[avr-gcc-list] Program Memory

[Nils Kristian Strom]

Hi!

I have a feeling that avr-gcc dislikes program memory. I have tried
different approaches to make avr-gcc know that a variable is const, so
that it can save RAM.

I have searched for information about this topic, and my conclusion is
that gcc ALWAYS puts everything in SRAM.  Unless, of course, the
attribute progmem directive is used.  In this case, avr-gcc puts the
contents in Flash, and one MUST use the various progmem.h macros to
access the variable contents.

Please, tell me that this is not the case ;-(


The more recent AVR devices have a lpm instruction that is almost as
flexible as ld.  Is there work going on to try to make the compiler more
elegant with regards to the different memories  - at least for the newer
devices ?




As an example

const unsigned char foo = 0x12;

void foobar( unsigned char ch ){ asm( "nop" ); }

int main( void ) {
        foobar( foo );
}

gives the following assembly output: (avr-objdump -S)

void foobar( unsigned char ch ){ asm( "nop" ); }
  96:   cf 93           push    r28
-- snip --
        foobar( foo );
  c8:   80 91 60 00     lds     r24, 0x0060
  cc:   0e 94 4b 00     call    0x96

The optimal assembly would clearly be
        ldi     r24, 0x12
        call    0x96




Changing the declaration to 
const unsigned char __attribute__ ((progmem)) foo = 0x12;

puts foo in program memory, but the main() assembly output is
        foobar( foo );          
  ca:   80 91 54 00     lds     r24, 0x0054
  ce:   0e 94 4c 00     call    0x98

0x0054 is foo's address in program memory.


Regards
Nils


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