eliminate the dead iterate branch in _unsafe_(get)setindex!.

[N5N3]

It's sad that compiler can't do this automatically.
Some benchmark with setindex!:

julia> a = zeros(Int, 100, 100);
julia> @btime $a[:,:] = $(1:10000);
  1.340 μs (0 allocations: 0 bytes) #master: 3.350 μs (0 allocations: 0 bytes)

julia> @btime $a[:,:] = $(view(LinearIndices(a), 1:100, 1:100));
  10.000 μs (0 allocations: 0 bytes) #master: 11.000 μs (0 allocations: 0 bytes)

So at least IndexLinear case get vectorized, which should be good for getindex as most of it results are IndexLinear.
Edit: Oops, just found that setindex! use an AbstractArray-based iteration. So the acceleration is quite limited.
Since #51071 we can't assuming all AbstractArray supports indexing, so switch to eachindex based iteration might be breaking.

BTW optimization for FastSubArray introduced in #45371 still work after this change as the parent array might have their own copyto! optimization.

[adienes]

Since #51071 we can't assuming all AbstractArray supports indexing

I mean, getindex is in the interface, so I think you can assume that.
if something subtypes AbstractArray without implementing getindex it's a bug on their side

[N5N3]

setindex! now use eachindex based iteration.
Update the latest benchmark:

julia> a = zeros(Int, 100, 100);
julia> @btime $a[:,:] = $(1:10000);
  1.410 μs (0 allocations: 0 bytes) #master: 3.350 μs (0 allocations: 0 bytes)

julia> @btime $a[:,:] = $(view(LinearIndices(a), 1:100, 1:100));
  7.475 μs (0 allocations: 0 bytes) #master: 11.000 μs (0 allocations: 0 bytes)

[N5N3]

@nanosoldier runtests()

[nanosoldier]

The package evaluation job you requested has completed - possible new issues were detected.
The full report is available.

[jishnub]

There appears to be a mild regression on this PR:

julia> using LinearAlgebra

julia> U = UpperTriangular(rand(100,100)); C = similar(U);

julia> @btime $C .= 2.0 .* $U;
  1.867 μs (0 allocations: 0 bytes) # master, commit 2afc20c18a8
  2.166 μs (0 allocations: 0 bytes) # PR, N5N3:simd_index/83cb107971f

julia> versioninfo()
Julia Version 1.11.0-DEV.1233
Commit 83cb107971f (2024-01-08 14:47 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
  WORD_SIZE: 64
  LLVM: libLLVM-15.0.7 (ORCJIT, tigerlake)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)
Environment:
  LD_LIBRARY_PATH = :/usr/lib/x86_64-linux-gnu/gtk-3.0/modules
  JULIA_EDITOR = subl

[N5N3]

IIUC, this pr didn't touch the broadcast (and scalar indexing) so this looks like a bench noise, or an improvement comes from LLVM upgrade (assuming you use the latest master)

[jishnub]

Gentle bump, is there work that remains in this?

[vtjnash]

LGTM. Should we call this _unchecked_iterate though, to indicate that it doesn't check for termination, or _prechecked_iterate to indicate the caller is supposed to have checked, instead of _checked_iterate?