Your new version is better, but still has a serious shortcoming:
the DestBitIndex should be able to have any range.
Your method assumes parameter DestBitIndex has range 0 to 31, this is too
restricted.
See specification in first posting.
Furthermore your method is less desireable, currently it falls in
classification A2B1
The best classification is A2B2.
Your method's lower classification explained:
Your method writes garbage bits from the value as well...
Suppose the value is:
LSB ------------------------------> MSB
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1111 1111 1111 0000 0000 0000 0000 0000
Suppose the bit count is: 3
Your method copies all set bits from value to output.
Buffer before call:
0000 0000 0000 0000 0000 0000 0000 0000
Buffer after call:
1111 1111 1111 0000 0000 0000 0000 0000
So you see, your method ignores the bit count in this case.
It would have been better/more desireable if your method preserves
the remaining bits in the buffer, and treats accessive bits in the value
as
garbage which should not be written to the buffer.
So the best/most desired result would be:
1110 0000 0000 0000 0000 0000 0000 0000
Your method will be classified as: A2B1
While my methods are classified as: A2B2
See first post to understand classifications :)
Here is your version translated to Delphi.
Your method cannot enter the contest because it fails a basic requirement
as
explained in the top of this posting and in the failure comment below and
in
the specification in the first posting ;)
// 59 instructions.
// Failure: writes to wrong memory locations when DestBitIndex is larger
than 31
procedure WriteLongwordBitsTerje2
(
Value : longword;
BitCount : longword;
DestAddress : pointer;
DestBitIndex : longword
);
var
vLastBit : longword;
vBottomMask : longword;
vTopMask : longword;
begin
// Normalize bit index
longword(DestAddress) := longword(DestAddress) + (DestBitIndex shr 5);
DestBitIndex := DestBitIndex and 31;
// Index of last inserted bit:
vLastBit := DestBitIndex + BitCount - 1;
// Generate a mask of 0 bits below the first inserted bit */
vBottomMask := ( longword(-1) ) shl DestBitIndex;
// There must be 0 to 31 unmodified bits after the last inserted: */
vTopMask := ( longword(-1) ) shr (31-(vLastBit and 31));
// One or two affected words? */
if (vLastBit >= 32) then
begin
Plongword(DestAddress)^ := (Plongword(DestAddress)^ and not vBottomMask)
or (Value shl DestBitIndex);
Longword(DestAddress) := Longword(DestAddress) + 4;
// Since we modify two words, DestBitIndex MUST be > 0! */
Plongword(DestAddress)^ := (Plongword(DestAddress)^ and not vTopMask) or
(Value shr (32-DestBitIndex));
end else
begin
// It all fits in a single word !
Plongword(DestAddress)^ := (Plongword(DestAddress)^ and not (vBottomMask
and vTopMask)) or (Value shl DestBitIndex); // topmask incorrect.
end;
end;
// Generated Assembler:
{
Project1.dpr.1833: begin
00409058 55 push ebp
00409059 8BEC mov ebp,esp
0040905B 83C4F0 add esp,-$10
0040905E 53 push ebx
0040905F 56 push esi
00409060 57 push edi
00409061 8BF1 mov esi,ecx
00409063 8955F8 mov [ebp-$08],edx
00409066 8945FC mov [ebp-$04],eax
00409069 8B5D08 mov ebx,[ebp+$08]
Project1.dpr.1835: longword(DestAddress) := longword(DestAddress) +
(DestBitIndex shr 5);
0040906C 8BC3 mov eax,ebx
0040906E C1E805 shr eax,$05
00409071 01C6 add esi,eax
Project1.dpr.1836: DestBitIndex := DestBitIndex and 31;
00409073 83E31F and ebx,$1f
Project1.dpr.1839: vLastBit := DestBitIndex + BitCount - 1;
00409076 8B45F8 mov eax,[ebp-$08]
00409079 03C3 add eax,ebx
0040907B 48 dec eax
Project1.dpr.1842: vBottomMask := ( longword(-1) ) shl DestBitIndex;
0040907C 8BCB mov ecx,ebx
0040907E 83CAFF or edx,-$01
00409081 D3E2 shl edx,cl
00409083 8955F4 mov [ebp-$0c],edx
Project1.dpr.1845: vTopMask := ( longword(-1) ) shr (31-(vLastBit and
31));
00409086 8BD0 mov edx,eax
00409088 83E21F and edx,$1f
0040908B B91F000000 mov ecx,$0000001f
00409090 2BCA sub ecx,edx
00409092 83CAFF or edx,-$01
00409095 D3EA shr edx,cl
00409097 8955F0 mov [ebp-$10],edx
Project1.dpr.1848: if (vLastBit >= 32) then
0040909A 83F820 cmp eax,$20
0040909D 7236 jb $004090d5
Project1.dpr.1850: Plongword(DestAddress)^ := (Plongword(DestAddress)^ and
not vBottomMask) or (Value shl DestBitIndex);
0040909F 8BCB mov ecx,ebx
004090A1 8B55FC mov edx,[ebp-$04]
004090A4 D3E2 shl edx,cl
004090A6 8BC6 mov eax,esi
004090A8 8B08 mov ecx,[eax]
004090AA 8B7DF4 mov edi,[ebp-$0c]
004090AD F7D7 not edi
004090AF 23CF and ecx,edi
004090B1 0BD1 or edx,ecx
004090B3 8910 mov [eax],edx
Project1.dpr.1852: Longword(DestAddress) := Longword(DestAddress) + 4;
004090B5 83C604 add esi,$04
Project1.dpr.1855: Plongword(DestAddress)^ := (Plongword(DestAddress)^ and
not vTopMask) or (Value shr (32-DestBitIndex));
004090B8 B920000000 mov ecx,$00000020
004090BD 2BCB sub ecx,ebx
004090BF 8B55FC mov edx,[ebp-$04]
004090C2 D3EA shr edx,cl
004090C4 8BC6 mov eax,esi
004090C6 8B08 mov ecx,[eax]
004090C8 8B5DF0 mov ebx,[ebp-$10]
004090CB F7D3 not ebx
004090CD 23CB and ecx,ebx
004090CF 0BD1 or edx,ecx
004090D1 8910 mov [eax],edx
004090D3 EB19 jmp $004090ee
Project1.dpr.1859: Plongword(DestAddress)^ := (Plongword(DestAddress)^ and
not (vBottomMask and vTopMask)) or (Value shl DestBitIndex);
004090D5 8BCB mov ecx,ebx
004090D7 8B55FC mov edx,[ebp-$04]
004090DA D3E2 shl edx,cl
004090DC 8BC6 mov eax,esi
004090DE 8B08 mov ecx,[eax]
004090E0 8B5DF4 mov ebx,[ebp-$0c]
004090E3 235DF0 and ebx,[ebp-$10]
004090E6 F7D3 not ebx
004090E8 23CB and ecx,ebx
004090EA 0BD1 or edx,ecx
004090EC 8910 mov [eax],edx
Project1.dpr.1861: end;
004090EE 5F pop edi
004090EF 5E pop esi
004090F0 5B pop ebx
004090F1 8BE5 mov esp,ebp
004090F3 5D pop ebp
004090F4 C20400 ret $0004
}
Thanks for trying anyway...
Bye,
Skybuck.
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