#3988 Z80 Incorrect code optimization

[Arjan Bakker]

I have this (bad) code:

#include <stdio.h>

unsigned short FadeColor(unsigned short color, unsigned char step)
{
    signed short c1 = (color & 0x07) - step;
    signed short c2 = ((color & 0x070)>>4) - step;

    if (c1<0) c1=0;
    if (c2<0) c2=0;

    return c1 + (c2<<4);
}

void main()
{
    unsigned short color = FadeColor(0x077, 4);

    printf("color %02x\n", color);
}

__sfr __at 0xff sif;
int putchar(int c) {

    sif = 'p';
    sif = c;

    return c;
}

I'm using sdcc --version
SDCC : mcs51/z80/z180/r2k/r2ka/r3ka/r4k/r5k/r6k/sm83/tlcs90/ez80/z80n/r800/ds390/pic16/pic14/TININative/ds400/hc08/s08/stm8/pdk13/pdk14/pdk15/mos6502/mos65c02/f8/f8l TD- 4.5.24 #16523 (MINGW64)

Running it with:
sdcc -mz80 --no-peep --opt-code-size --max-allocs-per-node 50000 ./bug.c -o ./bug.ihx && ucsim_z80 -I if=outputs[0xff] ./bug.ihx

I expect it to print: color 33
But instead I get this: color 03

This incorrect code has been generated:

;./bug.c:6: signed short c2 = ((color & 0x070)>>4) - step;
    ld  a, l
    and a, #0x70
    ld  h, #0x00
    ld  a, #0x04
00123$:
    srl h
    rr  a
    dec a
    jp  nz, 00123$
    sub a, e
    ld  l, a
    sbc a, a
    ld  h, a

After and a, #0x70 the result should be loaded back to L, right shift HL and load L back into A before sub a, e. If I lower the --max-allocs-per-node to 10000 it does exactly that. It also works correctly on SDCC 4.5.0 with --max-allocs-per-node set to 50000 (and even with higher settings) .

[Philipp Klaus Krause]

I can reproduce the issue using SDCC from current trunk on my Debian GNU/Linux testing on amd64 system.

[Philipp Klaus Krause]

Fixed in [r16532].