This smells remarkably like a school assignment. Why would you want to do
this sort of thing many times in real life since testing what touches your
memory block is probably more efficient?
John Dammeyer
Wireless CAN with the CANRF module now available.
Automation Artisans Inc.
Ph. 1 250 544 4950
Message
From: delphi-bounces (AT) elists (DOT) org
[mailto:delphi-bounces (AT) elists (DOT) org] Behalf Ross Levis
Sent: Thursday, November 03, 2005 2:54 AM
To: Delphi Discussion List
Subject: Test memory block
I'm looking for the fastest way to test if a block of memory is all
zeros or not. This needs to be as fast as possible since it is being
tested many times a second.
The memory block can be any length. Assuming Buffer is a
pointer to a
block of memory, and BufferSize is the length of the block, I'm
currently doing it like this.
type
TBufferArray = array[00] of Byte;
PBufferArray = ^TBufferArray;
var
BufArray: BufferArray;
i: Integer;
begin
BufArray := Buffer;
Result := False;
for i := 0 to BufferSize-1 do
if Buffer[i] <0 then Exit;
Result := True;
This is reasonably quick but I'm wondering if there is a
faster way that
does not need to increment an integer many times.
Thanks,
Ross.
Delphi mailing list -Delphi (AT) elists (DOT) org
Delphi mailing list -Delphi (AT) elists (DOT) org

No, it's a real-life commercial audio player application. At the end of
playing a song, I need to flush the remaining audio buffers from inside
a DLL. I send it a zeroed buffer and it returns the remaining audio in
the buffer, and the only way to know it's finished is by testing the
returned buffer is all zero.

Generally the buffer will be between 2k and 8k.

I'll give your CompareMem idea a go Rob and do some time tests. Many
thanks for that.

Ross.

Message
From: "John Dammeyer" <johnd (AT) autoartisans (DOT) com>
To: "'Borland's Delphi Discussion List'" <delphi (AT) elists (DOT) org>
Sent: Friday, November 04, 2005 5:07 AM
Subject: RE: Test memory block

This smells remarkably like a school assignment. Why would you want to
do
this sort of thing many times in real life since testing what touches
your
memory block is probably more efficient?

John Dammeyer

Delphi mailing list -Delphi (AT) elists (DOT) org

Just as a matter of interest, I came up with my own fast method working
along the lines of John's method but I think this is tidier.

type
TBuffer32 = array[00] of LongWord;
PBuffer32 = ^TBuffer32;

TBuffer8 = array[00] of Byte;
PBuffer8 = ^TBuffer8;

function IsZero(Buffer32: Pointer; BufferSize: Integer): Boolean;
var
Size32: Integer;
Buffer8: PBuffer8;
Buffer32: PBuffer32;
i: Integer;
begin
Result := False;
Buffer32 := Buffer;
Size32 := BufferSize shr 2;
for i := 0 to Size32-1 do
if Buffer32[i] <0 then // test 4 bytes at a time
Exit;
Buffer8 := Buffer;
for i := 0 to BufferSize - (Size32 shl 2) -1 do
if Buffer8[i] <0 then // test remaining 1,2, or 3 bytes
Exit;
Result := True;
end;

Regards,
Ross.

Delphi mailing list -Delphi (AT) elists (DOT) org

typo

function IsZero(Buffer: Pointer;

Message
From: "Ross Levis" <ross (AT) stationplaylist (DOT) com>
To: "Borland's Delphi Discussion List" <delphi (AT) elists (DOT) org>
Sent: Saturday, November 05, 2005 12:09 PM
Subject: Re: Test memory block

Just as a matter of interest, I came up with my own fast method working
along the lines of John's method but I think this is tidier.

type
TBuffer32 = array[00] of LongWord;
PBuffer32 = ^TBuffer32;

TBuffer8 = array[00] of Byte;
PBuffer8 = ^TBuffer8;

function IsZero(Buffer32: Pointer; BufferSize: Integer): Boolean;
var
Size32: Integer;
Buffer8: PBuffer8;
Buffer32: PBuffer32;
i: Integer;
begin
Result := False;
Buffer32 := Buffer;
Size32 := BufferSize shr 2;
for i := 0 to Size32-1 do
if Buffer32[i] <0 then // test 4 bytes at a time
Exit;
Buffer8 := Buffer;
for i := 0 to BufferSize - (Size32 shl 2) -1 do
if Buffer8[i] <0 then // test remaining 1,2, or 3 bytes
Exit;
Result := True;
end;

Delphi mailing list -Delphi (AT) elists (DOT) org

>Just as a matter of interest, I came up with my own fast method working
>along the lines of John's method but I think this is tidier.

This is starting to resemble a coding competition! Here's my suggestion:

function IsZero(const ADataToSearch:Pointer; const
LengthInBytes:LongWord) : boolean;
var
p:pLongWord; //pLongWord defined System unit in D7
i:LongWord;
begin
p:=pLongWord(ADataToSearch);
result:=false;
for i:=0 to ((LengthInBytes div 4)-1) do
if p^ <0 then
exit
else
inc(p);
case LengthInBytes mod 4 of
0: result:=true;
1: if pByte(p)^=0 then result:=true;
2: if pWord(p)^=0 then result:=true;
3: if ((pWord(p)^=0) and (PByte(LongWord(p)+2)^=0)) then result:=true;
end;
end;//IsZero

Delphi mailing list -Delphi (AT) elists (DOT) org

That's very good Melcome, I may use your exact code. The extra
0.000000001 millisecond saving at the end is excellent :-)

It appears to be similar to what the FastCode Project's CompareMem does
in assembler code.

Does the div 4 as apposed to shr 2 use the same number of CPU cycles?

Anyway, I think this thread should die about now
;

Ross.

Message
From: "Malcolm Clark" <malcolm (AT) post3 (DOT) tele.dk>
To: "Borland's Delphi Discussion List" <delphi (AT) elists (DOT) org>
Sent: Sunday, November 06, 2005 1:06 AM
Subject: Re: Test memory block


>Just as a matter of interest, I came up with my own fast method working
>along the lines of John's method but I think this is tidier.

This is starting to resemble a coding competition! Here's my suggestion:

function IsZero(const ADataToSearch:Pointer; const
LengthInBytes:LongWord) : boolean;
var
p:pLongWord; //pLongWord defined System unit in D7
i:LongWord;
begin
p:=pLongWord(ADataToSearch);
result:=false;
for i:=0 to ((LengthInBytes div 4)-1) do
if p^ <0 then
exit
else
inc(p);
case LengthInBytes mod 4 of
0: result:=true;
1: if pByte(p)^=0 then result:=true;
2: if pWord(p)^=0 then result:=true;
3: if ((pWord(p)^=0) and (PByte(LongWord(p)+2)^=0)) then
result:=true;
end;
end;//IsZero

Delphi mailing list -Delphi (AT) elists (DOT) org

>Does the div 4 as apposed to shr 2 use the same number of CPU cycles?
Yes, the compiler has generated a shift for division by powers of two as
far back as Turbo Pascal days.

>Anyway, I think this thread should die about now
I'm terminated:-)
-Malcolm

Delphi mailing list -Delphi (AT) elists (DOT) org

Malcolm Clark wrote:

p:=pLongWord(ADataToSearch);
result:=false;
for i:=0 to ((LengthInBytes div 4)-1) do
if p^ <0 then exit
else inc(p);

I had to activate this thread again as I'm not convinced that inc(p)
will increment 4 bytes at a time. I'm thinking that p^ will access 4
bytes, but inc(p) will only increment 1 byte of the block, and then
access the same 3 bytes overlapped plus 1 new byte. am I wrong?

I think it will need to be inc(p,4) ?

Ross.

Delphi mailing list -Delphi (AT) elists (DOT) org

Ross Levis wrote:
[]
I had to activate this thread again as I'm not convinced that inc(p)
will increment 4 bytes at a time. I'm thinking that p^ will access 4
bytes, but inc(p) will only increment 1 byte of the block, and then
access the same 3 bytes overlapped plus 1 new byte. am I wrong?

I think it will need to be inc(p,4) ?

If p is typed as ^Longword it will be incremented 4 bytes at a time. If
as ^byte, then one byte at a time.

David

Delphi mailing list -Delphi (AT) elists (DOT) org

I see. So effectively, inc(p) gets translated to inc(p,4) by the
compiler.

Ross.

Message
From: "David J Taylor" <david-taylor (AT) blueyonder (DOT) co.uk>
To: "Borland's Delphi Discussion List" <delphi (AT) elists (DOT) org>
Sent: Monday, November 07, 2005 11:27 PM
Subject: Re: Test memory block

Ross Levis wrote:
[]
I had to activate this thread again as I'm not convinced that inc(p)
will increment 4 bytes at a time. I'm thinking that p^ will access 4
bytes, but inc(p) will only increment 1 byte of the block, and then
access the same 3 bytes overlapped plus 1 new byte. am I wrong?

I think it will need to be inc(p,4) ?

If p is typed as ^Longword it will be incremented 4 bytes at a time. If
as ^byte, then one byte at a time.

David

Delphi mailing list -Delphi (AT) elists (DOT) org

Ross Levis wrote:
I see. So effectively, inc(p) gets translated to inc(p,4) by the
compiler.
Ross.
If P is a pointer to a four byte quantity, the operation is
Pointer (Integer (P) + 4)
David
Delphi mailing list -Delphi (AT) elists (DOT) org