Hi,

I was interested to see if bit packing works when a record member spans byte boundaries, and in general it appears to work. However on my i386 system I discovered a bug that the following illustrates.

//---------------------------------------------
//Tested on Intel Core 2 Duo running Windows XP Pro SP2.
// Compiler version 2.2.0 [2007/09/09] and 2.3.1  [2008/02/03]
uses SysUtils, Crt;
type
    bit = 0..1;
    t7bit = 0..$3F;
    // A record to test behaviour over byte boundaries.
    BitStruct = bitpacked record
        bit0    : bit;
        bit8_1  : byte;   // Assigning $FF to this member causes
problems...
        bit15_9 : t7bit;
    end;
    // Map the record to a word for convenience - but overlaying
    // a word using absolute instead of a variant record produces
    // the same result.
    MappedStruct = record
        case boolean of
            false : (AsWord : word);
            true  : (AsBits : BitStruct);
    end;
var
    BitTest : MappedStruct;
begin
    clrscr;
    // Load a bitpacked value via the members, that should map to $01FF
in AsWord member.
    BitTest.AsBits.bit0 := 1;
    BitTest.AsBits.bit8_1 := $FF;
    BitTest.AsBits.bit15_9 := $00;
    // Display the mapped word value.

Writeln(' Expected word value : $01FF, Got : $',
IntToHex(BitTest.AsWord,4),

           ' (I get $0129 V2.2.0, $0129 V2.3.1)');
    // Display a member value

Writeln(' Expected bit8_1 value : $FF, Got : $',
IntToHex(BitTest.AsBits.bit8_1,2),

           '  (I get $94 V2.2.0, $94 V2.3.1)');
    WriteLn;
    WriteLn(' Repeat same test, but now in ''with'' statement ...');
    Writeln;
    with BitTest, AsBits do begin
        //  Same bitpacked value.
        bit0 := 1;
        bit8_1 := $FF;
        bit15_9 := $00;
        // Display the word value.

Writeln(' Expected word value : $01FF, Got :
$',IntToHex(BitTest.AsWord, 4),

                ' (I get $0001 V2.2.0, $01F9 V2.3.1)');
                // Display a member value

Writeln(' Expected bit8_1 value : $FF, Got : $',
IntToHex(BitTest.AsBits.bit8_1, 2),

           '  (I get $00 V2.2.0, $FC V2.3.1)');
    end;
    BitTest.AsWord := $01FF;
    WriteLn;
    WriteLn(' Assign $01FF to ''AsWord'' member and check ''AsBit''
members (works on my system)');
    WriteLn;
    WriteLn('   Expected bit15..9 value : $00, Got :
$',IntToHex(BitTest.AsBits.bit15_9, 2));
    Writeln('   Expected bit8_1 value : $FF, Got : $',
IntToHex(BitTest.AsBits.bit8_1 ,2));
    Writeln('   Expected bit0 value : $01, Got : $',
IntToHex(BitTest.AsBits.bit0, 2));
    repeat until KeyPressed;
end.
//---------------------------------------------

The limited amount of testing done indicates that the record member bit8_1 only causes a problem with a value of $FF, but the interesting thing is that the result varies depending on program structure and compiler version: Different results are seen whith the same code in a procedure, in a 'with' statement, or if there is code in front of it etc. The result is NOT always incorrect.

Note that using a variant record to show the value is only a convenience here and the error can be seen without a variant record by examining the record directly in a watch window, or by overlaying the record using the absolute keyword.

I have attached slightly more comprehensive tests.

Best Regards,

Russell Davies

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On 17 Jun 2008, at 10:56, <

The problem can occur both when assigning 0 and when assigning field_type(-1). I'll look into fixing it tomorrow.

Jonas

Fixed. Note that
a) your test programs were specific to little endian systems (bitpacked records have a different layout on big endian systems) b) the internal layout of bitpacked records is not guaranteed to remain the same over different versions (although there are no particular intentions to change it either)
c) the second test program only defines 15 bits of the packed records, so in the tests when checking for expected results you have to mask out the left over bit (since it can contain garbage) d) there was one error in the second test: if bit9_15 is set to one and the rest to 0, the value of AsWord should be $0200 rather than $0100.

Thanks for the good test programs!

Jonas