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backport 1.11: fix _growbeg! unncessary resizing (#56029) #56090

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Oct 10, 2024
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backport 1.11: fix _growbeg! unncessary resizing (#56029) #56090

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9 changes: 7 additions & 2 deletions base/array.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -1062,10 +1062,11 @@ function _growbeg!(a::Vector, delta::Integer)
setfield!(a, :ref, @inbounds memoryref(ref, 1 - delta))
else
@noinline (function()
@_terminates_locally_meta
memlen = length(mem)
# since we will allocate the array in the middle of the memory we need at least 2*delta extra space
# the +1 is because I didn't want to have an off by 1 error.
newmemlen = max(overallocation(memlen), len + 2 * delta + 1)
newmemlen = max(overallocation(len), len + 2 * delta + 1)
newoffset = div(newmemlen - newlen, 2) + 1
# If there is extra data after the end of the array we can use that space so long as there is enough
# space at the end that there won't be quadratic behavior with a mix of growth from both ends.
Expand All @@ -1074,10 +1075,14 @@ function _growbeg!(a::Vector, delta::Integer)
if newoffset + newlen < memlen
newoffset = div(memlen - newlen, 2) + 1
newmem = mem
unsafe_copyto!(newmem, newoffset + delta, mem, offset, len)
for j in offset:newoffset+delta-1
@inbounds _unsetindex!(mem, j)
end
else
newmem = array_new_memory(mem, newmemlen)
unsafe_copyto!(newmem, newoffset + delta, mem, offset, len)
end
unsafe_copyto!(newmem, newoffset + delta, mem, offset, len)
setfield!(a, :ref, @inbounds memoryref(newmem, newoffset))
end)()
end
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5 changes: 5 additions & 0 deletions test/arrayops.jl
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Expand Up @@ -467,6 +467,11 @@ end
@test vc == [v[1:(i-1)]; 5; v[i:end]]
end
@test_throws BoundsError insert!(v, 5, 5)

# test that data is copied when there is plenty of room to do so
v = empty!(collect(1:100))
pushfirst!(v, 1)
@test length(v.ref.mem) == 100
end

@testset "popat!(::Vector, i, [default])" begin
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