# Regression between v0.6 and master with reinterpret from #21831
**URL:** https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040
**Category:** Internals & Design
**Created:** [June 1, 2017, 4:58pm UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040 "2017-06-01T16:58:06Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [June 1, 2017, 4:58pm UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040/1 "2017-06-01T16:58:06Z")
On v0.6.0-rc2, this works:
```julia
julia> primitive type Foo 24 end ; reinterpret(Foo, b"123")
1-element Array{Foo,1}:
Foo(0x333231)
```
On v0.7.0-DEV.427, it fails:
```julia
julia> primitive type Foo 24 end ; reinterpret(Foo, b"123")
ERROR: ArgumentError: reinterpret from alignment 1 bytes to alignment 4 bytes not allowed
Stacktrace:
[1] reinterpret(::Type{Foo}, ::Array{UInt8,1}, ::Tuple{Int64}) at ./array.jl:160
[2] reinterpret(::Type{Foo}, ::Array{UInt8,1}) at ./array.jl:139
```
This seems to be another regression caused by @yuyichao’s change [https://github.com/JuliaLang/julia/pull/21831](https://github.com/JuliaLang/julia/pull/21831)
---
### Author:  [@jameson](https://discourse.julialang.org/u/jameson)
#### Post date: [June 1, 2017, 5:07pm UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040/2 "2017-06-01T17:07:58Z")
This is intentional, and is the bug that change is intended to catch: computer hardware can’t be relied upon to load a string (with byte alignment) as an integer (with word alignment)
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [June 1, 2017, 5:17pm UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040/3 "2017-06-01T17:17:06Z")
I think the bug is that the alignment for a 24-bit value is set to be 4, not 1.
The alignment of a primitive type is not necessarily going to be a power of 2,
for example, a 12-byte primitive type might need anything from 1-byte alignment to 8 or 16 byte alignment (which would require padding when placed into an array).
Also, the change #21831 seems like overkill, when it is just ARM processors that have strict alignment requirements (of the supported Julia platforms).
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [June 1, 2017, 10:59pm UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040/4 "2017-06-01T22:59:08Z")
Note: another bit of evidence that it is a bug to ask for an alignment of 4 for a 24-bit primitive type, is that if you create a Vector of type Foo, and ask for sizeof, you get 3_n, not 4_n, so it is not forcing an alignment of 4 bytes after all.
If this is not going to be fixed, how are primitive types supposed to be constructed?
Also, #21831 has no warnings in NEWS.md, and there wasn’t any deprecation period for this change.
---
### Author:  [@jameson](https://discourse.julialang.org/u/jameson)
#### Post date: [June 2, 2017, 11:28pm UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040/5 "2017-06-02T23:28:46Z")
> [@ScottPJones](#):
>
> sizeof, you get 3_n, not 4_n, so it is not forcing an alignment of 4 bytes after all.
What’s sizeof got to do with it? These numbers can be different without being wrong or inconsistent.
> [@ScottPJones](#):
>
> how are primitive types supposed to be constructed?
The same way they always have: as numbers.
> [@ScottPJones](#):
>
> there wasn’t any deprecation period
It should be mentioned in NEWs. I know the author has also tried to help ensure all packages in METADATA were fixed before it was merged. We don’t usually do depreciations when fixing bugs / adding errors for broken code.
---
### Author:  [@ScottPJones](https://discourse.julialang.org/u/ScottPJones)
#### Post date: [June 3, 2017, 1:24am UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040/6 "2017-06-03T01:24:18Z")
> [@jameson](#):
>
> What’s sizeof got to do with it? These numbers can be different without being wrong or inconsistent.
Making a vector of a 24-bit primitive type, you can see they are not aligned on 4-byte boundaries.
The code generated to access them _is_ correct (and is reasonably efficient, unlike if you try to use `NTuple{3,UInt8}` instead (which was @mbauman’s suggestion on Gitter).
> [@jameson](#):
>
> The same way they always have: as numbers.
Can you give an example of that then? Previously, it was easy to reinterpret a vector of bytes as a vector of different primitive types (such as the 24-bit one that @anon94023334 has a use case for).
That’s not possible any longer.
> [@jameson](#):
>
> It should be mentioned in NEWs. I know the author has also tried to help ensure all packages in METADATA were fixed before it was merged. We don’t usually do depreciations when fixing bugs / adding errors for broken code.
This change can have serious consequences to performance - and seems to only have been done because of issues on ARM.
This doesn’t seem to really be fixing a bug - at least, not on the Intel/AMD or POWER platforms.
This change has broken code that worked perfectly well before. Where was the “bug” (except possibly on ARM platforms)?
---
### Author:  [@anon94023334](https://discourse.julialang.org/u/anon94023334)
#### Post date: [June 3, 2017, 1:52am UTC](https://discourse.julialang.org/t/regression-between-v0-6-and-master-with-reinterpret-from-21831/4040/7 "2017-06-03T01:52:52Z")
For reference: a 24-bit integer would be a really nice sweet spot for medium size graphs: 64k vertices is too small, but 2^32 is too large. 16.7 milllion vertices handles a lot of common graph structures on commodity hardware, and the 16-bit-per-edge cost savings (vs 32 bit ints) can really add up.