Argthtjtr

1. Let set R_eff always contain all registers that are effective at the current program position. R_eff := { r | r is output register }. Start at the last program instruction and move backwards.


2. Mark the next preceding operation in program with: destination register r_dest element-of R_eff. If such an instruction is not found then go to 5.


3. .If the operation directly follows a branch or a sequence of branches then mark these instructions too. Otherwise remove r_dest
   from R_eff .


4. Insert each source (operand) register r_op of newly marked
   instructions in R_eff if not already contained. Go to 2.


5. Stop. All unmarked instructions are introns.

This is the algorithm given , I know the question was asked before, but I'm not clear on a few things( and turns out there isn't much online material assisting with this stuff). I mainly need to know what we're comparing. You have a program, and within the program, are instructions with the form of :

r0= r1 + r2

Just as an example. So I understand we start at the last instruction and move backwards for our comparisons. But in step 2 when it says to move backwards are we going back the to the immediately previous instructions, or are we moving backwards to the instruction with a register in common?
For the example I gave, would I keep searching backwards till I found another r0, or would I go backwards until I found a r1 or r2?

I'm pretty confused with this simple thing and would appreciate any help I can get