Re: porting GCC to a micro with a very limited addressing mode --- what to write in LEGITIMATE_ADDRESS, LEGITIMIZE_ADDRESS and micro.md ?!

[Sergio Ruocco <sergio dot ruocco at gmail dot com>]

Now my GO_IF_LEGITIMATE_ADDRESS refuses anything that is not a REG
or a CONSTANT_ADDRESS:

int legitimate_address1(enum machine_mode MODE,rtx X)
{
	if(CONSTANT_ADDRESS_P(X))
                return 1;
	if(GET_CODE(X)==REG && is_base_reg(REGNO(X)))
                return 1;

	return 0; /* fails everything else */

} /* this is the strict version, the non strict version is similar */

but GCC (4.0.2, just in case the version is relevant) still aborts the
compilation.


Then I found this wiki note about forcing complex addresses into
registers: http://gcc.gnu.org/wiki/WritingANewBackEnd

...
rtx copy_addr_to_reg (rtx x)
    Equivalent to copy_to_mode_reg (Pmode, x). For example, this
function can be used to compute a complex address X in a register for an
instruction which supports only register indirect addressing. See also
replace_equiv_address() below.
...


Thus I changed in the .md file the movXX RTL expand macro to force any
MEM expression into a register:

(define_expand "movhi" /*  my micro is 16 bit... */
        [(set (match_operand:HI 0 "nonimmediate_operand" "")
              (match_operand:HI 1 "general_operand" "")
        )]
        ""
	{
	if(!no_new_pseudos && GET_CODE(operands[0])==MEM) {
		if( /* addr in operands[0] == base reg + offset */ )
			operands[0] = copy_addr_to_reg ( operands[0] );
	}
)

The GCC still fails to generate the assembly code to do the arithmetic
computation of the baseReg+offset-->tempReg, and then use (tempReg) as
address.

Note that with the current MD GCC is able to generate simple sums like
R1 = R2 + R3 and R1 = R2 + IMM, thus the basic math to compute an
address is there.

Any suggestion on what I am doing wrong ?

	Sergio


Michael Hope wrote:
> Hi Sergio.  My port has similar addressing modes - all memory must be
> accessed by one of two registers and can only be accessed indirectly,
> indirect with pre increment, and indirect with post increment.  The
> key is GO_IF_LEGITIMATE_ADDRESS and the legitimate address helper
> function.  Mine looks like this:
> 
> /* Return 1 if the address is OK, otherwise 0.
>    Used by GO_IF_LEGITIMATE_ADDRESS.  */
> 
> bool
> tomi_legitimate_address (enum machine_mode mode ATTRIBUTE_UNUSED,
> 			 rtx x,
> 			 bool strict_checking)
> {
>   /* (mem reg) */
>   if (REG_P (x)
>       && tomi_reg_ok (x, strict_checking)
>       )
>     {
>       return 1;
>     }
> 
>   if (GET_CODE(x) == PRE_DEC)
>     {
> ...
>     }
> 
>   if (GET_CODE(x) == POST_INC)
>     {
> ...
>     }
> 
>   return 0;
> }
> 
> tomi_reg_ok returns true if x is any register when strict checking is
> clear and true if x is one of my addressing registers when strict
> checking is set.
> 
> GCC will feed any memory accesses through this function to see if they
> are directly supported, and if not it will break them up into
> something smaller and try again.
> 
> Hope that helps,
> 
> -- Michael
> 
> 
> 2010/1/26 Sergio Ruocco <sergio.ruocco@gmail.com>:
>> Gabriel Paubert wrote:
>>> On Mon, Jan 25, 2010 at 01:34:09PM +0100, Sergio Ruocco wrote:
>>>> Hi everyone,
>>>>
>>>> I am porting GCC to a custom 16-bit microcontroller with very limited
>>>> addressing modes. Basically, it can only load/store using a (general
>>>> purpose) register as the address, without any offset:
>>>>
>>>>      LOAD (R2) R1    ; load R1 from memory at address (R2)
>>>>      STORE R1 (R2)   ; store R1 to memory at address (R2)
>>>>
>>>> As far as I can understand, this is more limited than the current
>>>> architectures supported by GCC that I found in the current gcc/config/*.
>>> The Itanium (ia64) has the same limited choice of addressing modes.
>>>
>>>       Gabriel
>> Thanks Gabriel.
>>
>> I dived into the ia64 md, but it is still unclear to me how the various
>> parts (macros, define_expand and define_insn in MD etc.) work together
>> to force the computation of a source/dest address plus offset into a
>> register... can anyone help me with this ?
>>
>> Thanks,
>>
>>        Sergio
>>