On 29/06/18 21:55, Sven Barth via fpc-pascal wrote:
Am 29.06.2018 um 18:45 schrieb Alan Krause:
I stumbled upon something the other day that was causing numerical
differences between compiled Delphi and FPC code. Executing the
following sample console application illustrates the issue clearly:
program test;
uses
math, SysUtils;
var
arg1 : double;
arg2 : double;
res : double;
begin
arg1 := 100000.00;
arg2 := 72500.51;
writeln( 'arg1 = ' + FormatFloat( '0.00000000', arg1 ) );
writeln( 'arg2 = ' + FormatFloat( '0.00000000', arg2 ) );
res := arg1 - arg2;
writeln( 'arg1 - arg2 = ' + FormatFloat( '0.00000000', res ) );
writeln( 'Max( arg1 - arg2, 0 ) = ' + FormatFloat( '0.00000000',
Max( res, 0 ) ) );
writeln( 'Max( arg1 - arg2, 0.0 ) = ' + FormatFloat( '0.00000000',
Max( res, 0.0 ) ) );
end.
--- begin output (Linux x86_64) ---
arg1 = 100000.00000000
arg2 = 72500.51000000
arg1 - arg2 = 27499.49000000
*Max( res, 0 ) = 27499.49023438*
Max( res, 0.0 ) = 27499.49000000
--- end output ---
I am guessing that the integer value of zero is causing the wrong
overloaded function to be called? I was able to solve the problem in
my code by replacing the 0 with 0.0.
The compiler converts the 0 to the type with the lowest precision that
can hold the value (or the largest if none can hold it exactly). For 0
this is already satisfied by Single, so the compiler essentially has the
parameter types Double and Single. For some reason (I don't know whether
it's due to a bug or by design) it picks the Single overload instead of
the Double one.
Someone who knows more about the compiler's overload handling would need
to answer why it favors (Single, Single) over (Double, Double) for
(Double, Single) parameters (or (Single, Double), the order doesn't
matter here).
Regards,
Sven
More confusingly, if a single variable is used, the expected Max(Double,
Double) is called:
function Max(a, b: Double): Double; overload;
begin
WriteLn('Double');
if a > b then Result := a else Result := b;
end;
function Max(a, b: Single): Single; overload;
begin
WriteLn('Single');
if a > b then Result := a else Result := b;
end;
var
v1: Double;
v2: Single;
begin
v1 := Pi;
v2 := 0;
WriteLn(v1);
WriteLn(Max(v1,0));
WriteLn(Max(v1,0.0));
WriteLn(Max(v1,v2));
end.
Prints:
3.1415926535897931E+000
Single
3.141592741E+00
Double
3.1415926535897931E+000
Double
3.1415926535897931E+000
If this is not a bug, it would be very helpful if the compiler could
print a warning whenever a value is implicitly converted from double to
single.
Colin
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