inlined ASM code: invalid output constraint

[4ntoine]

Clang issue related to backend most likely

/Applications/A.app/Contents/Resources/Java/hardware/a/cores/a/wiring.c:185:15: error: invalid
      output constraint '=w' in asm
                "brne 1b" : "=w" (us) : "0" (us) // 2 cycles

wiring.c (part):

/* Delay for the given number of microseconds.  Assumes a 8 or 16 MHz clock. */
void delayMicroseconds(unsigned int us)
{
    // calling avrlib's delay_us() function with low values (e.g. 1 or
    // 2 microseconds) gives delays longer than desired.
    //delay_us(us);
#if F_CPU >= 20000000L
    // for the 20 MHz clock on rare Arduino boards

    // for a one-microsecond delay, simply wait 2 cycle and return. The overhead
    // of the function call yields a delay of exactly a one microsecond.
    __asm__ __volatile__ (
        "nop" "\n\t"
        "nop"); //just waiting 2 cycle
    if (--us == 0)
        return;

    // the following loop takes a 1/5 of a microsecond (4 cycles)
    // per iteration, so execute it five times for each microsecond of
    // delay requested.
    us = (us<<2) + us; // x5 us

    // account for the time taken in the preceeding commands.
    us -= 2;

#elif F_CPU >= 16000000L
    // for the 16 MHz clock on most Arduino boards

    // for a one-microsecond delay, simply return.  the overhead
    // of the function call yields a delay of approximately 1 1/8 us.
    if (--us == 0)
        return;

    // the following loop takes a quarter of a microsecond (4 cycles)
    // per iteration, so execute it four times for each microsecond of
    // delay requested.
    us <<= 2;

    // account for the time taken in the preceeding commands.
    us -= 2;
#else
    // for the 8 MHz internal clock on the ATmega168

    // for a one- or two-microsecond delay, simply return.  the overhead of
    // the function calls takes more than two microseconds.  can't just
    // subtract two, since us is unsigned; we'd overflow.
    if (--us == 0)
        return;
    if (--us == 0)
        return;

    // the following loop takes half of a microsecond (4 cycles)
    // per iteration, so execute it twice for each microsecond of
    // delay requested.
    us <<= 1;

    // partially compensate for the time taken by the preceeding commands.
    // we can't subtract any more than this or we'd overflow w/ small delays.
    us--;
#endif

    // busy wait
    __asm__ __volatile__ (
        "1: sbiw %0,1" "\n\t" // 2 cycles
        "brne 1b" : "=w" (us) : "0" (us) // 2 cycles
    );
}

[4ntoine]

Unfortunately i can't understand at the moment what =w means.
If i comment that asm inline code or replace =w with =r (which seems to be correct) if i'm getting:

fatal error: error in backend: Inline asm not supported by this streamer because we don't have an asm parser for this target

[4ntoine]

I was wrong, "w" is AVR-related constraint and it's valid code:
http://stackoverflow.com/questions/27654116/what-does-w-in-gcc-inline-assembly-mean

[4ntoine]

back-end issue