[avr-gcc-list] [discussion] lpm support in avr-gcc

[Svein E. Seldal]

Hi,

Are there any plans on making the use of LPM built natively into gcc? 
When programming and using variables that are located into the 
program.data segment, accessing it is a *bit* troublesome. Look at the 
code on the bottom. It demonstrates how "complex" the avr-gcc's LPM 
function can make programming, while the second example shows my 
suggested implementation.

The principes of Harvard arch's isn't exactly new and other developers 
have solved this problem before and I've seen many excellent solutions 
to the challenges that the AVR architecture makes!

My suggestion is that when a string/struct/variable is declared as 
'const' it should be placed into the progmem.data segment. In 99% 
precent of the applications, things like strings are read-only and 
can/shold be placed in flash. And for the remaining 1% cases, just omit 
the const and the data will be placed in SRAM.

To be able to do it like this, changes to the compiler must be made. It 
needs to keep track of where the variable is stored and use either ld or 
lpm where appropriate, but my academic guess is that this is just a 
matter of attributes.

I know this has performance impacts, but wouldn't it be the coder's 
responsibility to decide when to use initialized data or using 
flash-stored consts?

Keep up the *good* work. avr-gcc is a very good acheivement and very 
useful tool. avr-gcc is being used in development ranging from private 
projects to commercial product development! Thanks.

Let the discussion begin.
Regards,
Svein


AVR Example:
-------------

  struct test {
    int number;
    const char *string;
  };

  const char PROGMEM string1[] = "string";
  const char PROGMEM string2[] = "string2";

  const struct test PROGMEM foo[] {
    { 344, string1 },
    { 945, string2 },
    { 0, NULL }
  };

  // This function prints the string 'str' from flash
  void puts_P(const char *str);

  void func(void) {
    struct test *f = foo;

    while(foo &&
          (str = (const char *)_LPM16(&foo->string)) )
    {
      puts_P(str);
      foo++;
    }
  }


Improved code:
--------------

  struct test {
    int number;
    const char *string;
  };

  const struct test foo[] {
    { 344, "string" },
    { 945, "string2" },
    { 0, NULL }
  };

  void func(void) {
    while(foo && foo->string)
    {
      puts(foo->string);
      foo++;
    }
  }

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