BT instruction?

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vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 22 Jan 2007, 06:05

so anyone could create thread for "Speed testing code" and put it all together? Wink

22 Jan 2007, 06:05

Borsuc

Joined: 29 Dec 2005
Posts: 2465
Location: Bucharest, Romania

Borsuc 24 Jan 2007, 10:52

I mainly use BT for small code.. speed is hard to measure because it varies on processor.

I remember some time ago I did this test, and bt was much slower on my Pentium 4, but almost tied on my AMD Mobile Sempron 2800+.

But I can't remember if I did it right, but I did use maximum priority (even the mouse froze during the test).

24 Jan 2007, 10:52

Plue

Joined: 15 Dec 2005
Posts: 151

Plue 24 Jan 2007, 16:10

> I mainly use BT for small code
But BT is larger than TEST?!?

24 Jan 2007, 16:10

f0dder

Joined: 19 Feb 2004
Posts: 3175
Location: Denmark

f0dder 24 Jan 2007, 16:43

Plue wrote:

> I mainly use BT for small code
But BT is larger than TEST?!?

Nah, you need more than just a TEST instruction to perform the sam as BT [mem32], reg32 - BT is small here.

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24 Jan 2007, 16:43

MCD

Joined: 21 Aug 2004
Posts: 602
Location: Germany

MCD 25 Jan 2007, 11:40

f0dder wrote:

Nah, you need more than just a TEST instruction to perform the sam as BT [mem32], reg32 - BT is small here.

You absolutely right. BTx instructions are useful when you need to test a bit at a position not know at compile-time and if you only have a few bits to test.

if bit position is know at compile time => use TEST or something

if you have lots of bits to test => try building a mask, maybe with some look up table and perform a TEST with that

BTW: BTx is almost as fast as TEST (except for the memory version) on my Athlon XP, but was way slower than TEST on a PII

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25 Jan 2007, 11:40

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8367
Location: Kraków, Poland

Tomasz Grysztar 25 Jan 2007, 11:42

BTx instructions other than BT are also useful because the allow to test the bit and alter it at the same time.

25 Jan 2007, 11:42

16bitPM

Joined: 08 Jul 2011
Posts: 30

16bitPM 23 Oct 2022, 15:44

Tomasz Grysztar wrote:

BTx instructions other than BT are also useful because the allow to test the bit and alter it at the same time.

I would also say for bit and bit string manipulation, because BT effectively copies a bit to the carry flag, you can use it directly in arithmetic and shift operations. Other than that specific use case, BT reg,imm seems rather superfluous to me.

23 Oct 2022, 15:44

Furs

Joined: 04 Mar 2016
Posts: 2595

Furs 24 Oct 2022, 13:15

16bitPM wrote:

Tomasz Grysztar wrote:
BTx instructions other than BT are also useful because the allow to test the bit and alter it at the same time.

I would also say for bit and bit string manipulation, because BT effectively copies a bit to the carry flag, you can use it directly in arithmetic and shift operations. Other than that specific use case, BT reg,imm seems rather superfluous to me.

Here's another one: in 64-bit mode, immediates are 32-bit sign extended, so if you want to test a bit in the upper 32-bit half of the 64-bit register, only BT can do that with an immediate.

24 Oct 2022, 13:15

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20517
Location: In your JS exploiting you and your system

revolution 24 Oct 2022, 13:20

Furs wrote:

Here's another one: in 64-bit mode, immediates are 32-bit sign extended, so if you want to test a bit in the upper 32-bit half of the 64-bit register, only BT can do that with an immediate.

I don't understand what you mean.

This instruction is valid:

Code:

bt rax,rcx

Are you confusing 32-bit immediate (+- 2GB) with 32-bit register length?

24 Oct 2022, 13:20

16bitPM

Joined: 08 Jul 2011
Posts: 30

16bitPM 24 Oct 2022, 14:20

revolution wrote:

Furs wrote:
Here's another one: in 64-bit mode, immediates are 32-bit sign extended, so if you want to test a bit in the upper 32-bit half of the 64-bit register, only BT can do that with an immediate.
I don't understand what you mean.

This instruction is valid:
Code:
bt rax,rcx    
Are you confusing 32-bit immediate (+- 2GB) with 32-bit register length?

No, he was replying on my comment on the specific case of BT with an immediate operand and a register.

24 Oct 2022, 14:20

Furs

Joined: 04 Mar 2016
Posts: 2595

Furs 25 Oct 2022, 13:01

Yeah, basically you can't do:

Code:

test rax, 1 shl 42

but you can do:

Code:

bt rax, 42

25 Oct 2022, 13:01

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