We are initializating incy inconditionally:

    if (y1 <= y2) {
        incy = 1;
    } else {
        incy = -1;
    }

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 hw/display/artist.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/hw/display/artist.c b/hw/display/artist.c
index abacb0e27d..47f0e9f0bc 100644
--- a/hw/display/artist.c
+++ b/hw/display/artist.c
@@ -557,91 +557,90 @@ static void fill_window(ARTISTState *s, int startx, int starty,
 static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
                       bool update_start, int skip_pix, int max_pix)
 {
     struct vram_buffer *buf;
     uint8_t color = artist_get_color(s);
     int dx, dy, t, e, x, y, incy, diago, horiz;
     bool c1;
     uint8_t *p;
 
     trace_artist_draw_line(x1, y1, x2, y2);
 
     if (update_start) {
         s->vram_start = (x2 << 16) | y2;
     }
 
     buf = &s->vram_buffer[ARTIST_BUFFER_AP];
 
     c1 = false;
-    incy = 1;
 
     if (x2 > x1) {
         dx = x2 - x1;
     } else {
         dx = x1 - x2;
     }
     if (y2 > y1) {
         dy = y2 - y1;
     } else {
         dy = y1 - y2;
     }
     if (dy > dx) {
         t = y2;
         y2 = x2;
         x2 = t;
 
         t = y1;
         y1 = x1;
         x1 = t;
 
         t = dx;
         dx = dy;
         dy = t;
 
         c1 = true;
     }
 
     if (x1 > x2) {
         t = y2;
         y2 = y1;
         y1 = t;
 
         t = x1;
         x1 = x2;
         x2 = t;
     }
 
     horiz = dy << 1;
     diago = (dy - dx) << 1;
     e = (dy << 1) - dx;
 
     if (y1 <= y2) {
         incy = 1;
     } else {
         incy = -1;
     }
     x = x1;
     y = y1;
 
     do {
         if (c1) {
             p = buf->data + x * s->width + y;
         } else {
             p = buf->data + y * s->width + x;
         }
 
         if (skip_pix > 0) {
             skip_pix--;
         } else {
             artist_rop8(s, p, color);
         }
 
         if (e > 0) {
             artist_invalidate_lines(buf, y, 1);
             y  += incy;
             e  += diago;
         } else {
             e += horiz;
         }
         x++;
     } while (x <= x2 && (max_pix == -1 || --max_pix > 0));
 }
-- 
2.21.1

On Fri, Feb 14, 2020 at 01:12:59AM +0100, Philippe Mathieu-Daudé wrote:
> We are initializating incy inconditionally:
> 
>     if (y1 <= y2) {
>         incy = 1;
>     } else {
>         incy = -1;
>     }
> 
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
>  hw/display/artist.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/hw/display/artist.c b/hw/display/artist.c
> index abacb0e27d..47f0e9f0bc 100644
> --- a/hw/display/artist.c
> +++ b/hw/display/artist.c
> @@ -557,91 +557,90 @@ static void fill_window(ARTISTState *s, int startx, int starty,
>  static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
>                        bool update_start, int skip_pix, int max_pix)
>  {
>      struct vram_buffer *buf;
>      uint8_t color = artist_get_color(s);
>      int dx, dy, t, e, x, y, incy, diago, horiz;
>      bool c1;
>      uint8_t *p;
>  
>      trace_artist_draw_line(x1, y1, x2, y2);
>  
>      if (update_start) {
>          s->vram_start = (x2 << 16) | y2;
>      }
>  
>      buf = &s->vram_buffer[ARTIST_BUFFER_AP];
>  
>      c1 = false;
> -    incy = 1;
>  
>      if (x2 > x1) {
>          dx = x2 - x1;
>      } else {
>          dx = x1 - x2;
>      }
>      if (y2 > y1) {
>          dy = y2 - y1;
>      } else {
>          dy = y1 - y2;
>      }
>      if (dy > dx) {
>          t = y2;
>          y2 = x2;
>          x2 = t;
>  
>          t = y1;
>          y1 = x1;
>          x1 = t;
>  
>          t = dx;
>          dx = dy;
>          dy = t;
>  
>          c1 = true;
>      }
>  
>      if (x1 > x2) {
>          t = y2;
>          y2 = y1;
>          y1 = t;
>  
>          t = x1;
>          x1 = x2;
>          x2 = t;
>      }
>  
>      horiz = dy << 1;
>      diago = (dy - dx) << 1;
>      e = (dy << 1) - dx;
>  
>      if (y1 <= y2) {
>          incy = 1;
>      } else {
>          incy = -1;
>      }
>      x = x1;
>      y = y1;
>  
>      do {
>          if (c1) {
>              p = buf->data + x * s->width + y;
>          } else {
>              p = buf->data + y * s->width + x;
>          }
>  
>          if (skip_pix > 0) {
>              skip_pix--;
>          } else {
>              artist_rop8(s, p, color);
>          }
>  
>          if (e > 0) {
>              artist_invalidate_lines(buf, y, 1);
>              y  += incy;
>              e  += diago;
>          } else {
>              e += horiz;
>          }
>          x++;
>      } while (x <= x2 && (max_pix == -1 || --max_pix > 0));
>  }
> -- 
> 2.21.1
> 

Acked-by: Sven Schnelle <svens@stackframe.org>