When do we need $v1 in MIPS?

Question

We all know that usually a program will just return one value, but I have noticed that we have two value returning registers $v0 and $v1 in MIPS. T

Answer 1

Another use case is returning 64-bit numbers on 32-bit MIPS (or 128-bit numbers in 64-bit MIPS).

Answer 2

I can think of plenty of examples, but it's probably just up to your discretion. The best example is probably a function that returns a value in $v0 and may set an error code in $v1.

Answer 3

There are lots of uses for that

• Returning a value twice the length of the register (64-bit values in MIPS32 and 128-bit values in MIPS64) as @Martin Rosenau mentioned

  long long mult(int x, int y) {
      return  (long long)x * y;
  }
  

  Output assembly:

  mult(int, int):
      mult    $4,$5
      mflo    $2
      mfhi    $3
      j       $31
      nop
  

• Returning structs that fit into 2 registers like these

  typedef struct {
      int a, b;
  } ret_struct2x32;
  
  ret_struct2x32 return_struct2x32() {
      return ret_struct2x32{2, 3};
  }
  
  typedef struct {
      short a, b, c, d;
  } ret_struct4x16;
  
  ret_struct4x16 return_struct4x16() {
      return ret_struct4x16{(short)2, (short)0, (short)3, (short)0};
  }
  

  or returning tuples that appear in some languages like python

  def func(x,y):
      # code to compute x and y
      return x,y
  
  a, b = 1, 2
  u, v = func(a, b)
  

  In C++ we have std::pair and std::tuple

  std::pair<int, int> return_pair()
  {
      return std::make_pair(2, 3);
  }
  
  std::tuple<short, short, short, short> return_tuple()
  {
      return std::make_tuple((short)2, (short)0, (short)3, (short)0);
  }
  

  See the demo on Compiler Explorer. Unfortunately the gcc version for MIPS is too old and can't make use the struct-in-register optimization, so look at the x86 output and you'll see that the whole tuples are returned in only a single instruction

It's similar to the pair (E/R)DX:(E/R)AX in x86 to return a value double the size of the registers

See also

• About returning more than one value in C/C++/Assembly
• Why MIPS assembler has more that one register for return value?