[Bug target/31644] [avr] can't find a register in class 'BASE_POINTER_REGS' while reloading 'asm'

["wvangulik at xs4all dot nl" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #5 from wvangulik at xs4all dot nl  2007-11-01 21:50 -------
It seems that this is caused by the fact that eeprom_read_word is actually a
piece of inline assembler returning it's value in the Z register, using z as
"pointer class" description.

This then somehow eliminates the compiler's ability allocate a pointer.

Here is my test file source:
compile using: -Wall, -Os -mmcu=atmega16

===========================================================
#define u16 unsigned int

//If both are defined it will not compile

#define EXTRA_VAR   //Use this to create extra variable parsing
#define USE_REG_Z   //Use this to make use of Z register

static inline u16 eeprom_read_word(u16* p)
{

#ifdef USE_REG_Z
  register u16 result asm("r30");
#else
  register u16 result asm("r24");
#endif  

  asm volatile (
  " ; Do nothing, just fake write to result"
      : "+x" (p),
         "=r" (result)
      :  );    
  return result;  

}


extern void foo(  u16 in0,
                  u16 in1, 
                  u16 in2, 
                  u16 in3, 
                  u16 in4, 
#ifdef EXTRA_VAR
                  u16 in5, 
#endif
                  u16 in6);


void bar(void) 
{
  u16 in0 = eeprom_read_word((u16*)0);
  u16 in1 = eeprom_read_word((u16*)1);
  u16 in2 = eeprom_read_word((u16*)2);
  u16 in3 = eeprom_read_word((u16*)3);
  u16 in4 = eeprom_read_word((u16*)4);
  u16 in5 = eeprom_read_word((u16*)5);
  u16 in6 = eeprom_read_word((u16*)6);

  foo(  in0,
        in1,
        in2,
        in3,
        in4,
#ifdef EXTRA_VAR
        in5, 
#endif
        in6 );

  foo(  in0,
        in1,
        in2,
        in3,
        in4,
#ifdef EXTRA_VAR
        in5, 
#endif
        in6  );  

}

When compiling you can see that adding an extra variable gives the problem. But
then changing the return variable to r24 solves the problem.

Hmm while typing I found another fishy thing:
Compiling for r24 with and without extra_var gives pushes of all registers.
However r12 is not used when compiled for non extra var. But using extra stack
instead

And trying -mcall-prologues does not invoke the prologue call when using extra
var and r24... it does for the other 2 ok compiling combinations.

I hope someone can make something out of this.

Oh and the r30 instead of z gives the following (more descriptive) error:

(insn 14 12 16 0 (set (reg/v:HI 54 [ in0 ])
        (reg/v:HI 30 r30 [ result ])) 12 {*movhi} (insn_list:REG_DEP_TRUE 12
(ni
l))
    (expr_list:REG_DEAD (reg/v:HI 30 r30 [ result ])
        (nil)))
test.c:67: confused by earlier errors, bailing out

This involves movhi* which is in other bugreports as well, maybe this is the
vital clue?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31644