Re: [fpc-devel] wrong result for abs(low(int64))
On Thu, Apr 4, 2024 at 9:49 PM Martin Frb via fpc-devel wrote: > The below writes: -9223372036854775808 > > Shouldn't absolute return a positive number? > > program Project1; > begin >writeln( abs(low(int64)) ); > end. See https://gitlab.com/freepascal.org/fpc/source/-/issues/40694 There were several commits about Abs w.r.t. this bug lately. -- Bart ___ fpc-devel maillist - fpc-devel@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
Re: [fpc-devel] wrong result for abs(low(int64))
On 04/04/2024 16:39, J. Gareth Moreton via fpc-devel wrote: Essentially, an arithmetic overflow is happening. Since the largest Int64 possible is 9,223,372,036,853,775,807, going one above that (the result to abs(low(int64))) wraps back around to -9,223,372,036,853,775,808. Internally, you can think about negating (positing?) a negative number as inverting the bits and then adding one (aka. two's complement), so low(int64) is 1000...000, which inverted becomes 0111...111, and then adding one results in 1000...000 again. I got that bit... Initially I was surprised to find "abs" to return a signed number. After all, it will never be negative. But, in the meantime I recalled that unsigned numbers will cause other issues when used in expressions. "abs(-1)-2" would behave different. ___ fpc-devel maillist - fpc-devel@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
Re: [fpc-devel] wrong result for abs(low(int64))
Essentially, an arithmetic overflow is happening. Since the largest Int64 possible is 9,223,372,036,853,775,807, going one above that (the result to abs(low(int64))) wraps back around to -9,223,372,036,853,775,808. Internally, you can think about negating (positing?) a negative number as inverting the bits and then adding one (aka. two's complement), so low(int64) is 1000...000, which inverted becomes 0111...111, and then adding one results in 1000...000 again. Kit On 04/04/2024 14:14, Martin Frb via fpc-devel wrote: The below writes: -9223372036854775808 Shouldn't absolute return a positive number? program Project1; begin writeln( abs(low(int64)) ); end. Seems writeln( abs(low(longint)) ); also returns negative... ___ fpc-devel maillist - fpc-devel@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel ___ fpc-devel maillist - fpc-devel@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
Re: [fpc-devel] wrong result for abs(low(int64))
Ahoy, hoy, On 2024‑04‑04 15:14:38 +0200, Martin Frb via fpc-devel wrote: > The below writes: -9223372036854775808 > > Shouldn't absolute return a positive number? No, see https://wiki.freepascal.org/Integer#characteristics -- Sincerely yours, Kai Burghardt signature.asc Description: PGP signature ___ fpc-devel maillist - fpc-devel@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
[fpc-devel] wrong result for abs(low(int64))
The below writes: -9223372036854775808 Shouldn't absolute return a positive number? program Project1; begin writeln( abs(low(int64)) ); end. Seems writeln( abs(low(longint)) ); also returns negative... ___ fpc-devel maillist - fpc-devel@lists.freepascal.org https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
