[swift-evolution] [proposal]Decouple definition of Int8 from target char type

[Dmitri Gribenko]

On Thu, Feb 25, 2016 at 7:47 PM, William Dillon via swift-evolution
<swift-evolution at swift.org> wrote:
> Hi All,
>
> (For best viewing, use a fixed-width font with this email)
>
> While porting Swift to ARM/Linux (and while observing other ports, such as SwiftAndroid) several of my collaborators and I have noticed inconsistencies in the handling of char.  I’ve taken the liberty of producing a table to summarize these differences that will hopefully illuminate the problem.
>
> char           ARM         mips         ppc        ppc64       i386      x86_64
> Linux/ELF  unsigned[1]  unsigned[2]  unsigned[3] unsigned[4] signed[5] signed[6]
> Mach-O       signed[7]     N/A        signed[7]   signed[7]  signed[7] signed[7]
> Windows    —=-=-=-=-=-=-=-=-=-=-=-=-=-=- signed —=-=-=-=-=-=-=-=-=-=-=-=-=-=-[8]
>
> Swift currently maps the Int8 type to be equal to the char type of the target platform.  On targets where char is unsigned by default, Int8 becomes an unsigned 8-bit integer, which is a clear violation of the Principle of Least Astonishment.  Furthermore, it is impossible to specify a signed 8-bit integer type on platforms with unsigned chars.

I'm probably misunderstanding you, but are you sure that's what is
happening?  I can't imagine how the standard library would just
silently make Int8 unsigned on Linux arm.

What I would expect to happen is that on Linux arm the Clang importer
would map 'char' to UInt8, instead of mapping it to Int8 like it does
on x86_64.

>     C type    |   Swift type
> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>          char |   CChar
> unsigned char |   UInt8
>   signed char |    Int8

This brings in the notion of the CChar type, and requires us to define
(hopefully!) some rules for type-based aliasing, since you want to be
able to freely cast UnsafePointer<CChar> to UnsafePointer<UInt8> or
UnsafePointer<Int8>.

What about a proposal where we would always map 'char' to Int8,
regardless of the C's idea of signedness?

Dmitri

-- 
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(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/