assembly - LEA or ADD instruction?

Question

When I'm handwriting assembly, I generally choose the form

lea eax, [eax+4]

Over the form..

add eax, 4

I have heard that lea is a "0-clock" instruction (like NOP), while 'add' isn't. However, when I look at compiler produced Assembly I often see the latter form used instead of the first. I'm smart enough to trust the compiler, so can anyone shed some light on which one is better? Which one is faster? Why is the compiler choosing the latter form over the former?

Answer 1

One significant difference between LEA and ADD on x86 CPUs is the execution unit which actually performs the instruction. Modern x86 CPUs are superscalar and have multiple execution units that operate in parallel, with the pipeline feeding them somewhat like round-robin (bar stalls). Thing is, LEA is processed by (one of) the unit(s) dealing with addressing (which happens at an early stage in the pipeline), while ADD goes to the ALU(s) (arithmetic / logical unit), and late in the pipeline. That means a superscalar x86 CPU can concurrently execute a LEA and an arithmetic/logical instruction.

The fact that LEA goes through the address generation logic instead of the arithmetic units is also the reason why it used to be called "zero-clocks"; it takes no time to execute because address generation has already happened by the time it would be / is executed.

It's not free, since address generation is a step in the execution pipeline, but it's got no execution overhead. And it doesn't occupy a slot in the ALU pipeline(s).

Edit: To clarify, LEA is not free. Even on CPUs that do not implement it via the arithmetic unit it takes time to execute due to instruction decode / dispatch / retire and/or other pipeline stages that all instructions go through. The time taken to do LEA just occurs in a different stage of the pipeline for CPUs that implement it via address generation.