While reviewing some Rust code that did something like this:

```
let a = n / d;
let b = n % d;
```

I lamented the lack of a `divmod`

method in Rust (that would return both the
quotient and remainder). My colleague Brendan pointed out that he actually
added it back in 2013 but it was moved out of the standard
library before the 1.0 release.

I also learned that the `div`

instruction on x86 provides the remainder
so there is potentially some benefit to combining the operation. I suspected
that LLVM was probably able to optimise the separate operations and a trip to
the Compiler Explorer confirmed it.

This function:

```
pub fn divmod(n: usize, d: usize) -> (usize, usize) {
(n / d, n % d)
}
```

Compiles to the following assembly, which I have annotated with my understanding of each line (Note: Iâ€™m still learning x86 assembly):

```
; rdi = numerator, rsi = denominator
example::divmod:
test rsi, rsi ; check for denominator of zero
je .LBB0_5 ; jump to div zero panic if zero
mov rax, rdi ; load rax with numerator
or rax, rsi ; or rax with denominator
shr rax, 32 ; shift rax right 32-bits
je .LBB0_2 ; if the result of the shift sets the zero flag then numerator and
; denominator are 32-bit since none of the upper 32-bits are set.
; jump to 32-bit division implementation
mov rax, rdi ; move numerator into rax
xor edx, edx ; zero edx (I'm not sure why, might be relevant to the calling
; convention and is used by the caller?)
div rsi ; divide rax by rsi
ret ; return, quotient is in rax, remainder in rdx
; 32 bit implementation
.LBB0_2:
mov eax, edi ; move edi to eax (32-bit regs)
xor edx, edx ; zero edx
div esi ; divide eax by esi
ret
; div zero panic
.LBB0_5:
push rax
lea rdi, [rip + str.0]
lea rdx, [rip + .L__unnamed_1]
mov esi, 25
call qword ptr [rip + core::panicking::panic@GOTPCREL]
ud2
.L__unnamed_2:
.ascii "/app/example.rs"
.L__unnamed_1:
.quad .L__unnamed_2
.asciz "\017\000\000\000\000\000\000\000\002\000\000\000\006\000\000"
str.0:
.ascii "attempt to divide by zero"
```

I found it interesting that after checking for a zero denominator thereâ€™s an
additional check to see if the values fit into 32-bits, and if so it jumps to an
instruction sequence that uses 32-bit registers. According to the testing done
in this report 32-bit `div`

has lower latencyâ€”particularly on older
models.

~~I wasnâ€™t able to work out why each implementation zeros ~~`edx`

. If you know,
send me a message and Iâ€™ll update the post.

**Update:** Brion Vibber on the Fediverse provided this explanation as to why `edx`

is being zeroed:

iirc rdx / edx is the top word for the x86 division operation, which takes a double-word numerator â€“ the inverse of multiplication producing a double-word output.

This makes sense and looking back at the docs it does say that:

32-bit: Unsigned divide EDX:EAX by r/m32, with result stored in EAX := Quotient, EDX := Remainder.

64-bit: Unsigned divide RDX:RAX by r/m64, with result stored in RAX := Quotient, RDX := Remainder.