Janko Stamenović - 2025-10-13

It seems to me that SDCC resolves the accesses to the globals and static like they would constantly have to be updated.

Starting with the function:

int gOffsetX = 100;

int calc(int i) {
    static int dx;
    dx = gOffsetX >> 4;
    dx += i;
    return dx;
}

even with

sdcc -mz80 --opt-code-speed --reserve-regs-iy \
    --max-allocs-per-node2900000 -c t.c

we get a lot of updates of the memory locations reserved for dx even if the temporary register pair would be better:

_calc::
    ex  de, hl
;t.c:5: dx = gOffsetX >> 4;
    ld  a, (#_gOffsetX)
    ld  (#_calc_dx_10000_2),a
    ld  a, (#_gOffsetX + 1)
    ld  hl, #_calc_dx_10000_2 + 1
    ld  (hl), a
    sra (hl)
    dec hl
    rr  (hl)
    inc hl
    sra (hl)
    dec hl
    rr  (hl)
    inc hl
    sra (hl)
    dec hl
    rr  (hl)
    inc hl
    sra (hl)
    dec hl
    rr  (hl)
;t.c:6: dx += i;
    ld  a, (hl)
    add a, e
    ld  (hl), a
    inc hl
    ld  a, (hl)
    adc a, d
    ld  (hl), a
;t.c:7: return dx;
    dec hl
    ld  e, (hl)
    inc hl
    ld  d, (hl)
;t.c:8: }
    ret

Explicitly using the temporary variable produces much better code:

int gOffsetX = 100;

int calc(int i) {
    static int dx;
    dx = gOffsetX >> 4;
    dx += i;
    return dx;
}

_calc_t::
    ld  c, l
    ld  b, h
;t.c:20: int tdx = gOffsetX >> 4;
    ld  de, (_gOffsetX)
    sra d
    rr  e
    sra d
    rr  e
    sra d
    rr  e
    sra d
    rr  e
;t.c:21: tdx += i;
    ex  de, hl
    add hl, bc
;t.c:22: dx = tdx;
    ld  (_calc_t_dx_10000_6), hl
;t.c:23: return tdx;
    ex  de, hl
;t.c:24: }
    ret

And to confirm the initial report as soon as --reserve-regs-iy is not specified, the calc is even with -max-allocs-per-node2900000 compiled (TD- 4.5.2 #15383) to:

_calc::
    ex  de, hl
;t.c:5: dx = gOffsetX >> 4;
    ld  a, (_gOffsetX+0)
    ld  (_calc_dx_10000_2+0), a
    ld  a, (_gOffsetX+1)
    ld  (_calc_dx_10000_2+1), a
    ld  iy, #_calc_dx_10000_2
    sra 1 (iy)
    rr  0 (iy)
    sra 1 (iy)
    rr  0 (iy)
    sra 1 (iy)
    rr  0 (iy)
    sra 1 (iy)
    rr  0 (iy)
;t.c:6: dx += i;
    ld  hl, #_calc_dx_10000_2
    ld  a, (hl)
    add a, e
    ld  (hl), a
    inc hl
    ld  a, (hl)
    adc a, d
    ld  (hl), a
;t.c:7: return dx;
    ld  de, (_calc_dx_10000_2)
;t.c:8: }
    ret
 

Last edit: Janko Stamenović 2025-10-13