Question

I am working on a very low level part of the application in which performance is critical.

While investigating the generated assembly, I noticed the following instruction:

lea eax,[edx*8+8]

I am used to seeing additions when using memory references (e.g. [edx+4]), but this is the first time I see a multiplication.

• Does this mean that the x86 processor can perform simple multiplications in the lea instruction?
• Does this multiplication have an impact on the number of cycles needed to execute the instruction?
• Is the multiplication limited to powers of 2 (I would assume this is the case)?

Thanks in advance.

Answer 1

To expand on my comment and to answer the rest of the question…

Yes, it’s limited to powers of two. (2, 4, and 8 specifically) So no multiplier is needed since it’s just a shift. The point of it is to quickly generate an address from an index variable and a pointer – where the datatype is a simple 2, 4, or 8 byte word. (Though it’s often abused for other uses as well.)

As for the number of cycles that are needed: According to Agner Fog’s tables it looks like the lea instruction is constant on some machines and variable on others.

On Sandy Bridge there’s a 2-cycle penalty if it’s “complex or rip relative”. But it doesn’t say what “complex” means… So we can only guess unless you do a benchmark.