[Vm-dev] VM Maker: ImageFormat-dtl.31.mcz

[Eliot Miranda]

Hi David,

    I have to observe that storing the version number in one width for 32-bits and a wider one for 64-bits is a very bad idea:
- the wider version number must avoid half widths that are ambiguous
- tools using the narrower width (which was adequate when the narrower format was defined) are broken by the wider width

I think an additional fix is the have the non-Spur 64-bit vm ignore the upper bits of to store the same bits in either half.

> On Apr 15, 2018, at 7:51 AM, commits at source.squeak.org wrote:
> 
> 
> David T. Lewis uploaded a new version of ImageFormat to project VM Maker:
> http://source.squeak.org/VMMaker/ImageFormat-dtl.31.mcz
> 
> ==================== Summary ====================
> 
> Name: ImageFormat-dtl.31
> Author: dtl
> Time: 15 April 2018, 11:51:24.795 am
> UUID: 0d9cd87d-1743-4962-b490-7da14c1d0366
> Ancestors: ImageFormat-dtl.30
> 
> Fix ckformat for 64 bit Spur, which saves format number in first 4 bytes of the header, versus 8 bytes for 64 bit V3. Prior version worked by accident for little endian host using strncmp() check, but failed when correct memcmp() was used without limiting check to 4 bytes for spur and 8 for V3.
> 
> =============== Diff against ImageFormat-dtl.30 ===============
> 
> Item was changed:
>  ----- Method: ImageFormat class>>versionNumberByteArrays (in category 'utility') -----
>  versionNumberByteArrays
>      "All byte array expressions of known version numbers. These are the possible values
> +    that may appear in the first 4 or 8 bytes of a saved image file. All 32 bit images have
> +    this number in the first 4 bytes of the image file header. A 64 bit V3 image has this
> +    number saved in the first 8 bytes of the header (only 4 bytes of which are significant).
> +    For a 64 bit Spur image, the number is saved in the first 4 bytes. In all cases, the value
> +    may be stored in little endian or big endian byte ordering depending on the host
> +    platform (although all currently supported VMs are for little endian host platforms)."
> -    that may appear in the first 4 or 8 bytes of a saved image file. A 64 bit image saves
> -    its image format number as a 64 bit value in the file header, and a 32 bit image saves
> -    its image format as a 32 bit value. The value may be stored in little endian or big endian
> -    byte ordering depending on the host platform."
> 
>      "ImageFormat versionNumberByteArrays do: [:e |
>          Transcript cr; show: e printString , ': ', (ImageFormat fromBytes: e) description]"
> 
>      ^self allVersionNumberByteArrays select: [:e |
> +        e size = 4
> +            or: [ (self fromBytes: e) requiresSpurSupport not ]].
> -        e size = (self fromBytes: e) wordSize]
>  !
>