[cc65] Strange behaviour of a simple code

In these last days i'm really puzzled with a strange behaviour with a cc65 simple program, don't know if is some kind of bug or a bad code from my part but can't find where the problem can be.

The result of the main code is not right:

1.  FROM:  17600
1.  TO:  30719
3.  FROM:  17473
3.  TO:  30209

but if uncomment the two commented printf lines then the result is the expected:

1.  FROM:  17600
1.  TO:  30719
2.  FROM:  17600
2.  TO:  30336
3.  FROM:  17728
3.  TO:  30464


why is the behaviour changed only for the printf lines? Any ideas?


test.c
------

#include <stdio.h>

void inc_test(void);

unsigned int from, to;


// MAIN
void main(void)
{
 from = 17600;
 to = 30719;
 
 printf("1. FROM: %u\n", from);
 printf("1. TO: %u\n", to);
 
 if (to >= 0x7740)   // 30528
 {
  to = 0x7680;  // 30336
 }
 else
 {
  to = 0x7740;
 }
 
 //printf("2. FROM: %u\n", from);
 //printf("2. TO: %u\n", to);
 
 inc_test();
 ++from;
 ++to;
 
 inc_test();
 ++from;
 ++to;

 printf("3. FROM: %u\n", from);
 printf("3. TO: %u\n", to);
}



inctest.s
---------

.export _inc_test

.import _from
.import _to


.CODE

; ---------------------------------------------------------------
; _inc_test
; ---------------------------------------------------------------
_inc_test:

 ldx #$15

@LGO: 

 lda _from
 adc #$03
 sta _from
 lda _to
 adc #$03
 sta _to

 dex
 bne @LGO

 rts



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