Your compiler are using the following formula:

m = -4 - (Trunc(-4 / 5) * 5)
m = -4 - (Trunc(-0.8) * 5)
m = -4 - (0 * 5)
m = -4 - 0
m = -4

Instead of:

m = -4 - (Floor(-4 / 5) * 5)
m = -4 - (Floor(-0.8) * 5)
m = -4 - (-1 * 5)
m = -4 - (-5)
m = 1