digitalmars.D.learn - Best way in D2 to rotate a ubyte[4] array

• Tom (4/4) Mar 09 2011 What is the most efficient way of implement a rotation of ubyte[4] array...
• bearophile (100/103) Mar 09 2011 Two versions, I have done no benchmarks so far:
• Tom (3/106) Mar 09 2011 Wow, thanks b, didn't know of the rol instruction...
• Kai Meyer (34/137) Mar 09 2011 I love it.
• U2 fan (2/38) Mar 09 2011 I am offend!
• Andrew Wiley (3/41) Mar 09 2011 Once I figured it out, I lol'd quite a bit.
• bearophile (53/53) Mar 10 2011 While creating the rotation code I have found two things I don't underst...
• spir (31/84) Mar 10 2011 About first point: looks like a bug at the interface between union and a...
• Kai Meyer (12/16) Mar 09 2011 I don't know of anything more efficient than:
• Jonathan M Davis (11/30) Mar 09 2011 I'm stunned that this works. I'd even consider reporting it as a bug. Yo...
• spir (7/12) Mar 09 2011 This works for other arrays as well. dmd understands.
Tom <tom nospam.com> writes:
```What is the most efficient way of implement a rotation of ubyte[4] array?

By rotation I mean: rotateRight([1, 2, 3, 4]) -> [4, 1, 2, 3]

TIA,
Tom;
```
Mar 09 2011
bearophile <bearophileHUGS lycos.com> writes:
```Tom:

What is the most efficient way of implement a rotation of ubyte[4] array?

By rotation I mean: rotateRight([1, 2, 3, 4]) -> [4, 1, 2, 3]

Two versions, I have done no benchmarks so far:

import std.c.stdio: printf;

union Four {
ubyte[4] a;
uint u;
}

void showFour(Four f) {
printf("f.u: %u\n", f.u);
printf("f.a: [%d, %d, %d, %d]\n",
cast(int)f.a[0], cast(int)f.a[1],
cast(int)f.a[2], cast(int)f.a[3]);
}

void main() {
Four f;
f.a[] = [1, 2, 3, 4];
showFour(f);
f.u = (f.u << 8) | (f.u >> 24);
showFour(f);
printf("\n");

// alternative
f.a[] = [1, 2, 3, 4];
uint u2 = f.u;
showFour(f);
printf("u2: %u\n", u2);
asm {
rol  u2, 8;
}
f.u = u2;
showFour(f);
}

/*

dmd -O -release test.d

__Dmain comdat
push    EBP
mov EBP,ESP
sub ESP,8
push    4
mov EAX,offset FLAT:_D12TypeInfo_xAh6__initZ
push    4
push    3
push    2
push    1
push    4
mov dword ptr -8[EBP],0
push    EAX
call    near ptr __d_arrayliteralT
push    EAX
lea EAX,-8[EBP]
push    EAX
call    near ptr _memcpy
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,-8[EBP]
mov ECX,-8[EBP]
shl EAX,8        ; <=========
shr ECX,018h
or  EAX,ECX
mov -8[EBP],EAX
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,offset FLAT:_DATA[024h]
push    EAX
call    near ptr _printf
mov EAX,offset FLAT:_D12TypeInfo_xAh6__initZ
push    4
push    4
push    3
push    2
push    1
push    4
push    EAX
call    near ptr __d_arrayliteralT
push    EAX
lea EAX,-8[EBP]
push    EAX
call    near ptr _memcpy
mov EAX,-8[EBP]
mov -4[EBP],EAX
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,offset FLAT:_DATA[028h]
push    dword ptr -4[EBP]
push    EAX
call    near ptr _printf
rol -4[EBP],8   ; <=========
mov EAX,-4[EBP]
mov -8[EBP],EAX
mov EAX,-4[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov ESP,EBP
pop EBP
ret
*/

In theory a C/C++/D compiler has to compile an expression like (x<< 8)|(x>>24)
with a ROL instruction, in practice DMD doesn't do it. Months ago I have asked
the two (four in X86) roll instructions to be added to the Phobos core
intrinsics module, but I am not sure what Walter answered me.

Bye,
bearophile
```
Mar 09 2011
Tom <tom nospam.com> writes:
```El 09/03/2011 20:25, bearophile escribió:
Tom:

What is the most efficient way of implement a rotation of ubyte[4] array?

By rotation I mean: rotateRight([1, 2, 3, 4]) ->  [4, 1, 2, 3]

Two versions, I have done no benchmarks so far:

import std.c.stdio: printf;

union Four {
ubyte[4] a;
uint u;
}

void showFour(Four f) {
printf("f.u: %u\n", f.u);
printf("f.a: [%d, %d, %d, %d]\n",
cast(int)f.a[0], cast(int)f.a[1],
cast(int)f.a[2], cast(int)f.a[3]);
}

void main() {
Four f;
f.a[] = [1, 2, 3, 4];
showFour(f);
f.u = (f.u<<  8) | (f.u>>  24);
showFour(f);
printf("\n");

// alternative
f.a[] = [1, 2, 3, 4];
uint u2 = f.u;
showFour(f);
printf("u2: %u\n", u2);
asm {
rol  u2, 8;
}
f.u = u2;
showFour(f);
}

/*

dmd -O -release test.d

__Dmain comdat
push    EBP
mov EBP,ESP
sub ESP,8
push    4
mov EAX,offset FLAT:_D12TypeInfo_xAh6__initZ
push    4
push    3
push    2
push    1
push    4
mov dword ptr -8[EBP],0
push    EAX
call    near ptr __d_arrayliteralT
push    EAX
lea EAX,-8[EBP]
push    EAX
call    near ptr _memcpy
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,-8[EBP]
mov ECX,-8[EBP]
shl EAX,8        ;<=========
shr ECX,018h
or  EAX,ECX
mov -8[EBP],EAX
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,offset FLAT:_DATA[024h]
push    EAX
call    near ptr _printf
mov EAX,offset FLAT:_D12TypeInfo_xAh6__initZ
push    4
push    4
push    3
push    2
push    1
push    4
push    EAX
call    near ptr __d_arrayliteralT
push    EAX
lea EAX,-8[EBP]
push    EAX
call    near ptr _memcpy
mov EAX,-8[EBP]
mov -4[EBP],EAX
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,offset FLAT:_DATA[028h]
push    dword ptr -4[EBP]
push    EAX
call    near ptr _printf
rol -4[EBP],8   ;<=========
mov EAX,-4[EBP]
mov -8[EBP],EAX
mov EAX,-4[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov ESP,EBP
pop EBP
ret
*/

In theory a C/C++/D compiler has to compile an expression like (x<<
8)|(x>>24) with a ROL instruction, in practice DMD doesn't do it. Months ago I
have asked the two (four in X86) roll instructions to be added to the Phobos
core intrinsics module, but I am not sure what Walter answered me.

Bye,
bearophile

Wow, thanks b, didn't know of the rol instruction...

Tom;
```
Mar 09 2011
Kai Meyer <kai unixlords.com> writes:
```On 03/09/2011 04:25 PM, bearophile wrote:
Tom:

What is the most efficient way of implement a rotation of ubyte[4] array?

By rotation I mean: rotateRight([1, 2, 3, 4]) ->  [4, 1, 2, 3]

Two versions, I have done no benchmarks so far:

import std.c.stdio: printf;

union Four {
ubyte[4] a;
uint u;
}

void showFour(Four f) {
printf("f.u: %u\n", f.u);
printf("f.a: [%d, %d, %d, %d]\n",
cast(int)f.a[0], cast(int)f.a[1],
cast(int)f.a[2], cast(int)f.a[3]);
}

void main() {
Four f;
f.a[] = [1, 2, 3, 4];
showFour(f);
f.u = (f.u<<  8) | (f.u>>  24);
showFour(f);
printf("\n");

// alternative
f.a[] = [1, 2, 3, 4];
uint u2 = f.u;
showFour(f);
printf("u2: %u\n", u2);
asm {
rol  u2, 8;
}
f.u = u2;
showFour(f);
}

/*

dmd -O -release test.d

__Dmain comdat
push    EBP
mov EBP,ESP
sub ESP,8
push    4
mov EAX,offset FLAT:_D12TypeInfo_xAh6__initZ
push    4
push    3
push    2
push    1
push    4
mov dword ptr -8[EBP],0
push    EAX
call    near ptr __d_arrayliteralT
push    EAX
lea EAX,-8[EBP]
push    EAX
call    near ptr _memcpy
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,-8[EBP]
mov ECX,-8[EBP]
shl EAX,8        ;<=========
shr ECX,018h
or  EAX,ECX
mov -8[EBP],EAX
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,offset FLAT:_DATA[024h]
push    EAX
call    near ptr _printf
mov EAX,offset FLAT:_D12TypeInfo_xAh6__initZ
push    4
push    4
push    3
push    2
push    1
push    4
push    EAX
call    near ptr __d_arrayliteralT
push    EAX
lea EAX,-8[EBP]
push    EAX
call    near ptr _memcpy
mov EAX,-8[EBP]
mov -4[EBP],EAX
mov EAX,-8[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov EAX,offset FLAT:_DATA[028h]
push    dword ptr -4[EBP]
push    EAX
call    near ptr _printf
rol -4[EBP],8   ;<=========
mov EAX,-4[EBP]
mov -8[EBP],EAX
mov EAX,-4[EBP]
call    near ptr _D4test8showFourFS4test4FourZv
mov ESP,EBP
pop EBP
ret
*/

In theory a C/C++/D compiler has to compile an expression like (x<<
8)|(x>>24) with a ROL instruction, in practice DMD doesn't do it. Months ago I
have asked the two (four in X86) roll instructions to be added to the Phobos
core intrinsics module, but I am not sure what Walter answered me.

Bye,
bearophile

I love it.
I've done a little benchmark that just repeats the rotate left a certain
number of times, and then a rotate right a certain number of times. It
looks like shifting ( << | >> ) is faster than the process of copying
the uint value, shifting it, and copying it back. If I move the
assignment for the rol outside of the for loop, the rol is about twice
as fast.
http://dl.dropbox.com/u/12135920/rotate.d

Both are anywhere from 30 to 80 times faster than the slicing method I
proposed (also included in the rotate.d file).

------
Rotating static array left to right 5000000 times
Finished in    1971.46 milliseconds
Rotating static array right to left 5000000 times
Finished in    1987.60 milliseconds
Rotating dynamic array left to right 5000000 times
Finished in    1932.40 milliseconds
Rotating dynamic array right to left 5000000 times
Finished in    1981.71 milliseconds
Shifting Union left to right 5000000 times
Finished in      33.46 milliseconds
Shifting Union right to left 5000000 times
Finished in      34.26 milliseconds
Rolling Union left to right 5000000 times
Finished in      67.51 milliseconds
Rolling Union right to left 5000000 times
Finished in      67.47 milliseconds
Rolling Union left to right 5000000 times with assignment with temporary
variable outside of the loop
Finished in      28.81 milliseconds
Rolling Union right to left 5000000 times with assignment with temporary
variable outside of the loop
Finished in      25.57 milliseconds
```
Mar 09 2011
U2 fan <iam u2fan.com> writes:
```== Quote from bearophile (bearophileHUGS lycos.com)'s article
Tom:
What is the most efficient way of implement a rotation of ubyte[4] array?

By rotation I mean: rotateRight([1, 2, 3, 4]) -> [4, 1, 2, 3]

Two versions, I have done no benchmarks so far:
import std.c.stdio: printf;
union Four {
ubyte[4] a;
uint u;
}
void showFour(Four f) {
printf("f.u: %u\n", f.u);
printf("f.a: [%d, %d, %d, %d]\n",
cast(int)f.a[0], cast(int)f.a[1],
cast(int)f.a[2], cast(int)f.a[3]);
}
void main() {
Four f;
f.a[] = [1, 2, 3, 4];
showFour(f);
f.u = (f.u << 8) | (f.u >> 24);
showFour(f);
printf("\n");
// alternative
f.a[] = [1, 2, 3, 4];
uint u2 = f.u;
showFour(f);
printf("u2: %u\n", u2);
asm {
rol  u2, 8;
}
f.u = u2;
showFour(f);
}
Bye,
bearophile

I am offend!
```
Mar 09 2011
Andrew Wiley <debio264 gmail.com> writes:
```On Wed, Mar 9, 2011 at 7:25 PM, U2 fan <iam u2fan.com> wrote:
=3D=3D Quote from bearophile (bearophileHUGS lycos.com)'s article
Tom:
What is the most efficient way of implement a rotation of ubyte[4] arr=

ay?
By rotation I mean: rotateRight([1, 2, 3, 4]) -> [4, 1, 2, 3]

Two versions, I have done no benchmarks so far:
import std.c.stdio: printf;
union Four {
=A0 =A0 ubyte[4] a;
=A0 =A0 uint u;
}
void showFour(Four f) {
=A0 =A0 printf("f.u: %u\n", f.u);
=A0 =A0 printf("f.a: [%d, %d, %d, %d]\n",
=A0 =A0 =A0 =A0 =A0 =A0cast(int)f.a[0], cast(int)f.a[1],
=A0 =A0 =A0 =A0 =A0 =A0cast(int)f.a[2], cast(int)f.a[3]);
}
void main() {
=A0 =A0 Four f;
=A0 =A0 f.a[] =3D [1, 2, 3, 4];
=A0 =A0 showFour(f);
=A0 =A0 f.u =3D (f.u << 8) | (f.u >> 24);
=A0 =A0 showFour(f);
=A0 =A0 printf("\n");
=A0 =A0 // alternative
=A0 =A0 f.a[] =3D [1, 2, 3, 4];
=A0 =A0 uint u2 =3D f.u;
=A0 =A0 showFour(f);
=A0 =A0 printf("u2: %u\n", u2);
=A0 =A0 asm {
=A0 =A0 =A0 =A0 rol =A0u2, 8;
=A0 =A0 }
=A0 =A0 f.u =3D u2;
=A0 =A0 showFour(f);
}
Bye,
bearophile

I am offend!

Once I figured it out, I lol'd quite a bit.
```
Mar 09 2011
bearophile <bearophileHUGS lycos.com> writes:
```While creating the rotation code I have found two things I don't understand.
Maybe some of you is able to help me understand.

This version of the code:

union Four {
uint u;
ubyte[4] a;
}
void main() {
Four f;
asm {
rol f.u, 8;
}
}

DMD 2.052 gives this error, do you know why?

test.d(8): bad type/size of operands 'f.u'

----------------------------------

So to avoid wasting load asm instructions I have tried to write it like this:

union Four {
ubyte[4] arr;
uint ui;
}
void main() {
Four fo;
fo.arr[0] = 1;
fo.arr[1] = 2;
fo.arr[2] = 3;
fo.arr[3] = 4;
uint* uptr = &(fo.ui);
asm {
rol [uptr], 8;
}
asm {
rol uptr, 8;
}
}

but looking at the asm it produces, do you know why the rol with [uptr] and
uptr are translated to the same instruction (so it rotates the pointer instead
of the pointed uint)?

__Dmain comdat
push EBP
mov EBP,ESP
sub ESP,8
mov dword ptr -8[EBP],0
lea EAX,-8[EBP]
mov byte ptr -8[EBP],1
mov byte ptr -7[EBP],2
mov byte ptr -6[EBP],3
mov byte ptr -5[EBP],4
mov -4[EBP],EAX
rol -4[EBP],8  ; <======
rol -4[EBP],8  ; <======
mov ESP,EBP
pop EBP
ret

Bye and thank you,
bearophile
```
Mar 10 2011
spir <denis.spir gmail.com> writes:
```On 03/10/2011 11:01 PM, bearophile wrote:
While creating the rotation code I have found two things I don't understand.
Maybe some of you is able to help me understand.

This version of the code:

union Four {
uint u;
ubyte[4] a;
}
void main() {
Four f;
asm {
rol f.u, 8;
}
}

DMD 2.052 gives this error, do you know why?

test.d(8): bad type/size of operands 'f.u'

----------------------------------

So to avoid wasting load asm instructions I have tried to write it like this:

union Four {
ubyte[4] arr;
uint ui;
}
void main() {
Four fo;
fo.arr[0] = 1;
fo.arr[1] = 2;
fo.arr[2] = 3;
fo.arr[3] = 4;
uint* uptr =&(fo.ui);
asm {
rol [uptr], 8;
}
asm {
rol uptr, 8;
}
}

but looking at the asm it produces, do you know why the rol with [uptr] and
uptr are translated to the same instruction (so it rotates the pointer instead
of the pointed uint)?

__Dmain comdat
push EBP
mov EBP,ESP
sub ESP,8
mov dword ptr -8[EBP],0
lea EAX,-8[EBP]
mov byte ptr -8[EBP],1
mov byte ptr -7[EBP],2
mov byte ptr -6[EBP],3
mov byte ptr -5[EBP],4
mov -4[EBP],EAX
rol -4[EBP],8  ;<======
rol -4[EBP],8  ;<======
mov ESP,EBP
pop EBP
ret

Bye and thank you,
bearophile

About first point: looks like a bug at the interface between union and asm. I
could rol either a pointer or an uint, but none of them member of a union.
Dunno why. Indeed, using '<<' works as expected.
But I could have this work (which rather confirms the above interpretation):

union Four {
uint u;
ubyte[4] a;
}

void main() {
Four f;

f.u = 0x_01_02_03_04u;
writefln ("'0x%08X'" , f.u);

// deceiving D:
auto p = &(f.u);
auto u = *p;

asm {
rol u, 8;
}
writefln ("'0x%08X'" , u);

/*  output:
'0x01020304'
'0x02030401'
*/
}

I have no idea about the second point.

Denis
--
_________________
vita es estrany
spir.wikidot.com
```
Mar 10 2011
Kai Meyer <kai unixlords.com> writes:
```On 03/09/2011 03:41 PM, Tom wrote:
What is the most efficient way of implement a rotation of ubyte[4] array?

By rotation I mean: rotateRight([1, 2, 3, 4]) -> [4, 1, 2, 3]

TIA,
Tom;

I don't know of anything more efficient than:
ubyte[4] bytes = [1,2,3,4];
bytes = bytes[\$-1] ~ bytes[0..\$-1]; // Rotate left
bytes = bytes[1..\$] ~ bytes[0]; // Rotate right

Both static arrays and dynamic arrays (ubyte[] bytes = [1,2,3,4];)
perform about the same between 1 and 10 milling rotations in either
direction. I think a temporary array might be created for the rhs, and
then have the values of the rhs array copied to the lhs array, but I
don't know. With static arrays, I'm not sure there would be a way to get
around it with out at least a temporary value for the one that's moving
between the first and last positions.
```
Mar 09 2011
Jonathan M Davis <jmdavisProg gmx.com> writes:
```On Wednesday, March 09, 2011 15:35:29 Kai Meyer wrote:
On 03/09/2011 03:41 PM, Tom wrote:
What is the most efficient way of implement a rotation of ubyte[4] array?

By rotation I mean: rotateRight([1, 2, 3, 4]) -> [4, 1, 2, 3]

TIA,
Tom;

I don't know of anything more efficient than:
ubyte[4] bytes = [1,2,3,4];
bytes = bytes[\$-1] ~ bytes[0..\$-1]; // Rotate left

I'm stunned that this works. I'd even consider reporting it as a bug. You're
concatenating a ubyte[] ont a ubyte...

bytes = bytes[1..\$] ~ bytes[0]; // Rotate right

You're concatenating a ubyte onto a slice of the array (so it's ubyte[] instead
of ubyte[4]). That will result in a temporary whose value will then be assigned
to the original ubyte[4].

Both static arrays and dynamic arrays (ubyte[] bytes = [1,2,3,4];)
perform about the same between 1 and 10 milling rotations in either
direction. I think a temporary array might be created for the rhs, and
then have the values of the rhs array copied to the lhs array, but I
don't know. With static arrays, I'm not sure there would be a way to get
around it with out at least a temporary value for the one that's moving
between the first and last positions.

Honestly, given that this is 4 ubytes, I would fully expect that the fastest
way
to do this would involve casting it to a unit and shifting it - something along
the lines of what Bearophile suggested. I'd be _very_ suprised if this
implementation were faster, since it involves creating a temporary array.

- Jonathan M Davis
```
Mar 09 2011
spir <denis.spir gmail.com> writes:
```On 03/10/2011 12:55 AM, Jonathan M Davis wrote:
I don't know of anything more efficient than:
ubyte[4] bytes = [1,2,3,4];
bytes = bytes[\$-1] ~ bytes[0..\$-1]; // Rotate left

I'm stunned that this works. I'd even consider reporting it as a bug. You're
concatenating a ubyte[] ont a ubyte...

This works for other arrays as well. dmd understands.

Denis
--
_________________
vita es estrany
spir.wikidot.com
```
Mar 09 2011