[fpc-devel] Assignment to bitpacked record member that crosses byte boundary does not always work correctly (i386).

RusselD at denelaviation.co.za RusselD at denelaviation.co.za
Tue Jun 17 10:56:18 CEST 2008



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;



    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

        bit15_9 : t7bit;



    // 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);




    BitTest : MappedStruct;




    // 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 : $',

           ' (I get $0129 V2.2.0, $0129 V2.3.1)');

    // Display a member value

    Writeln(' Expected bit8_1 value : $FF, Got : $',

           '  (I get $94 V2.2.0, $94 V2.3.1)');


    WriteLn(' Repeat same test, but now in ''with'' statement ...');


    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)');



    BitTest.AsWord := $01FF;


    WriteLn(' Assign $01FF to ''AsWord'' member and check ''AsBit''
members (works on my system)');


    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;





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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