Re: [avr-gcc-list] Stack use - possible bug

[Wouter van Gulik]

Jan Waclawek schreef:
> > Yes, I can agree to that, but I don't know if I could fully characterize =
> > this as a bug.

Considering this code:

#include <avr/io.h>
#include <stdio.h>

static void test0(void) { char buf[100]; puts(buf);}
static void test1(void) { char buf[110]; puts(buf);}
static void test2(void) { char buf[120]; puts(buf);}
static void test3(void) { char buf[130]; puts(buf);}
static void test4(void) { char buf[140]; puts(buf);}
static void test5(void) { char buf[150]; puts(buf);}
static void test6(void) { char buf[160]; puts(buf);}
static void test7(void) { char buf[170]; puts(buf);}
static void test8(void) { char buf[180]; puts(buf);}
static void test9(void) { char buf[190]; puts(buf);}

int main(void)
{
    test0();
    test1();
    test2();
    test3();
    test4();
    test5();
    test6();
    test7();
    test8();
    test9();
}

Running on an atmega48 with only 512 bytes this will "work" (at first 
glance). But the optimizer creates this:

        push r29
        push r28
        in r28,__SP_L__
        in r29,__SP_H__
        subi r28,lo8(-(-1450))
        sbci r29,hi8(-(-1450))
        in __tmp_reg__,__SREG__
        cli
        out __SP_H__,r29
        out __SREG__,__tmp_reg__
        out __SP_L__,r28

Which just is bogus since it is beyond memory.
I would consider this a bug.

Interestingly making all functions use buf[100] the optimizer gets smart 
and only uses 100 once:

        push r16
        push r17
        push r29
        push r28
        in r28,__SP_L__
        in r29,__SP_H__
        subi r28,lo8(-(-100))
        sbci r29,hi8(-(-100))
        in __tmp_reg__,__SREG__
        cli
        out __SP_H__,r29
        out __SREG__,__tmp_reg__
        out __SP_L__,r28

Here my knowledge of GCC stack optimization stops.

HTH,

Wouter