Use of SHR with signed integers [Archive]

Ñuño Martínez

01-02-2007, 12:54 PM

I'm using SHR and SHL to convert fixed point <-> integers but I've find a problem.

program project2;

FUNCTION integer_to_fixed (v: LONGINT): LONGINT;
BEGIN
integer_to_fixed := v SHL 16;
END;

FUNCTION fixed_to_integer (v: LONGINT): LONGINT;
BEGIN
fixed_to_integer := v SHR 16;
END;

VAR
v: LONGINT;
begin
v := integer_to_fixed (-1); { Gets -1 in 16.16 fixed point. }
WriteLn (v); { Writes the value as stored in memory. }
WriteLn (fixed_to_integer (v)); { Should write '-1' but writes '65535'. }
ReadLn;
end.

As you see, SHR doesn't add the '1' bit. What can I use in the "fixed_to_integer" function?

[edit] Found a solution:

FUNCTION fixed_to_integer (v: LONGINT): LONGINT;
BEGIN
IF v < 0 THEN
fixed_to_integer := (v SHR 16) OR $FFFF0000
ELSE
fixed_to_integer := v SHR 16;
END;

Is this the bets one?

Mirage

01-02-2007, 01:17 PM

You can use DIV 65536 of course, but it's slow.

Another way is to use assembler:

FUNCTION fixed_to_integer (v: LONGINT): LONGINT;
asm
sar eax, 16
END;

LP

01-02-2007, 04:38 PM

You can use DIV 65536 of course, but it's slow.
Actually when optimizations are enabled, this division will be translated into "sar" (at least in Delphi; is it the same in FPC?)

dmantione

01-02-2007, 06:53 PM

Yes, FPC uses sar, optimized or unoptimized. However, -3 div 2=-1, while -3 sar 1=-2. To compensate FPC generates this code (register variables off):

; [12] fixed_to_integer := v div 65536;
mov edx,dword [ebp-4]
mov eax,edx
sar eax,31
and eax,65535
add edx,eax
sar edx,16
mov dword [ebp-8],edx

BeRo

03-02-2007, 09:06 PM