[fpc-devel] Implicit Type Conversion when running with range and overflow checking

[Nikolay Nikolov]

On 02/25/2015 01:47 AM, Louis Salkind wrote:
> Pardon the stupid question(s)...  I was surprised by the behavior of 
> the following code when range checking and overflow checking was 
> enabled on win32:
>
> procedure test_rangeoverflow;
> var
>   a, b: BYTE;
>   i: Integer;
> begin
>   a := 128;
>   b := 129;
>   i := a + b;   // no errors, i is 257; a and b have been promoted to 
> integers
>                 // but shouldn't there be a range check, as a+b has 
> overflowed a byte and should not be promoted?
No, because both are implicitly converted to the natural integer type 
for the platform (32-bit on 32-bit platforms, 64-bit on 64-bit 
platforms, 16-bit on i8086), then the addition is done using that type.
>   i := BYTE(a + b);    // no errors, and i is 1.  Even if a+b is 
> promoted to integer,
>                        // shouldn't there be range check when coercing 
> and integer of 257 to byte?
No, because a typecast to a different integer type explicitly disables 
range checking. IOW, it means "I know what I'm doing, truncate this to a 
byte and just ignore the higher bits." If you want range checking, don't 
use a typecast.
>   a := a + b;    // gives range check when storing result, but not an 
> overflow error first
An overflow error is like a range check error, but is produced when you 
exceed the natural integer size of the processor. So, on a 32-bit 
platform, adding two 32-bit integers may produce an overflow error, 
while adding 16-bit integers may produce a range check error. AFAIK, 
this distinction is done for BP7 and Delphi compatibility reasons, but 
anyhow, you, as a programmer, can think of them as being the same thing.
> end;
>
> Comments?  I'm guessing this is either specified in the Pascal 
> standard, or fpc is emulating Delphi behavior.  Nevertheless, the 
> result seemed non-intuitive enough to me that I wanted to confirm this 
> was the intended behavior.
I'm not familiar with Pascal standards (although I thing at least 
Standard Pascal has only one integer type, so the standard says nothing 
about mixing different integer types), but this is the BP7 and Delphi 
behavior.

Nikolay