On 29 Nov 2013, at 11:00, Klaus Hartnegg wrote:
Explanation: I'm trying to port code from Turbo Pascal. There word1-
word2 does wrap around with {$Q-}{$R+}. And dec(word1) never checks
for overflow, even with {$Q+}{$R+}.
FreePascal seems to have a wider definition of Range Check, and the
inc() and dec() functions do check for overflow.
The longint trick prevents runtime error 201, but causes runtime
error 215 if the longword value does not fit into longint.
On closer look this is all behaving like documented, just different
from TurboPascal (maybe it would have worked in TP-compatibility-
mode).
The correct fix is to simply add {$R-} to the code wherever it
currently has {$Q-}. I have just done this, and now it passes the
test.
It's due to the fact that (then) Borland initially implemented range
checking in an incomplete way, and didn't detect range errors that
were caused by overflow in a register. They probably decided to add
this to a separate check because just adding this functionality to {$R
+} could break existing programs. However, semantically there is no
difference between a range error and an overflow error: in both cases
a value out of range is produced.
The reason that you sometimes get range errors in FPC while you would
get overflow errors in TP, is because in Pascal by default all
operations are carried out using the CPU native integer type. In TP
this is 16 bit, in FPC 32 or 64 bit. As a result, several calculations
that would overflow in TP become range errors in FPC.
Jonas
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