RE: [avr-gcc-list] -fshort-enums are not so short

[Ben Mann]

I "suppose" I assume that it is related to the size of int which is 16 bits
in AVR, however I never thought twice about it - I always assumed it implied
"short" was 16-bits, as in the "short" data type.. Theoretically it could be
changeable using -mint8, but this option has the intriguing comment in the
avr-libc manual:
"Note that this is not really supported by avr-libc, so it should normally
not be used."

I had a similar problem with something like this:

unsigned char a = something;
switch(a) {
        case 1:
                foo();
                break;
        case 2:
                bar();
                break;
}

Which sets a to (say) r24/r25 with r25=0, then does a 2-byte comparison on
each value.

For my speed-critical state machine I was a little miffed.

I ended up "happy" with

unsigned char a = something;
if (0)
else if (a == 1) goto CASE_1;
else if (a == 2) goto CASE_2;

and you can imagine the rest - the resulting jump table is much smaller when
you have to iterate a large number of cases.

The solution seemed quick and easy (though dirty) and once again I'd be keen
to see the outcome of your query.

Ben Mann



-----Original Message-----
From: address@hidden [mailto:address@hidden
On Behalf Of Bernard Fouché
Sent: Wednesday, 24 November 2004 7:58 PM
To: address@hidden
Subject: [avr-gcc-list] -fshort-enums are not so short


Hi.

Compiling with gcc 3.4.2 with -Os -fshort-enums, it appears that enums are
processed on 16 bits and not 8 when they could be on 8 bits. (when looking
at 'man gcc' it is said that this option will make the compiler use the
smallest possible type of int to process the enum, I assume that for the avr
it is 8 bits)

C code:

enum Foo { FOO_1, FOO_2 } Foo;
uint8_t v;

main()
{
  switch(Foo){
  case FOO_1:
    v=1;
    break;
  case FOO_2:
    v=2;
    break;
  }
}

compilation result:

000000ca <main>:
enum Foo { FOO_1, FOO_2 } Foo;
uint8_t v;

main()
{
  ca:   cf ef           ldi     r28, 0xFF       ; 255
  cc:   d0 e1           ldi     r29, 0x10       ; 16
  ce:   de bf           out     0x3e, r29       ; 62
  d0:   cd bf           out     0x3d, r28       ; 61
  switch(Foo){
  d2:   80 91 02 01     lds     r24, 0x0102
  d6:   99 27           eor     r25, r25
  d8:   00 97           sbiw    r24, 0x00       ; 0
  da:   19 f0           breq    .+6             ; 0xe2
  dc:   01 97           sbiw    r24, 0x01       ; 1
  de:   19 f0           breq    .+6             ; 0xe6
  e0:   05 c0           rjmp    .+10            ; 0xec
  case FOO_1:
    v=1;
  e2:   81 e0           ldi     r24, 0x01       ; 1
  e4:   01 c0           rjmp    .+2             ; 0xe8
    break;
  case FOO_2:
    v=2;
  e6:   82 e0           ldi     r24, 0x02       ; 2
  e8:   80 93 01 01     sts     0x0101, r24
    break;
  }
}
...


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