AVR Freaks

Optimisation breaks (sic) debug code

Note that getting precise or even consistent timing from C source can be difficult.

Incidentally, his macros could be made 'timing-safe'.
You can avoid ASM. Simply disable interrupts and avoid -Os making subroutines.

		DEBUG(0x37);
     3ba:	a9 98       	cbi	0x15, 1	; 21
.
.
.
		DEBUG(0x38);
     3c0:	a9 98       	cbi	0x15, 1	; 21
     3c2:	a9 98       	cbi	0x15, 1	; 21
     3c4:	a9 98       	cbi	0x15, 1	; 21
     3c6:	a9 9a       	sbi	0x15, 1	; 21
     3c8:	a9 9a       	sbi	0x15, 1	; 21
     3ca:	a9 9a       	sbi	0x15, 1	; 21
     3cc:	a9 98       	cbi	0x15, 1	; 21
     3ce:	a9 9a       	sbi	0x15, 1	; 21

   // Startbit, a bunch of data bits and a stop bit.
   #define DEBUG(X) (DEBUG_((X<<1) | 0x0100))
   #define DEBUG_(X)   (_BIT(X,0), \
         _BIT(X,1), _BIT(X,2), _BIT(X,3),_BIT(X,4),\
         _BIT(X,5), _BIT(X,6), _BIT(X,7), \
         _BIT(X,8))

#include 

int main(void) {

  int byte, bit;
  
  fputs("#define DEBUG(X) \\\n", stdout);
  fputs("  switch (X) { \\\n", stdout);
  for (byte=0; byte<128; byte++) {
    printf("    case 0x%02X: \\\n", byte);
    fputs ("      __asm__ __volatile__ ( \\\n", stdout);
    // Start bit
    fputs("          \"cbi %[port], %[pin] \\n\\t\" \\\n", stdout);
    // Data bits
    for (bit=0; bit<7; bit++) {
      if (byte & (1<<bit)) {
        fputs("          \"sbi %[port], %[pin] \\n\\t\" \\\n", stdout);
      }
      else {
        fputs("          \"cbi %[port], %[pin] \\n\\t\" \\\n", stdout);
      }
    }
    // Stop bit
    fputs("          \"sbi %[port], %[pin] \\n\\t\" \\\n", stdout);
    fputs("       :: [port] \"I\" (_SFR_IO_ADDR(DEBUG_PORT)), \\\n", stdout);
    fputs("          [pin]  \"I\" (DEBUG_BIT)); \\\n", stdout);
    fputs("      break; \\\n", stdout);
  }
  fputs("  }\n\n", stdout);

  return 0;
  
}

///////////////////////////////////////////////////////////////////////////////

I do not get the point. Syntax? File level? Something else?

No need for clobber: only a port register changes.

In-line assembly implicitly clobbers R0.

I should have written "avr-gcc implicitly puts R0 in the clobber list."
...
To me, this is a requirements flaw.
If the compiler is using R0 for pretty much anything, the flaw will cost at least two cycles.

Unlike Extended asm, all Basic asm blocks are implicitly volatile. See Volatile. Similarly, Basic asm blocks are not treated as though they used a "memory" clobber (see Clobbers).