What is register %eiz?

assembly x86 gnu-assembler objdump
Summer_More_More_Tea picture Summer_More_More_Tea · Mar 31, 2010 · Viewed 9k times · Source

In the following assembly code that I dumped out using objdump:

lea    0x0(%esi,%eiz,1),%esi

What is register %eiz? What does the preceding code mean?

Answer

Sinan Ünür picture Sinan Ünür · Mar 31, 2010

See Why Does GCC LEA EIZ?:

Apparently %eiz is a pseudo-register that just evaluates to zero at all times (like r0 on MIPS).

...

I eventually found a mailing list post by binutils guru Ian Lance Taylor that reveals the answer. Sometimes GCC inserts NOP instructions into the code stream to ensure proper alignment and stuff like that. The NOP instruction takes one byte, so you would think that you could just add as many as needed. But according to Ian Lance Taylor, it’s faster for the chip to execute one long instruction than many short instructions. So rather than inserting seven NOP instructions, they instead use one bizarro LEA, which uses up seven bytes and is semantically equivalent to a NOP.

Related questions

What does cltq do in assembly?

0x0000000000400553 <main+59>: mov -0x4(%rbp),%eax 0x0000000000400556 <main+62>: cltq 0x0000000000400558 <main+64>: shl $0x3,%rax 0x000000000040055c <main+68>: mov %rax,%rdx In fact my programe is as simple as : 5 int main(int …

Read More
What does the bracket in `movl (%eax), %eax` mean?

I have googled enough but could not figure out what the bracket () means. Also, I see some syntax as movl 8(%ebp), %eax Could some someone suggest me some good reference? I have not been able to find any in the …

Read More
NASM Vs GAS (Practical differences)

I'm not trying to prompt an Intel vs AT&T war (moot point anyway, now that they both support Intel syntax) or ask which one is "better" per se, I just want to know the practical differences in choosing …

Read More