Both tools translate assembly instructions directly into machine code, but is it possible to determine which one produces the fastest and cleanest code?
+13
A:
When you're writing in assembler, you are precisely describing the instructions to generate so it doesn't depend on the assembler. It depends on you. There's a one-to-one correspondence between the mnemonics you write and actual instructions in machine code.
Mehrdad Afshari
2009-08-15 19:27:41
Technically that's not quite true; for things like addressing modes, there can be more than one way of encoding the same location. There are other isomorphisms that I've forgotten about; an x86 shareware assembler from back in the day used these isomorphisms to "fingerprint" code assembled with it, with a view to catching cheats.
Barry Kelly
2009-08-15 19:35:22
I understand what you are saying. There can be different machine code instructions with similar effect. Technically, an assembler can provide two different representations for those instructions (`int3` and `int 3`) or use some hints to choose which one to generate. These stuff are usually documented for each assembler. I guess for the purpose of this question, it's reasonable to ignore these issues since theoretically, **they are really different instructions** and the assembler could allow you to choose from one.
Mehrdad Afshari
2009-08-15 19:39:55
Even in assembler there is some room for optimzing. A lot of architectures allow short modes for adressing, for example using a byte value for relative displacment instead of a long. Since this optimizations change the size of the object code, some displacments may be used in the short form, so a multi pass optimization is required. AFAIK yasm can do this.
drhirsch
2009-08-16 00:02:46
The point is, while assemblers can help, most of the time, you can still force the exact instruction you want to generate.
Mehrdad Afshari
2009-08-16 02:59:09
If I remember correctly, NASM in particular is of the "what you write is what you get" persuasion, and will generate precisely the opcodes you request; if there's an ambiguity anywhere, you will have to disambiguate explicitly.
Pavel Minaev
2009-08-17 22:46:28
It may also matter if you may need to write assembly on other platforms. It's nice to have them be in the same format, and NASM only works for x86/x64.
Christopher
2009-08-19 20:00:08
@Pinael: NASM lets you specify an "optimisation level" which actually indicates how many passes over the code to make. As more addresses are resolved, long addresses can be replaced with short ones and so forth.
Artelius
2009-11-08 05:38:34
+2
A:
It is assember... it does not optimize code. It just translates as is. So the fastest and cleanest code is produced by programmer or compiler
Artyom
2009-08-15 19:27:41
Ok got it!Now, the assembler is also a compiler right?Could it save a CPU cycle here and there? that was my doubt...
cyber988348
2009-08-16 19:52:41
+2
A:
Obviously nasm because Intel syntax looks much cleaner than AT&T syntax.
Brian
2009-08-15 19:28:08
A:
@Brian: that was not the question ...
@cyber98834: Well, an assembler does what every assembler must do : translate every instruction to its opcode .
There's no optimization .
Oh and also, there's not such a thing as a "fastest code" ... Can I ask you a question ? The CPU's speed is static, isn't it ?
So, you can't make a code run faster because you can't change the CPU's speed .
But, you can shrink the code so that the CPU handles less amount of instructions, and so takes less time to run .
I hope you understand what I'm trying to say .
I suggest you to buy ( or to look for some pdf's, but I don't know if that's legal ) Michael Abrash's Graphics Programming Black Book which covers many optimization lessons .
SBouazza
2009-08-15 19:38:10
Hi SBouazza!I understand your point. Thanks!I`ve already got Michael Abrash`s Graphics Programming Black Book ready (http://www.gamedev.net/reference/articles/article1698.asp) for later :)
cyber988348
2009-08-16 19:44:17
+4
A:
I don't know about these two specific tools, but there are some instructions that can be encoded differently:
ADD AX,1is either05 01or81 c0 01orfe c0INT 3is eitherccorcd 03- New AVX instructions that extend two-byte SSE instructions will either have a 2-byte or 3-byte prefix. All 2-byte prefixes can be encoded as 3-byte prefixes as well.
These are just a few examples off the top of my head of how assemblers can encode the same instruction differently, so the question does in fact make sense.
Nathan Fellman
2009-08-15 20:07:18
Most assemblers usually choose the instruction with minimum size and they can provide hints to let you choose specifically which one you'd like to generate. For instance, `cc` and `cd 03` are two distinct instructions from the perspective of CPU. It's completely on behalf of the assembler to choose not to provide a way to generate an instruction.
Mehrdad Afshari
2009-08-15 20:21:19
I agree that a good assembler *should* do that. I'm just pointing out ways in which an assembler can do something different. For instance, `INC AX` encoded as `fe c0` is exactly the same as `ADD AX, 1` encoded as `05 01` as far as functionality and instruction length are concerned, though they are indeed decoded separately in the CPU.
Nathan Fellman
2009-08-16 04:02:35
That's what I was actually looking for with my question.I'm starting to learn x86 assembly (been reading the Intel manual) and want to use Linux tools for that, but was in doubt about which tool to start with.Since it seems no real difference exists between gas and nasm I'll pick up nasm because of the friendlier sintax.Thanks for the help!
cyber988348
2009-08-16 19:39:12
+4
A:
As a sidenote on the syntax-matter. You can have GAS work perfectly fine with Intel syntax by putting the following line at the top of your source file:
.intel_syntax noprefix
I am using Intel syntax too for all my assmebly needs. It seems far more natural than the AT&T syntax. And it saves some keystrokes :-).
RoaldFre
2009-08-17 22:40:22