Re: [avr-libc-dev] Suggested improvement to <avr/pgmspace.h>

[Marko Mäkelä]

On Mon, Jan 02, 2017 at 05:27:19PM +0100, Georg-Johann Lay wrote:
> On 02.01.2017 11:58, Marko Mäkelä wrote:
> You can use the __flash address space instead of PROGMEM + pgm_read, 
> cf.
>
> https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html
>
> In your case, the function would read:
>
> static void msg (const __flash char *msg)
> {
>  char c;
>  while ((c = *msg++))
>  {
>     UDR0 = c;
>     loop_until_bit_is_set (UCSR0A, UDRE0);
>  }
> }
>
> The __flash address space is implemented by the compiler as a GNU 
> extension to the C language, and hence -std=gnu99 (or higher) is 
> needed.

Thank you, this is much cleaner, and like you pointed out, this allows 
portable code to be written when using typedef or #define. I guess 
<avr/pgmspace.h> now only exists for backward compatibility with old 
code?

I would like to follow up with some avr-gcc or avr-g++ remarks (sorry 
for the off-topic discussion).

It looks like avr-g++ does not accept the __flash qualifier. Are there 
plans to support it?

> There is no guarantee for an optimal code being generated for that 
> example (as applies to any other C code).

It turns out that the compiled code is identical with my asm-accelerated 
version. The entry into the loop looks like this:

 ae:    85 91           lpm     r24, Z+
 b0:    81 11           cpse    r24, r1
 b2:    f7 cf           rjmp    .-18            ; 0xa2 <main+0x12>
 b4:    f3 cf           rjmp    .-26            ; 0x9c <main+0xc>

The above is also emitted for -mmcu=attiny2313. With -mmcu=at90s2313, 
the "mov" from the temporary register r0 could be omitted:

 48:    c8 95           lpm
 4a:    80 2d           mov     r24, r0
 4c:    31 96           adiw    r30, 0x01       ; 1
 4e:    81 11           cpse    r24, r1
 50:    f8 cf           rjmp    .-16            ; 0x42 <__SREG__+0x3>
 52:    f4 cf           rjmp    .-24            ; 0x3c <main+0x8>

Best regards,

        Marko