Choosing between accumulator, SIB and ModR/M

[Marat Dukhan]

I have few related questions:
1. Is there a way to force NASM to use SIB form for memory operands which are normally encoded with only ModR/M?
2. Is there a way to force NASM to use ModR/M form for instructions which normally use different encoding (e.g. PUSH ecx; ADD eax, 10)?

[Frank Kotler]

I guess you could rewrite Nasm. I don't think there's any "easy" way. Is there something wrong with the way Nasm's doing it?

[Marat Dukhan]

I want to do alignment of instructions in a better way than just inserting NOPs or prefixes. Manipulating instruction forms is an obvious way to achieve that.

[Frank Kotler]

Okay. So you'd need to switch back and forth "at will"? I think that would take a major re-write. Do you have reason to think that longer instructions would be "better" than a few "nop"s? (I don't know)

Best,
Frank

[Rob Neff]

Within the .TEXT segment use a sequence of db "YOUR_OPCODE" statements to insert exactly what you want where you want it.

[Marat Dukhan]

Okay. So you'd need to switch back and forth "at will"?

I would prefer it to work on per-instruction basis. For example, now I can write

Code: [Select]

MOVDQA xmm0, [esi]or

Code: [Select]

MOVDQA xmm0, [byte esi + 0]or

Code: [Select]

MOVDQA xmm0, [dword esi + 0]
to switch between no offset, byte offset, and dword offset. It would be good if I could also specify something like

Code: [Select]

MOVDQA xmm0, [sib esi]
to force NASM to use SIB.

Similarly, I can write

Code: [Select]

DS ADD eax, 10
to add a DS segment prefix to this instruction. I want to also have an option to write e.g.

Code: [Select]

MODRM ADD eax, 10
to encode ADD eax, 10 using ModR/M scheme.

Do you have reason to think that longer instructions would be "better" than a few "nop"s? (I don't know)

For high-performance code modern processors often require certain alignment of instructions, e.g. for Intel Atom pairs of instructions should be aligned on 8-byte boundary, and for Intel Nehalem they should be aligned on 16 bytes. However, adding NOPs decreases the efficiency of instruction decoder (which is exactly what I want to improve with this optimization), and the same problem arises when using prefixes for aligning instructions. So changing instruction forms is the best option.

[Marat Dukhan]

Within the .TEXT segment use a sequence of db "YOUR_OPCODE" statements to insert exactly what you want where you want it.

I want the code to be human-readable, so this is not an option for me. Besides that, it will not generate relocs.

[Rob Neff]

Feel free to post your suggestion to the dev mail list:
https://lists.sourceforge.net/lists/listinfo/nasm-devel