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. The code I have seen all only have $v0 but $v1, and I haven't found any assembly language example code that uses $v1.

Could anyone tell me why we have two $v registers in MIPS and show me a example of when it could be used?

The CPU registers don't just have a single usage. Nearly all of the registers can be used internally by a function for just about any purpose; and the more of them you've got, the better. — Michael, Oct 06 '13 at 15:01
possible duplicate of [Why MIPS assembler has more that one register for return value?](http://stackoverflow.com/questions/20131986/why-mips-assembler-has-more-that-one-register-for-return-value) — phuclv, Jul 19 '15 at 03:17
examples where multiple return registers are used can easily been seen here [About returning more than one value in C/C++/Assembly](https://stackoverflow.com/q/31497152/995714) — phuclv, Sep 26 '18 at 02:17

phuclv · Accepted Answer · 2019-07-29T15:58:10.493

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

When do we need $v1 in MIPS?

3 Answers3

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