#3329 --callee-saves-bc broken since at least 4.1.0

[Sebastian Riedel]

I noticed that sm83’s calling convention would make --callee-saves-bc basically impossible. (2B return in BC)
But my tests showed that --callee-saves-bc is generally ignored on sm83 and z80.

hello.c:

#include <assert.h>
unsigned char func (unsigned int a) {
    return (a>>8)&0xF0 | (a&0x0F);
}
int main (void) {
    unsigned int b = 0;
    for (unsigned char i = 8; i > 0; --i) {
        b += func(0x1231);
    }
    assert(b == 0x88);return b;
}

./4.2.0/sdcc -S -msm83 hello.c and ./4.2.0/sdcc -S -msm83 --callee-saves-bc hello.c produce:

_func::
    ld  a, d
    and a, #0xf0
    ld  c, a
    ld  a, e
    and a, #0x0f
    or  a, c
    ret
...
    push    bc
    ld  de, #0x1231
    call    _func
    pop bc
...

./4.2.0/sdcc -S -msm83 --sdcccall 0 hello.c and ./4.2.0/sdcc -S -msm83 --sdcccall 0 --callee-saves-bc hello.c produce:

_func::
    ldhl    sp, #3
    ld  a, (hl-)
    and a, #0xf0
    ld  c, a
    ld  a, (hl)
    and a, #0x0f
    or  a, c
    ld  e, a
    ret
...
    push    bc
    push    de
    ld  hl, #0x1231
    push    hl
    call    _func
    pop hl
    ld  l, e
    pop de
    pop bc
...

./4.1.0/sdcc -S -mgbz80 hello.c and ./4.1.0/sdcc -S -mgbz80 --callee-saves-bc hello.c produce:

_func::
    ldhl    sp, #3
    ld  a, (hl)
    and a, #0xf0
    ld  c, a
    dec hl
    ld  a, (hl)
    and a, #0x0f
    or  a, c
    ld  e, a
    ret
...
    push    bc
    push    de
    ld  hl, #0x1231
    push    hl
    call    _func
    add sp, #2
    ld  a, e
    pop de
    pop bc
...

It looks similar on z80.