GCC Bugzilla Bug 39510

Code:

#include <avr/io.h>

register uint8_t test asm("r2");

int main(void)
{
    test = TCNT0;
}

Compiled with -Os results in the following instruction sequence:

00000092 <main>:
  92:   82 b7           in      r24, 0x32       ; 50
  94:   28 2e           mov     r2, r24
  96:   08 95           ret

This is particularly nasty for naked interrupts, where one would expect the
compiler to generate just a single instruction:

  in  r2, 0x32

Apparently the compiler behaves correctly when opposite transfer is requested,
generating single OUT opcode.

This bug is invalid.

Your test code is in main() which returns an int. The return value is stored in
r24. So the code is correct to store the value in both r2 (test) and r24.

See the avr-libc user manual, FAQ, for more information on the calling
convention:
http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_reg_usage

Perhaps you need to try this in a function that has void return.

I can't see how register usage and calling convention affect this.

As I said before, this behaviour bit me in a naked interrupt handler, with no
arguments and no return value.

I'm attaching a fixed test case where functions have no arguments and void
return type.

#include <avr/io.h>

register uint8_t test asm("r2");

void fun1(void)
{
    test = TCNT0;
}

void fun2(void)
{
    TCNT0 = test;
}

Compile with -Os. Generated instruction sequence:

00000000 <fun1>:
   0:   82 b7           in      r24, 0x32       ; 50
   2:   28 2e           mov     r2, r24
   4:   08 95           ret

00000006 <fun2>:
   6:   22 be           out     0x32, r2        ; 50
   8:   08 95           ret

Created an attachment (id=17545) [edit]
Second test case with void functions.

This might be the case hard registers are not always copy propogated except
into inline-asm.  Can you provide the preprocessed source?

Created an attachment (id=17553) [edit]
Preprocessed source of test case

Created with "avr-gcc -v -save-temps -mmcu=attiny13 -Os -c test.c -o test.o"

Confirmed on 4.3.2.