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Free Pascal => General => Topic started by: smaniok on August 09, 2020, 05:30:21 am

Title: Result mismatch on division by exponential notation and literal notation.
Post by: smaniok on August 09, 2020, 05:30:21 am
Hello everyone.

I found an inconsistency in results when dividing a float number by an integer in exponential notation and literal notation.

Ex, the following code used to round and crop a float number to six decimal places, using the division first by 1E6 (exponential notation) and last for 1000000 (literal notation as I mean):

Code: Pascal  [Select][+][-]
  1. program NumberTest;
  2.  
  3. const
  4.   NUM_CONST = 2.123456789;
  5. var
  6.   NumVar: Double;
  7.   Res1, Res2, Res3, Res4: Double;
  8. begin
  9.   NumVar := NUM_CONST;
  10.   Res1 := Round(NUM_CONST * 1E6) / 1E6;
  11.   Res2 := Round(NumVar    * 1E6) / 1E6;
  12.   Res3 := Round(NUM_CONST * 1E6) / 1000000;
  13.   Res4 := Round(NumVar    * 1E6) / 1000000;
  14.  
  15.   WriteLn('Res1 = ', Res1);
  16.   WriteLn('Res2 = ', Res2);
  17.   WriteLn('Res3 = ', Res3);
  18.   WriteLn('Res4 = ', Res4);
  19. end.
  20.  

I expected the same result in both, but it is not the case.

The result output on Windows 10 (64 bit) is:
Quote
Res1 =  2.1234570000000002E+000
Res2 =  2.1234569549560547E+000
Res3 =  2.1234570000000002E+000
Res4 =  2.1234570000000002E+000

Note that the operation using the const number results as expected (2.123457),  but when using a variable it results in (2.1234569549560547).

The same code on Linux (Ubuntu 20.04) results in:
Quote
Res1 =  2.1234569549560547E+000
Res2 =  2.1234569549560547E+000
Res3 =  2.1234570000000001E+000
Res4 =  2.1234570000000001E+000

Note that on Linux, the operations using const and var result the same (2.1234569549560547).

Why does an operation using the expenential notation differ from using the literal one?

Edit: I am using Free Pascal 3.2.0, but the same occurs in 3.0.4
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: julkas on August 09, 2020, 11:13:27 am
Try also following code -
Code: Pascal  [Select][+][-]
  1. program NumberTest;
  2. {$IFDEF FPC}
  3. {$MODE Delphi}
  4. {$ENDIF}
  5. uses SysUtils;
  6.  
  7.     const
  8.       NUM_CONST = 2.123456789;
  9.     var
  10.       NumVar: Double;
  11.       Res1, Res2, Res3, Res4: Double;
  12.     begin
  13.       NumVar := NUM_CONST;
  14.       Res1 := Round(NUM_CONST * 1E6);
  15.       Res2 := Round(NumVar    * 1E6);
  16.       WriteLn(PUInt64(@Res1)^);
  17.       WriteLn(PUInt64(@Res2)^);  
  18.  
  19.       Res1 := Res1 / 1E6;
  20.       Res2 := Res2 / 1E6;
  21.       WriteLn(PUInt64(@Res1)^);
  22.       WriteLn(PUInt64(@Res2)^);  
  23.  
  24.       Res3 := Round(NUM_CONST * 1E6) / 1000000;
  25.       Res4 := Round(NumVar    * 1E6) / 1000000;
  26.  
  27.       WriteLn('Res1 = ', Res1);
  28.       WriteLn('Res2 = ', Res2);
  29.       WriteLn('Res3 = ', Res3);
  30.       WriteLn('Res4 = ', Res4);
  31.  
  32.       Res1 := Round(NUM_CONST * 1E6) / 1E6;
  33.       Res2 := Round(NumVar    * 1E6) / 1E6;
  34.       WriteLn(PUInt64(@Res1)^);
  35.       WriteLn(PUInt64(@Res2)^);  
  36.       WriteLn('Res1 = ', Res1);
  37.       WriteLn('Res2 = ', Res2);
  38.                  
  39.     end.
  40.  
FPC output -
Code: Bash  [Select][+][-]
  1. $ ./consta.exe
  2. 4701814500532158464
  3. 4701814500532158464
  4. 4611964018876986044
  5. 4611964018876986044
  6. Res1 =  2.1234570000000002E+000
  7. Res2 =  2.1234570000000002E+000
  8. Res3 =  2.1234570000000002E+000
  9. Res4 =  2.1234570000000002E+000
  10. 4611964018876986044
  11. 4611964018775556096
  12. Res1 =  2.1234570000000002E+000
  13. Res2 =  2.1234569549560547E+000
  14.  
Delphi output -
Code: Bash  [Select][+][-]
  1. $ ./consta.exe
  2. 4701814500532158464
  3. 4701814500532158464
  4. 4611964018876986044
  5. 4611964018876986044
  6. Res1 =  2.12345700000000E+0000
  7. Res2 =  2.12345700000000E+0000
  8. Res3 =  2.12345700000000E+0000
  9. Res4 =  2.12345700000000E+0000
  10. 4611964018876986044
  11. 4611964018876986044
  12. Res1 =  2.12345700000000E+0000
  13. Res2 =  2.12345700000000E+0000
  14.  
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: ASerge on August 09, 2020, 11:21:38 am
I found an inconsistency in results when dividing a float number by an integer in exponential notation and literal notation.
For the 1E6 constant, the compiler uses the Single value type. And calculations of the corresponding accuracy. When an integer is used (1000000), the constant is converted to the Double type, and double-precision calculations are applied accordingly.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: julkas on August 09, 2020, 11:57:06 am
@ASerge How you explain this -
Code: Pascal  [Select][+][-]
  1. program NumberTest;
  2. {$IFDEF FPC}
  3. {$MODE Delphi}
  4. {$ENDIF}
  5.  
  6.     var
  7.       NumVar: Double;
  8.       Res1: Double;
  9.     begin
  10.       NumVar := 2.123456789;
  11.          
  12.       Res1 := Round(NumVar * 1.0) / 1E6;
  13.       WriteLn(PUInt64(@Res1)^);
  14.          
  15.           Res1 := Round(NumVar * 1.0);
  16.           Res1 := Res1 / 1E6;
  17.       WriteLn(PUInt64(@Res1)^);
  18.     end.
  19.  

FPC result -
Code: Bash  [Select][+][-]
  1. $ ./consta.exe
  2. 4521832792723554304
  3. 4521832792735477133
  4.  

Delphi -
Code: Bash  [Select][+][-]
  1. $ ./consta.exe
  2. 4521832792735477133
  3. 4521832792735477133
  4.  

FPC bug ?
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: julkas on August 09, 2020, 02:11:14 pm
Another - 
Code: Pascal  [Select][+][-]
  1. program NumberTest;
  2. {$IFDEF FPC}
  3. {$MODE Delphi}
  4. {$ENDIF}
  5. uses Math;
  6.  
  7.     var
  8.       NumVar: Double;
  9.  
  10.     begin
  11.       NumVar := 2.0;
  12.                  
  13.       NumVar := Round(NumVar) / 10.0;
  14.       WriteLn(PUInt64(@NumVar)^);
  15.  
  16.       NumVar := Round(Double(2.0)) / 10.0;
  17.       WriteLn(PUInt64(@NumVar)^);
  18.  
  19.     end.
  20.  
Result -
Code: Bash  [Select][+][-]
  1. $ ./consta.exe
  2. 4596373779801702400
  3. 4596373779694328218
  4.  
C lang - https://ideone.com/SKRJ2Q
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: ASerge on August 09, 2020, 08:54:18 pm
@ASerge How you explain this -
Same. Change 1E6 to Double(1E6).
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: smaniok on August 09, 2020, 11:16:32 pm
For the 1E6 constant, the compiler uses the Single value type. And calculations of the corresponding accuracy. When an integer is used (1000000), the constant is converted to the Double type, and double-precision calculations are applied accordingly.

Yes, typecast solved the difference. In addition, converting the left operand also has the same effect.

Code: Bash  [Select][+][-]
  1. Res2 := Double(Round(NumVar * 1E6)) / 1E6;
  2.  

@ASerge How you explain this -
Code: Pascal  [Select][+][-]
  1. program NumberTest;
  2. {$IFDEF FPC}
  3. {$MODE Delphi}
  4. {$ENDIF}
  5.  
  6.     var
  7.       NumVar: Double;
  8.       Res1: Double;
  9.     begin
  10.       NumVar := 2.123456789;
  11.          
  12.       Res1 := Round(NumVar * 1.0) / 1E6;
  13.       WriteLn(PUInt64(@Res1)^);
  14.          
  15.           Res1 := Round(NumVar * 1.0);
  16.           Res1 := Res1 / 1E6;
  17.       WriteLn(PUInt64(@Res1)^);
  18.     end.
  19.  

FPC result -
Code: Bash  [Select][+][-]
  1. $ ./consta.exe
  2. 4521832792723554304
  3. 4521832792735477133
  4.  


The same here, Res1 is explicitly declared as Double.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: jamie on August 10, 2020, 12:58:53 am
Just move on, don't expect this to ever get fixed.

Get use to Type casting the hell out of everything or do your coding in Delphi and save the agony of bug hunting.

Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: ASerge on August 10, 2020, 06:15:56 pm
The same here, Res1 is explicitly declared as Double.
Constant (literal)! Not a variable, result, or anything else. The constant is defined as the Single type.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Thaddy on August 10, 2020, 07:26:23 pm
Constant (literal)! Not a variable, result, or anything else. The constant is defined as the Single type.
In that case it is a bug. An untyped const should take the highest possible resolution for the platform.
Plz report.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: ASerge on August 10, 2020, 07:53:35 pm
In that case it is a bug. An untyped const should take the highest possible resolution for the platform.
Is this documented somewhere?
I have seen other things that the Single type is the only one that is guaranteed on all platforms.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Jonas Maebe on August 10, 2020, 08:41:59 pm
See https://wiki.freepascal.org/User_Changes_2.2.0#Floating_point_constants for more information. Back then, the current behaviour was considered Delphi-compatible (but perhaps an error was made). It also mentions a way to specify a higher minimal precision for floating point constants.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Thaddy on August 10, 2020, 09:06:37 pm
See https://wiki.freepascal.org/User_Changes_2.2.0#Floating_point_constants for more information. Back then, the current behaviour was considered Delphi-compatible (but perhaps an error was made). It also mentions a way to specify a higher minimal precision for floating point constants.
So that should be reverted.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Jonas Maebe on August 10, 2020, 09:23:42 pm
More likely refined.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Alextp on August 11, 2020, 06:53:56 pm
>An untyped const should take the highest possible resolution for the platform.
Plz report.

Do I need to write to bugtracker about it? or Jonas made a fix?

Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Jonas Maebe on August 11, 2020, 07:00:45 pm
I did not change anything. A bug report would be good, because just using the highest precision in all cases is what we originally did and that caused also compatibility issues. It would also prevent the use of SSE on all x86-64 platforms apart from Win64 as soon as you have a constant, because then the whole expression will suddenly have to be evaluated using extended precision (unless you start explicitly typecasting all of your constants to single/double), and in on all other platforms also have similar effects with expressions only containing single precision values and constants.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Alextp on August 11, 2020, 07:24:12 pm
Posted
https://bugs.freepascal.org/view.php?id=37549
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: FPK on August 11, 2020, 08:25:18 pm
The bug report tells nothing. Point out some documentation how delphi handles it. Everything else is guessing: it depends too much on the platform: is a real extended type available, is maybe one type handled in software etc. This is all a very slippery slope. For example defaulting to double is random if there is also extended. Further double might be very slow on some platforms as they are only emulated (some arm CPUs, Xtensa) etc.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Alextp on August 11, 2020, 08:36:12 pm
At least "default to Double" will be the improvement. IMO

(I don't have link to docs)
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: FPK on August 11, 2020, 08:39:57 pm
At least "default to Double" will be the improvement. IMO

People using CPUs without hardware double support will see it differently if all operations with constants get suddenly carried out in double instead of hardware supported single.
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: Alextp on August 11, 2020, 08:43:35 pm
(I don't know what to write, then).
Title: Re: Result mismatch on division by exponential notation and literal notation.
Post by: PascalDragon on August 12, 2020, 09:35:56 am
In that case the status quo is as good as it can get.
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