Question

i have bit of code that causes an underflow:

var
    t1, t2, delta: DWORD:
begin
   t1 := 0xffffff00;
   t2 := 0x00000037;

   delta := (t2 - t1);

The subtraction itself does generate an overflow (underflow), but i don’t want Delphi to throw an EIntOverflow exception. So i try disabling the generation of overflow checking code by disabling overflow checking:

var
    t1, t2, delta: DWORD:
begin
   t1 := 0xffffff00;
   t2 := 0x00000037;

{$OVERFLOWCHECKS OFF}
   delta := (t2 - t1);
{$OVERFLOWCHECKS ON}

Yet even with the OVERFLOWCHECKS OFF option, it still throws an exception. And the generated code still contains the check:

A reminder of the documentation on $Q:

Overflow checking

Type Switch
Syntax {$Q+} or {$Q-}
{$OVERFLOWCHECKS ON} or {$OVERFLOWCHECKS OFF}
Default {$Q-}
{$OVERFLOWCHECKS OFF}
Scope Local

Remarks

The $Q directive controls the
generation of overflow checking code.
In the {$Q+} state, certain integer
arithmetic operations (+, -, *, Abs,
Sqr, Succ, Pred, Inc, and Dec) are
checked for overflow. The code for
each of these integer arithmetic
operations is followed by additional
code that verifies that the result is
within the supported range. If an
overflow check fails, an EIntOverflow
exception is raised (or the program is
terminated if exception handling is
not enabled).

The $Q switch is usually used in
conjunction with the $R switch, which
enables and disables the generation of
range-checking code. Enabling overflow
checking slows down your program and
makes it somewhat larger, so use {$Q+}
only for debugging.

How do i use $OVERFLOWCHECKS OFF to disable the generation of overflow checking code?

---

Mason’s answer worked. The revised code is:

var
    t1, t2, delta: DWORD:
begin
   t1 := 0xffffff00;
   t2 := 0x00000037;

   delta := Subtract(t2, t1);


{$OVERFLOWCHECKS OFF}
function Subtract(const B, A: DWORD): DWORD; //subtract B-A
begin
   {
      Disabling overflow checking does not work at the line level,
      only the routine level. 
      Hence the function to subtract two numbers.
   }
   Result := (B-A);
end;
{$OVERFLOWCHECKS ON}

For google crawler, alternate question phrasing: How to temporarily disable overflow checking in Delphi?

Answer 1

It doesn’t work at the line level. You need to turn it off for the entire function.