Shifting and sign

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axlucas

Joined: 02 May 2014
Posts: 70
Location: Argentina

axlucas 19 Oct 2017, 07:03

I need to divide by 256, but I want to do it fast. I thought of shifting right 8 bits, but the problem is I'm using signed integers. Then suddenly, I wonder if shr actually preserves the higher bit or not or whether there's a signed and an unsigned version of this mnemonic. Funny how I never put attention to this before. Is this possible?

19 Oct 2017, 07:03

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8351
Location: Kraków, Poland

Tomasz Grysztar 19 Oct 2017, 07:12

Take a look at SAR instruction.

19 Oct 2017, 07:12

Hrstka

Joined: 05 May 2008
Posts: 56
Location: Czech republic

Hrstka 27 Oct 2017, 13:43

Keep in mind that div and sar don't produce the same result for negative values.

-257 idiv 256 = -1
-257 sar 8 = -2

27 Oct 2017, 13:43

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 27 Oct 2017, 13:56

Hrstka wrote:

Keep in mind that div and sar don't produce the same result for negative values.

-257 div 256 = -1
-257 sar 8 = -2

-257 div 256 = -1 and remainder -1

And it is ok because:
-1 * 256 + (-1) = -257

-257 sar 8 = -2 and remainder 255.

Which is OK as well, because -2 * 256 + 255 = -257 Wink

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27 Oct 2017, 13:56

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 27 Oct 2017, 13:59

JohnFound wrote:

-257 sar 8 = -2 and remainder 255.

Me is wondering where the remainder is found after using SAR? Confused

27 Oct 2017, 13:59

tthsqe

Joined: 20 May 2009
Posts: 767

tthsqe 28 Oct 2017, 09:57

The compiler folks have had this one figured out for a long time.
https://godbolt.org/g/rhpn7y

28 Oct 2017, 09:57

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 28 Oct 2017, 10:45

revolution wrote:

JohnFound wrote:
-257 sar 8 = -2 and remainder 255.
Me is wondering where the remainder is found after using SAR?

The remainder is there before using SAR, because, you know, most instructions return only one result, not two like DIV.

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28 Oct 2017, 10:45

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 28 Oct 2017, 14:21

tthsqe wrote:

The compiler folks have had this one figured out for a long time.
https://godbolt.org/g/rhpn7y

All I see is a mostly blank page with a small dark grey header at the top. Question

28 Oct 2017, 14:21

tthsqe

Joined: 20 May 2009
Posts: 767

tthsqe 28 Oct 2017, 16:43

Quote:

All I see is a mostly blank page with a small dark grey header at the top.

For all you non-JS users:

Code:

int f(int a){return a/256;}

f(int):
  lea eax, [rdi+255]
  test edi, edi
  cmovns eax, edi
  sar eax, 8
  ret

28 Oct 2017, 16:43

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 29 Oct 2017, 13:35

tthsqe wrote:

For all you non-JS users:

Code:

int f(int a){return a/256;}

f(int):
  lea eax, [rdi+255]
  test edi, edi
  cmovns eax, edi
  sar eax, 8
  ret

Thanks.

That code is good, but it assumes the existence of CMOVcc. For 64-bit code that is fine, but there are still some 32-bit CPUs that don't have it.

29 Oct 2017, 13:35

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