterpret_cast<intptr_t>(&PORTB);
> typedef Register<volatile uint8_t, reg_value_b> RegisterPortB;
>
> instead of this:
>
> typedef Register<volatile uint8_t, reinterpret_cast<intptr_t>(&PORTB)>
> RegisterPortB;
>
> // second case will tell you that reinterpret_cast is not constexpr.
>
> Overall in my case non-constexpr for reinterpret_cast sucks. I'm
> taking few more bytes in final binary (just for very simple code).
> If I will use in every part of my code there can be a case when I will
> not have enough memory.
>
> Would be great to disable that somehow in gcc. For now I'm not able
> change my code.
>
> Best Regards
> Zygmunt
>
> 2017-09-05 11:52 GMT+02:00 Kilian, Jens <jens.kilian@xxxxxxxxxxxxx>:
>> Hi Zygmunt,
>> You could try changing the second template parameter to an actual pointer (the register address before casting) and apply the reinterpret cast to it in the reg() function.
>>
>> Hope this helps,
>>
>> Jens.
>>
>>> -----Original Message-----
>>> From: Zygmunt Ptak [mailto:zygmuntptak@xxxxxxxxx]
>>> Sent: Sunday, 03 September, 2017 22:20
>>> To: gcc-help@xxxxxxxxxxx
>>> Subject: reinterpret_cast is not a constant expression - needed help to
>>> find solution
>>>
>>> Hi,
>>>
>>> I have a code for AVR written in c++.
>>> To get the same size result as C code I'm using template classes.
>>> For example:
>>>
>>> #define AVR_REG_TO_U16(REG) (reinterpret_cast<u16>(& REG))
>>>
>>> template<typename RegType, u16 reg_r>
>>> struct Register
>>> {
>>> typedef Register<RegType, reg_r> Type;
>>> constexpr Register<RegType, reg_r>() {}
>>> static RegType & reg() { return *reinterpret_cast<RegType*>(reg_r); }
>>> void operator = (RegType value) { reg() = value; }
>>> void setBit(RegType value) { reg() = static_cast<RegType>(reg()
>>> | 1 << value); }
>>> void dropBit(RegType value) { reg() = static_cast<RegType>(reg()
>>> & ~(1 << value)); }
>>> void clear() { reg() = 0; }
>>> void setBits(RegType mask) { reg() = static_cast<RegType>(reg()
>>> | mask); }
>>> void clearBits(RegType mask) { reg() = static_cast<RegType>(reg()
>>> & ~mask); }
>>> operator RegType() { return reg(); }
>>> } __attribute__((packed));
>>>
>>> // and finally:
>>>
>>> Register<volatile uint8_t, AVR_REG_TO_U16(PIND)> pin_d;
>>> pin_d.setBit(1);
>>>
>>> Everything works pretty cool on old compiler.
>>> But on newer (in my case gcc-7.1.0) there is an error:
>>>
>>> error: 'reinterpret_cast<volatile uint8_t* {aka volatile unsigned
>>> char*}>(41)' is not a constant expression
>>>
>>> I tried found some solution, but nothing.
>>> All the time the same problem.
>>> Is it possible to turn off that option?
>>>
>>> I understand that this type of the code doesn't make a sens on PC, but
>>> on AVR it really does.
>>>
>>> Of course that simple example looks like not having any benefit.
>>> But it has, compiler nicely optimize code and as I said I'm able to
>>> get the same size of the code like in pure C.
>>> Size is really matter here with 2kB of the SRAM on atmega328p.
>>> I don't want to use pure C. I want to use C++ which is better for my
>>> project.
>>>
>>> Best regards
>>> Zygmunt Ptak
