[avr-gcc-list] subroutine and 8bit types...

[Dmitry]

Fellows,
I have a question about compiler attributes and so.

First of all - small part of code:
=========================================

#include <inttypes.h>

uint32_t        __last;

uint32_t getlast()
{
        return __last;
}


==========================================

the avr-gcc produces:
---------------------------------
       .text
.global getlast
        .type   getlast,@function
getlast:
/* prologue: frame size=0 */
/* prologue end (size=0) */
        lds r24,__last
        lds r25,(__last)+1
        lds r26,(__last)+2
        lds r27,(__last)+3
        mov r22,r24
        mov r23,r25
        mov r24,r26
        mov r25,r27
/* epilogue: frame size=0 */
        ret
/* epilogue end (size=1) */
/* function getlast size 13 (12) */
-----------------------------------

It seems to me that there are little overhead:
The subroutine returns value of __last in registers r22-r25. 
But it loads r24-r27 first and the moves them ro r22-r25.

"lds" allows to load data from data space to any register. So, it seems to me 
that

uint32_t getlast()
{
        asm volatile(
                "lds r22,__last         \n\t"
                "lds r23,(__last)+1     \n\t"
                "lds r24,(__last)+2     \n\t"
                "lds r25,(__last)+3     "
        );
}

will be more efficient. But compiler complains about void return.

The question is: is it possible to tell the compiler, that within a particular 
call only
r22-r25 must be used ?

Thanks in advance


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