On 13/10/2021 18:05, Marian Buschsieweke wrote:
> Hi together,
>
> parts of the avrlibc headers are not compatible with clang. (The use case is
> not to compile avrlibc itself with clang, but rather an application using
> a vanilla GCC compiled avrlibc.) However, the issues are not always trivial to
> fix.
>
> E.g. in avr/wdt.h there are several instances of code like this:
>
>> static __inline__
>> __attribute__ ((__always_inline__))
>> void wdt_enable (const uint8_t value)
>> {
>> if (_SFR_IO_REG_P (_WD_CONTROL_REG))
>> {
>> __asm__ __volatile__ (
>> [...]
>> : /* no outputs */
>> : "I" (_SFR_IO_ADDR(_WD_CONTROL_REG)),
>> [...]
>> : "r0"
>> );
>> }
>> else {
>> /* variant not using _WD_CONTROL_REG as immediate */
>> [...]
>>
>> }
>
> For targets where _SFR_IO_REG_P (_WD_CONTROL_REG) evaluates to false, the
> inline assembly in the then-clause is syntactically not correct, as the
> constraints "I" are not fulfilled. But GCC will never check the constraints,
> because GCC eliminates the dead branches prior to verifying correctness of
> inline assembly. With clang these syntax checks are done prior to the
> optimization of dead branches. I'm aware that inline assembly isn't part of
> the
> C language, still performing syntax checks prior to the optimizations seems
> more consistent to me. (If there were C syntax errors in the dead branch, the
> compiler would also refuse to compile despite the lines in question being
> unreachable.)
>
It's been a /long/ time since I have done anything with the AVR, and I
don't have a modern avr gcc compiler (or any clang avr compiler at all)
handy for testing. However, I have an idea you can try:
"I" (_SFR_IO_ADDR(_WD_CONTROL_REG) || 1)
If "_SFR_IO_ADDR(_WD_CONTROL_REG)" evaluates to false, then this
expression would give "1", which clang might be happier with.
