aliasing

[David Brown]

On 18/03/2024 14:54, Andreas Schwab via Gcc wrote:
> On Mär 18 2024, David Brown wrote:
> 
>> I think it would be possible to have an implementation where "signed
>> char" was 8-bit two's complement except that 0x80 would be a trap
>> representation rather than -128.
> 
> signed char cannot have padding bits, thus it cannot have a trap
> representation.
> 

The premise is correct (no padding bits are allowed in signed char), but 
does it follow that it cannot have a trap representation?  I don't think 
the standards are clear either way here - I think the committee missed a 
chance to tidy up the description a bit more when C23 removed formats 
other than two's complement for signed integer types.

I also feel slightly uneasy using signed char for accessing object 
representations since the object representation is defined in terms of 
an unsigned char array, and conversion from unsigned char to signed char 
is implementation-defined.  (This too could have been tightened in C23, 
as there is unlikely to be any implementation that does not do the 
conversion in the obvious manner.)

But I am perhaps worrying too much here.