Compiler bug - Fast Code does not give same results as No Optmization

[kreniska]

The following program gives different results when comiled for Fast Code versus No Optimization.
When running in the IDE the results are the same as No Optmization.
But, when compiled for Fast Code, the exe gives different results.
In ths example, the after setting the FltData array value, when reading the same value it does not return the value that was just set.
Changing little things in the code changes the results - like removing the for loop, for example.

If this one example shows that the results can be incorrect with FAST CODE optimization, how can I trust Fast Code for any of my projects?

Option Explicit

Const COL_FLTS = 1
Const COL_FTIME = 2

Public FltData(100, 7, 1) As Long
Public xCOL_FTIME As Long, xCOL_FLTS As Long


Sub Main()
    Dim x As Long, j As Long
    Dim rw As Long, mon As Long, comment As String
    rw = 1
    mon = 0
    comment = "main "
    xCOL_FLTS = 1: xCOL_FTIME = 2
    
Call testLongArray(rw, mon, "testlong #1")
'Call testLongArray(rw, mon, "testlong #2")


FltData(rw, COL_FLTS, mon) = FltData(rw, COL_FLTS, mon) + 1000
x = FltData(rw, COL_FTIME, mon) + 100
FltData(rw, COL_FTIME, mon) = x
For j = 1 To 12
Next j
If x <> FltData(rw, COL_FTIME, mon) Then
    MsgBox (comment & " no joy " + Str(x) + Str(FltData(rw, COL_FTIME, mon)) + Str(FltData(rw, COL_FLTS, mon)))
Else
    MsgBox (comment & " OK " + Str(x) + Str(FltData(rw, COL_FTIME, mon)) + Str(FltData(rw, COL_FLTS, mon)))
End If

Call testLongArray(rw, mon, "testlong #3 ")
End Sub

Function testLongArray(ByVal rw As Long, ByVal mon As Long, ByVal comment As String)
    Dim j As Long, x As Long
    
FltData(rw, COL_FLTS, mon) = FltData(rw, COL_FLTS, mon) + 10
x = FltData(rw, COL_FTIME, mon) + 100
FltData(rw, COL_FTIME, mon) = x
For j = 1 To 12
Next j
If x <> FltData(rw, COL_FTIME, mon) Then
    MsgBox (comment & " no joy" + Str(x) + Str(FltData(rw, COL_FTIME, mon)) + Str(FltData(rw, COL_FLTS, mon)))
Else
    MsgBox (comment & " OK " + Str(x) + Str(FltData(rw, COL_FTIME, mon)) + Str(FltData(rw, COL_FLTS, mon)))
End If

End Function