最近在优化一段代码的时候，遇到一个有关 @inbounds 的问题。
using CuArrays
CuArrays.allowscalar(false) # 这里disable了
x = cu(rand(2,3,4))
y = cu([CartesianIndex(i, i) for i in 1:4])
x[:, y]
这里会触发错误：
scalar getindex is disallowed
Stacktrace:
[1] error(::String) at ./error.jl:33
[2] assertscalar(::String) at /home/tj/.julia/packages/GPUArrays/1wgPO/src/indexing.jl:14
[3] getindex at /home/tj/.julia/packages/GPUArrays/1wgPO/src/indexing.jl:54 [inlined]
[4] iterate at ./abstractarray.jl:914 [inlined]
[5] iterate at ./abstractarray.jl:912 [inlined]
[6] checkindex(::Type{Bool}, ::Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}, ::CuArray{CartesianIndex{2},1,Nothing}) at ./multidimensional.jl:514
[7] checkbounds_indices at ./multidimensional.jl:509 [inlined]
[8] checkbounds_indices at ./abstractarray.jl:529 [inlined]
[9] checkbounds at ./abstractarray.jl:482 [inlined]
[10] checkbounds at ./abstractarray.jl:503 [inlined]
[11] _getindex at ./multidimensional.jl:669 [inlined]
[12] getindex(::CuArray{Float32,3,Nothing}, ::Function, ::CuArray{CartesianIndex{2},1,Nothing}) at ./abstractarray.jl:981
其主要原因是，_getindex里有 checkbounds
文档里有介绍checkbounds的实现，可以看到这里会触发cuarray 的 scalar 访问
checkbounds_indices(Bool, (IA1, IA...), (I1, I...)) = checkindex(Bool, IA1, I1) & checkbounds_indices(Bool, IA, I)
由于我的代码逻辑里可以保证这里不会出现异常访问，于是我希望直接 ignore boundscheck，于是定义了下面的函数：
@inline f(a, b) = @inbounds a[:, b]
f(x, y)
2×4 CuArray{Float32,2,Nothing}:
0.94804 0.683741 0.203128 0.582778
0.461565 0.36298 0.537245 0.469615
so far so good，但是，我需要在Zygote里用到这个函数：
gradient(f, x, y)
然后又触发了跟前面一样的error，这应该怎么解决？
scalar getindex is disallowed
Stacktrace:
[1] error(::String) at ./error.jl:33
[2] assertscalar(::String) at /home/tj/.julia/packages/GPUArrays/1wgPO/src/indexing.jl:14
[3] getindex at /home/tj/.julia/packages/GPUArrays/1wgPO/src/indexing.jl:54 [inlined]
[4] iterate at ./abstractarray.jl:914 [inlined]
[5] iterate at ./abstractarray.jl:912 [inlined]
[6] checkindex(::Type{Bool}, ::Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}, ::CuArray{CartesianIndex{2},1,Nothing}) at ./multidimensional.jl:514
[7] checkbounds_indices at ./multidimensional.jl:509 [inlined]
[8] checkbounds_indices at ./abstractarray.jl:529 [inlined]
[9] checkbounds at ./abstractarray.jl:482 [inlined]
[10] checkbounds at ./abstractarray.jl:503 [inlined]
[11] _getindex at ./multidimensional.jl:669 [inlined]
[12] getindex at ./abstractarray.jl:981 [inlined]
[13] adjoint at /home/tj/.julia/packages/Zygote/8dVxG/src/lib/array.jl:30 [inlined]
[14] _pullback at /home/tj/.julia/packages/ZygoteRules/6nssF/src/adjoint.jl:47 [inlined]
[15] f at ./In[36]:1 [inlined]
[16] _pullback(::Zygote.Context, ::typeof(f), ::CuArray{Float32,3,Nothing}, ::CuArray{CartesianIndex{2},1,Nothing}) at /home/tj/.julia/packages/Zygote/8dVxG/src/compiler/interface2.jl:0
[17] _pullback(::Function, ::CuArray{Float32,3,Nothing}, ::CuArray{CartesianIndex{2},1,Nothing}) at /home/tj/.julia/packages/Zygote/8dVxG/src/compiler/interface.jl:31
[18] pullback(::Function, ::CuArray{Float32,3,Nothing}, ::CuArray{CartesianIndex{2},1,Nothing}) at /home/tj/.julia/packages/Zygote/8dVxG/src/compiler/interface.jl:37
[19] gradient(::Function, ::CuArray{Float32,3,Nothing}, ::CuArray{CartesianIndex{2},1,Nothing}) at /home/tj/.julia/packages/Zygote/8dVxG/src/compiler/interface.jl:46
[20] top-level scope at In[40]:
···