Assembly and RISC-V Calling Convention

Question

A nested function calls another function (plot_function with parameter a0) and is also called by another function. Given the code that follows RISCV calling convention, which sets of registers should be saved to make this program run more optimally.  All the function knows is that the plot_function uses a0 as a parameter, and its own caller passes a value in a0. mv s1, s2  is the pseudo instruction equivalent of: addi s1, s2, 0

   add_memory_values:

      # save reg_set1 on stack here

      mv s1, a0   # parameter used by this function, holds an address

      li  s2, 1000  # down counter

      li s3, 0     # accumulator  

  loop1:

      lwu t1, 0(s1) # load from memory

      addw s1, 4  # go to to next word

      add s3, t1  # add memory value to accumulator

      mv a0, s3   # pass a parameter to plot_numbers

      # save reg_set2 here

      jal plot_numbers

      # restore reg_set2 here

      addi s2, -1  # decrement counter

      bgtz s2, loop1

      mv a0, s3

      # restore reg_set1 here

      ret

       

What combinations should apply (mark as all that are true):    

  t1 needs to be saved.

 

  ra does not need saving

 

  only ra needs to be saved

 

  all s# registers should be saved

 

  reg_set2 is: should include t1

 

  reg_set1 is empty

 

  a0 doesn't need to be saved on the stack

 

  only t1 and ra need to be saved

 

  all t# should be saved

 

  reg_set1 should include ra, s1, s2, s3

 

  reg_set2 is empty

 

Answer 1

here the goal is to save registers we loaded before making nested call (reg_set2 and reg set restored at end of call better have saved return address and all registers used in this function. hence right choices are:

* t1 needs to be saved

* reg_set2 is: should include t1

* reg_set1 should include ra, s1, s2, s3

let me know if any question

Sanjay