[avr-gcc-list] Uninitialized warning in 3.4.3

[David Kelly]

Another little issue I've run across recently:

This warning made me look twice at the generated code. Unless my brain
is totally fried the warning is wrong. The code within my
"#ifdef/#endif" gets optimized to nothing if enabled, and also mutes the
warning:

#define __bs_16(x) (((x) >> 8) | ((x) << 8))

extern void do_something( unsigned, unsigned );

void function( unsigned a, unsigned b )
{
        unsigned mbr, cnt;

#ifdef MUTE_WARNING
        mbr = cnt = 0;
#endif

        mbr = __bs_16(a);
        cnt = __bs_16(b);

        do_something( mbr, cnt );
}


Built like this:

% avr-gcc -O -gdwarf-2 -Wall -ffreestanding -mmcu=atmega128 -c junk.c
junk.c: In function `function':
junk.c:7: warning: 'mbr' might be used uninitialized in this function
junk.c:7: warning: 'cnt' might be used uninitialized in this function
%

Or quietened like this:
% avr-gcc -DMUTE_WARNING -O -gdwarf-2 -Wall -ffreestanding 
-mmcu=atmega128 -c junk.c
%

Either way the disassembly looks the same:

% avr-objdump -S junk.o

junk.o:     file format elf32-avr

Disassembly of section .text:

00000000 <function>:
void function( unsigned a, unsigned b )
{
        unsigned mbr, cnt;

        mbr = __bs_16(a);
   0:   49 2f           mov     r20, r25
   2:   58 2f           mov     r21, r24
        cnt = __bs_16(b);
   4:   27 2f           mov     r18, r23
   6:   36 2f           mov     r19, r22

        do_something( mbr, cnt );
   8:   b9 01           movw    r22, r18
   a:   ca 01           movw    r24, r20
   c:   0e 94 00 00     call    0x0
  10:   08 95           ret
%

Apparently my byte-swap macro is confusing avr-gcc. It doesn't confuse
FreeBSD gcc 3.4.2. OTOH its quite impressive how the optimizer did the
right thing.

--
David Kelly N4HHE, address@hidden
========================================================================
Whom computers would destroy, they must first drive mad.