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Add a more efficient implementation for String(::ReinterpretArray) #58438

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Add a more efficient implementation for String(::ReinterpretArray) #58438

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a76df84
Add a more efficient implementation for String(::ReinterpretArray)
lntricate1 May 16, 2025
6007ffa
Fix issue pointed out by jakobnissen
lntricate1 May 19, 2025
dff029d
Add tests
lntricate1 May 20, 2025
f2ea9b8
Forgot to refill v
lntricate1 May 20, 2025
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14 changes: 14 additions & 0 deletions base/strings/string.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,20 @@ function String(v::Vector{UInt8})
setfield!(v, :ref, memoryref(Memory{UInt8}()))
return str
end
function String(v::ReinterpretArray{UInt8, 1, S, Vector{S}, IsReshaped}) where {S, IsReshaped}
len = length(v)
len == 0 && return ""
ref = v.parent.ref
if ref.ptr_or_offset == ref.mem.ptr
str = ccall(:jl_genericmemory_to_string, Ref{String}, (Any, Int), ref.mem, len)
else
str = ccall(:jl_pchar_to_string, Ref{String}, (Ptr{S}, Int), ref, len)
end
# optimized empty!(v.parent); sizehint!(v.parent, 0) calls
setfield!(v.parent, :size, (0,))
setfield!(v.parent, :ref, memoryref(Memory{S}()))
return str
end
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I believe this is invalid, because String(::AbstractVector) is documented to copy, unless the vector is a Vector. So you'd need to make a stringvector, then copy to it.

Suggested change
function String(v::ReinterpretArray{UInt8, 1, S, Vector{S}, IsReshaped}) where {S, IsReshaped}
len = length(v)
len == 0 && return ""
ref = v.parent.ref
if ref.ptr_or_offset == ref.mem.ptr
str = ccall(:jl_genericmemory_to_string, Ref{String}, (Any, Int), ref.mem, len)
else
str = ccall(:jl_pchar_to_string, Ref{String}, (Ptr{S}, Int), ref, len)
end
# optimized empty!(v.parent); sizehint!(v.parent, 0) calls
setfield!(v.parent, :size, (0,))
setfield!(v.parent, :ref, memoryref(Memory{S}()))
return str
end
function String(v::ReinterpretArray{UInt8, 1, S, Vector{S}, IsReshaped}) where {S, IsReshaped}
len = length(v)
iszero(len) && return ""
par = parent(v)
mem = StringMemory(len % UInt)
GC.@preserve mem par begin
unsafe_copyto!(pointer(mem), Ptr{UInt8}(pointer(par)), len % UInt)
end
return unsafe_takestring(mem)
end

Another issue is whether you can make the signature broader - what about reinterprets of Memory?

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I've implemented this fix, but using jl_pchar_to_string (which always copies) to remove the extra allocation from StringMemory. I also implemented it for reinterprets of Memory.

This has made me realize that the docstring for String could be clearer. Perhaps splitting up the docstring for Vector{UInt8} and AbstractVector{UInt8} would be good. There's also a few missing entries that might be good to have (like the one for AbstractVector{Char}. Do you have any thoughts on this?

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Sure! Better to do that in a separate PR, in my opinion.


"Create a string re-using the memory, if possible.
Mutating or reading the memory after calling this function is undefined behaviour."
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