Array allocation elimination regression

[nsajko]

With this code:

function f_noalias!(x, y)
  Base.mightalias(x, y) && throw("No aliasing allowed!")
  x .= y
end
using AllocCheck
versioninfo()
check_allocs(f_noalias!, Tuple{Vector{Int}, Vector{Int}})

The allocating code is eliminated on v1.10, but not on v1.11:

julia> versioninfo()
Julia Version 1.10.0-rc1
Commit 5aaa9485436 (2023-11-03 07:44 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, znver2)
  Threads: 1 on 8 virtual cores
Environment:
  JULIA_NUM_PRECOMPILE_TASKS = 3
  JULIA_PKG_PRECOMPILE_AUTO = 0

julia> check_allocs(f_noalias!, Tuple{Vector{Int}, Vector{Int}})
Any[]

julia> versioninfo()
Julia Version 1.11.0-DEV.972
Commit 9884e447e79 (2023-11-23 16:16 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 8 × AMD Ryzen 3 5300U with Radeon Graphics
  WORD_SIZE: 64
  LLVM: libLLVM-15.0.7 (ORCJIT, znver2)
  Threads: 1 on 8 virtual cores
Environment:
  JULIA_NUM_PRECOMPILE_TASKS = 3
  JULIA_PKG_PRECOMPILE_AUTO = 0

julia> check_allocs(f_noalias!, Tuple{Vector{Int}, Vector{Int}})
3-element Vector{Any}:
 Allocating runtime call to "jl_genericmemory_copyto" in ./genericmemory.jl:76
  | ccall(:jl_genericmemory_copyto, Cvoid, (Any, Ptr{Cvoid}, Any, Ptr{Cvoid}, Int), dest.mem, dest.ptr_or_offset, src.mem, src.ptr_or_offset, Int(n))

Stacktrace:
 [1] unsafe_copyto!
   @ ./genericmemory.jl:76 [inlined]
 [2] _copyto_impl!
   @ ./array.jl:308 [inlined]
 [3] copyto!
   @ ./array.jl:299 [inlined]
 [4] copyto!
   @ ./array.jl:322 [inlined]
 [5] copyto!
   @ ./broadcast.jl:965 [inlined]
 [6] copyto!
   @ ./broadcast.jl:924 [inlined]
 [7] materialize!
   @ ./broadcast.jl:882 [inlined]
 [8] materialize!
   @ ./broadcast.jl:879 [inlined]
 [9] f_noalias!(x::Vector{Int64}, y::Vector{Int64})
   @ Main ./REPL[1]:3

 Allocating runtime call to "jl_genericmemory_copy_slice" in ./array.jl:347
  | newmem = ccall(:jl_genericmemory_copy_slice, Ref{Memory{T}}, (Any, Ptr{Cvoid}, Int), ref.mem, ref.ptr_or_offset, length(a))

Stacktrace:
  [1] copy
    @ ./array.jl:347 [inlined]
  [2] unaliascopy
    @ ./abstractarray.jl:1518 [inlined]
  [3] unalias
    @ ./abstractarray.jl:1502 [inlined]
  [4] broadcast_unalias
    @ ./broadcast.jl:945 [inlined]
  [5] preprocess
    @ ./broadcast.jl:952 [inlined]
  [6] preprocess_args
    @ ./broadcast.jl:955 [inlined]
  [7] preprocess
    @ ./broadcast.jl:951 [inlined]
  [8] copyto!
    @ ./broadcast.jl:968 [inlined]
  [9] copyto!
    @ ./broadcast.jl:924 [inlined]
 [10] materialize!
    @ ./broadcast.jl:882 [inlined]
 [11] materialize!
    @ ./broadcast.jl:879 [inlined]
 [12] f_noalias!(x::Vector{Int64}, y::Vector{Int64})
    @ Main ./REPL[1]:3

 Allocation of Vector{Int64} in ./array.jl:348
  | return $(Expr(:new, :(typeof(a)), :(Core.memoryref(newmem)), :(a.size)))

Stacktrace:
  [1] copy
    @ ./array.jl:348 [inlined]
  [2] unaliascopy
    @ ./abstractarray.jl:1518 [inlined]
  [3] unalias
    @ ./abstractarray.jl:1502 [inlined]
  [4] broadcast_unalias
    @ ./broadcast.jl:945 [inlined]
  [5] preprocess
    @ ./broadcast.jl:952 [inlined]
  [6] preprocess_args
    @ ./broadcast.jl:955 [inlined]
  [7] preprocess
    @ ./broadcast.jl:951 [inlined]
  [8] copyto!
    @ ./broadcast.jl:968 [inlined]
  [9] copyto!
    @ ./broadcast.jl:924 [inlined]
 [10] materialize!
    @ ./broadcast.jl:882 [inlined]
 [11] materialize!
    @ ./broadcast.jl:879 [inlined]
 [12] f_noalias!(x::Vector{Int64}, y::Vector{Int64})
    @ Main ./REPL[1]:3

Related Discourse discussion:

https://discourse.julialang.org/t/ann-alloccheck-jl-static-code-analysis-to-prove-allocation-free-behavior/106414/19?u=nsajko

[vtjnash]

jl_genericmemory_copyto doesn't allocate, so you should file this on AllocCheck instead