Why can assembly instructions contain multiplications in the “lea” instruction?

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 instructio

Answer 1

To expand on your last question:

Is the multiplication limited to powers of 2 (I would assume this is the case)?

Note that you get the result of base + scale * index, so while scale has to be 1, 2, 4 or 8 (the size of x86 integer datatypes), you can get the equivalent of a multiplication by some different constants by using the same register as base and index, e.g.:

lea eax, [eax*4 + eax]   ; multiply by 5

This is used by the compiler to do strength reduction, e.g: for a multiplication by 100, depending on compiler options (target CPU model, optimization options), you may get:

lea    (%edx,%edx,4),%eax   ; eax = orig_edx * 5
lea    (%eax,%eax,4),%eax   ; eax = eax * 5 = orig_edx * 25
shl    $0x2,%eax            ; eax = eax * 4 = orig_edx * 100