It's been a year now since I first asked this question below, and I'm inquiring has anything been done on introducing a fill or clear method in Rust that can do a fast bulk zeroing (setting mem elements to '0') of arrays/slices?
Sure looks like a fill methods exists now.
Edit: It seems to be slightly suboptimal for your use case in current stable rust, but that’s already fixed on nightly.
The codegen for .fill(0) is very different to .iter_mut().for_each(|m| *m = 0). Here is the .iter_mut().for_each(|m| *m = 0) assembly:
test rsi, rsi
je .LBB0_1
mov rdx, rsi
xor esi, esi
jmp qword ptr [rip + memset@GOTPCREL]
.LBB0_1:
ret
And here is the fill(0) assembly:
test rsi, rsi
je .LBB0_4
push r14
push rbx
push rax
mov rbx, rsi
mov r14, rdi
dec rbx
je .LBB0_3
mov rdi, r14
xor esi, esi
mov rdx, rbx
call qword ptr [rip + memset@GOTPCREL]
.LBB0_3:
mov byte ptr [r14 + rbx], 0
add rsp, 8
pop rbx
pop r14
.LBB0_4:
ret
They're both optimized. The fill(0) version includes an extra branch for when the slice length is 1, whereas the iterator version just calls memset directly.
However, on Nightly the situation is different. fill(0) compiles to:
xor edx, edx
jmp qword ptr [rip + _ZN76_$LT$$u5b$u8$u5d$$u20$as$u20$core..slice..specialize..SpecFill$LT$u8$GT$$GT$9spec_fill17h84e9f892edee5b96E@GOTPCREL]
That will invoke this specialized implementation, which I'm guessing will just become a memset. It's a shame that isn't inlined though - there's one extra function call :(.
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Well, let’s see if someone picks this up
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Thanks for the quick response to my question.
I'm using Rust 1.50, and tried out fill in my code.
I did a quick test with the original code for two inputs that took ~13 mins to run, to see accumulative affects between versions. In one case using fill was about 6 secs slower, while the other case was no appreciable difference. I'll do more testing to assess any performance differences.
Since I'm using Rustup how do I use it to install nightlies if fill eventually gets the inlined PR done?
You can add the nightly toolchain rustup install nightly and then you can use it manually by adding the +nightly argument to cargo (or other rust-related commands). Eg cargo +nightly build or cargo +nightly run, etc, will use the nightly toolchain.
You can also set your default toolchain to nightly for a specific project by using rustup override. You navigate (cd) into the top directory of a project and call rustup override set nightly. Go back to default (usually stable) with rustup override unset in the same directory. This doesn’t actually modify any files in the project itself but instead globally stores that the path of the project is associated with the nightly toolchain; this will not automatically get updated when you move the project or removed when you delete it. You can list all those overrides with rustup override list, if you do this, it also tells you which command you would need to remove all overrides for deleted directories.
All of this and more can be explored further with the included help commands. E.g. rustup help gives you a general help, and things like rustup toolchain help or rustup override help give more specific help pages. rustup install seems to be a (not too well-documented) shortcut for rustup toolchain install.
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Thanks again. 