Add divmod post

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+title = "2023"
+sort_by = "date"
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v2/content/posts/2023/divmod.md

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+title = "divmod, Rust, x86, and Optimisation"
+date = 2023-01-11T19:48:09+10:00
+
+#[extra]
+#updated = 2022-04-21T09:07:57+10:00
++++
+
+While reviewing some Rust code that did something like this:
+
+```rust
+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][rust-div-mod] back in 2013 but it was moved out of the standard
+library before the 1.0 release.
+
+<!-- more -->
+
+I also learned that the [`div` instruction on x86][div] 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][compiler-explorer] confirmed it.
+
+This function:
+
+```rust
+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: I'm still learning x86 assembly):
+
+```asm
+; 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 there'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][timing] 32-bit `div` has lower latency—particularly on older
+models.
+
+I wasn't able to work out why each implementation zeros `edx`. If you know,
+send me a message and I'll update the post.
+
+[View the Example on Compiler Explorer](https://rust.godbolt.org/z/hj9rb4Txa)
+
+[Brendan]: https://github.com/brendanzab
+[rust-div-mod]: https://github.com/rust-lang/rust/commit/f39152e07baf03fc1ff4c8b2c1678ac857b4a512
+[div]: https://www.felixcloutier.com/x86/div
+[compiler-explorer]: https://rust.godbolt.org/
+[timing]: https://gmplib.org/~tege/x86-timing.pdf