Cannot call pointerset at toplevel

[timholy]

julia> TT = Union{UInt8, Int8}
Union{Int8, UInt8}

julia> a = TT[0x0, 0x1]
2-element Array{Union{Int8, UInt8},1}:
 0x00
 0x01

julia> pa = pointer(a)
Ptr{Union{Int8, UInt8}} @0x00007fc52a508bf0

julia> Base.pointerset(pa, 0x01, 2, 1)
ERROR: pointerset: invalid pointer
Stacktrace:
 [1] top-level scope at none:0

but

julia> unsafe_store!(pa, 0x01, 2)
Ptr{Union{Int8, UInt8}} @0x00007fc52a508bf0

Ref JuliaDebug/JuliaInterpreter.jl#76

[timholy]

If this isn't trivially fixable, "calls to pointerset must be compiled" would be a clearer error message.

[Keno]

Seems like this should be disallowed in the compiled version also and whatever the users are of this should be fixed.

[timholy]

Comes from

julia/test/arrayops.jl

Lines 2506 to 2519 in 4b01e6a

	let TT = Union{UInt8, Int8}
	a = TT[0x0, 0x1]
	b = TT[0x0, 0x0]
	pa = pointer(a)
	pb = pointer(b)
	resize!(a, 1) # sets a[2] = 0
	resize!(b, 1)
	@assert pointer(a) == pa
	@assert pointer(b) == pb
	unsafe_store!(pa, 0x1, 2) # reset a[2] to 1
	@test length(a) == length(b) == 1
	@test a[1] == b[1] == 0x0
	@test a == b
	end

[JeffBezanson]

Yeah, that code doesn't even work:

julia> a = TT[0x0,0x0];

julia> pa = pointer(a);

julia> unsafe_store!(pa, 0x1, 2);

julia> a
2-element Array{Union{Int8, UInt8},1}:
 0x00
 0x00

[JeffBezanson]

I think we could allow it though, since we do have a defined memory layout for this. It would be a bit silly to allow this for references (we do, though of course there's a strong case against it) but not bits unions.

[vtjnash]

We can't entirely make it valid there—we have no possible way to know where the metadata for the value is located.

[JeffBezanson]

Right, it would not update the metadata. It would just be a way to manipulate the raw data representation. One can of course question the wisdom of that, but that's what the unsafe part is for :)