C to assembly - leaq instruction

Question

As far as I understood:

The 3rd line on the assembly code leaq (%rsi, %rsi, 2), %rdx, which as I translate it: z = y + 2*y.

I'm confused, is this code wrong, or is it an assembly magic?

Yes, I've just noticed that, I think it goes like `3y << 4 = y * 48`. — dud3, Mar 22 '17 at 23:44
@Jester some of these instructions are quite hard to digest(for me), do you happen to have any good references? — dud3, Mar 22 '17 at 23:47
For others, analyzed it one more time, this code seems to be correct. — dud3, Mar 22 '17 at 23:54
Apart from the official intel instruction set reference? What else do you need :) — Jester, Mar 23 '17 at 00:14

score 1 · Answer 1 · answered Jan 16 '19 at 22:53

1

simple explanation

it moves the address not the values

answered Jan 16 '19 at 22:53

Arun Kumar

1

Can you quote the most relevant portion of the PDF link? – Will Jan 17 '19 at 17:14
2

leaq (%rsi, %rsi, 2), %rdx means %rdx=2*rsi+rsi which is equal to 3*rsi in binary we write 3 as 000011 so after 4 left sfift (sal-shift acc. left) 110000 which is 48 – Arun Kumar Jan 17 '19 at 21:00

1 Answers1