wezm/wezm.net
1
0
Fork 1
mirror of https://github.com/wezm/wezm.net.git synced 2024-12-18 18:29:54 +00:00
Code Issues Projects Releases Packages Wiki Activity

Update divmod post

Browse source
This commit is contained in:
Wesley Moore 2023-01-11 21:11:43 +10:00
parent e00bb7868c
commit 0a4f1d2a45
No known key found for this signature in database
1 changed files with 18 additions and 4 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

22
v2/content/posts/2023/divmod.md
View file

@ -2,8 +2,8 @@
title = "divmod, Rust, x86, and Optimisation" title = "divmod, Rust, x86, and Optimisation"
date = 2023-01-11T19:48:09+10:00 date = 2023-01-11T19:48:09+10:00
#[extra] [extra]
#updated = 2022-04-21T09:07:57+10:00 updated = 2023-01-11T21:11:28+10:00
+++ +++
While reviewing some Rust code that did something like this: While reviewing some Rust code that did something like this:
@ -86,8 +86,21 @@ 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 in this report][timing] 32-bit `div` has lower latency—particularly on older
models. models.
I wasn't able to work out why each implementation zeros `edx`. If you know, ~~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. send me a message and I'll update the post.~~
**Update:** [Brion Vibber on the Fediverse][edx] 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 -- the inverse of multiplication producing a double-word output.
This makes sense and looking back at [the docs][div] 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.
[View the Example on Compiler Explorer](https://rust.godbolt.org/z/hj9rb4Txa) [View the Example on Compiler Explorer](https://rust.godbolt.org/z/hj9rb4Txa)
@ -96,3 +109,4 @@ send me a message and I'll update the post.
[div]: https://www.felixcloutier.com/x86/div [div]: https://www.felixcloutier.com/x86/div
[compiler-explorer]: https://rust.godbolt.org/ [compiler-explorer]: https://rust.godbolt.org/
[timing]: https://gmplib.org/~tege/x86-timing.pdf [timing]: https://gmplib.org/~tege/x86-timing.pdf
[edx]: https://bikeshed.vibber.net/@brion/109670222269686433