Synchronization in a nested while loop causes loss of threadID

I ran across a method that asked for me to use a shared memory pool of size blocksize and pull from it a few times in a for/while loop. I found that the CPU had problems with this. Here is a mwe (without the shmem shenanigans):

using Test
using CUDA
using CUDAKernels
using KernelAbstractions

@kernel function f_test_kernel!(a)
    tid = @index(Global, Linear)

    @uniform N = length(a)

    @uniform b = 0
    for i = 1:10
        if tid < N
            b += 1
            @synchronize()
        end
    end
end

a = zeros(1024)

# works
wait(f_test_kernel!(CUDADevice(),256)(CuArray(a), ndrange=1024))

# doesn't work
wait(f_test_kernel!(CPU(),4)(a, ndrange=1024))

Note: without the if statement, everything works fine. I also tried a few different nested if statements to see if a similar error occurred, but could not replicate it. It seems to be specifically a loop after a conditional (although maybe a loop in a loop would also trigger it? Still digging).

Error message (tid not defined):

ERROR: LoadError: TaskFailedException
Stacktrace:
 [1] wait
   @ ./task.jl:322 [inlined]
 [2] wait
   @ ~/projects/KernelAbstractions.jl/src/cpu.jl:65 [inlined]
 [3] wait (repeats 2 times)
   @ ~/projects/KernelAbstractions.jl/src/cpu.jl:29 [inlined]
 [4] top-level scope
   @ ~/projects/simuleios/histograms/mwe4.jl:22
 [5] include(fname::String)
   @ Base.MainInclude ./client.jl:444
 [6] top-level scope
   @ REPL[7]:1
 [7] top-level scope
   @ ~/.julia/packages/CUDA/YpW0k/src/initialization.jl:52

    nested task error: UndefVarError: tid not defined
    Stacktrace:
     [1] cpu_f_test_kernel!(::KernelAbstractions.CompilerMetadata{KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.NoDynamicCheck, CartesianIndex{1}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}}, ::Vector{Float64})
       @ ./none:0 [inlined]
     [2] overdub
       @ ./none:0 [inlined]
     [3] __thread_run(tid::Int64, len::Int64, rem::Int64, obj::KernelAbstractions.Kernel{CPU, KernelAbstractions.NDIteration.StaticSize{(4,)}, KernelAbstractions.NDIteration.DynamicSize, typeof(cpu_f_test_kernel!)}, ndrange::Tuple{Int64}, iterspace::KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}, args::Tuple{Vector{Float64}}, dynamic::KernelAbstractions.NDIteration.NoDynamicCheck)
       @ KernelAbstractions ~/projects/KernelAbstractions.jl/src/cpu.jl:157
     [4] __run(obj::KernelAbstractions.Kernel{CPU, KernelAbstractions.NDIteration.StaticSize{(4,)}, KernelAbstractions.NDIteration.DynamicSize, typeof(cpu_f_test_kernel!)}, ndrange::Tuple{Int64}, iterspace::KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}, args::Tuple{Vector{Float64}}, dynamic::KernelAbstractions.NDIteration.NoDynamicCheck)
       @ KernelAbstractions ~/projects/KernelAbstractions.jl/src/cpu.jl:130
     [5] (::KernelAbstractions.var"#33#34"{Nothing, Nothing, typeof(KernelAbstractions.__run), Tuple{KernelAbstractions.Kernel{CPU, KernelAbstractions.NDIteration.StaticSize{(4,)}, KernelAbstractions.NDIteration.DynamicSize, typeof(cpu_f_test_kernel!)}, Tuple{Int64}, KernelAbstractions.NDIteration.NDRange{1, KernelAbstractions.NDIteration.DynamicSize, KernelAbstractions.NDIteration.StaticSize{(4,)}, CartesianIndices{1, Tuple{Base.OneTo{Int64}}}, Nothing}, Tuple{Vector{Float64}}, KernelAbstractions.NDIteration.NoDynamicCheck}})()
       @ KernelAbstractions ~/projects/KernelAbstractions.jl/src/cpu.jl:22
in expression starting at /home/leios/projects/simuleios/histograms/mwe4.jl:22

I'll try my hand at it if I cannot find a workaround, but I figured I would create an issue here first.

Start by running the provided MWE with the CPU backend and inspect src/cpu.jl, especially __thread_run and the reported tid failure. Compare the nested conditional and synchronization behavior with the working CUDA case. Done means the CPU kernel completes without UndefVarError when synchronization is inside the conditional loop.