How to set the top nonzero bit of integer to zero?

[LemonParty]

For example:
Reset(7) = 3
Reset(3) = 1
Reset(1) = 0

I know in x86 that can be done with Bsr and Btr instruction. Is there a non loop decision that is crosspatform?

[Warfley]

Something like:

Code: Pascal  [Select][+][-]

1. function ResetMSBit(const x: QWord): QWord; inline;
2. begin
3.   Result := x And not (1 shl BsrQWord(x));
4. end;

BsrXYZ is provided by the system unit and will provide functionality on each platform. Where supported it directly uses the assembly instruction, but on platforms it doesn't have it, it's a regular implementation

Alternatively:

Code: Pascal  [Select][+][-]

1. function ResetMSBit(const x: QWord): QWord; inline;
2. var
3.   i: Integer;
4. begin
5.   for i:=63 downto 0 do
6.     if ((1 shl i) And x) <> 0 then
7.       Exit(Result and not (1 shl i));
8.   Result := 0;
9. end;

[LemonParty]

I guess I found the solution:

Code: Pascal  [Select][+][-]

1. function ResetMSBit(u: UInt32): UInt32;inline;
2. begin
3.   if u <> 0
4.     then Result:= u and RolDWord(-2, BsrDWord(u))
5.     else Result:= 0;
6. end;

[LemonParty]

This is also work:

Code: Pascal  [Select][+][-]

1. function ResetMSBit(u: UInt32): UInt32;inline;
2. begin
3.   Result:= u and RolDWord(-2, BsrDWord(u));
4. end;

[avra]

If you use trunk fpc, then just include sysutils and syshelpers and you can do something like:

Code: Pascal  [Select][+][-]

1. ...
2. var
3.   MyDword: dword;
4. begin
5.   MyDword            := %111000111000111000111;
6.   MyDword.Words[0]   := 77;
7.   MyDword.Bits[9]    := true;
8.   MyDword.Nibbles[0] := $A;
9.   WriteLn('MyDword.ToBinString      = ', MyDword.ToBinString);
10.   WriteLn('MyDword.HighestSetBitPos = ' + MyDword.HighestSetBitPos.ToString);
11.   WriteLn('MyDword.SetBitsCount     = ' + MyDword.SetBitsCount.ToString);
12.   WriteLn('MyDword.ToHexString      = ', MyDword.ToHexString);
13. ...
14.  

Full example can be found in \fpcsrc\packages\rtl-objpas\examples\syshelpersdemo.pas.
 
If you are not using trunk, then just install bithelpers package from my signature (you can find it in OPM as well), and you will be able to do the same.

[Thaddy]

@Avra
Yes, that's why we wrote it ...