Re: stdc_bit_ceil(3) and wrapping

[Joseph Myers <joseph@codesourcery.com>]

On Fri, 30 Dec 2022, Alejandro Colomar via Gcc wrote:

> I was wondering if there was any reason to make that UB in the standard, when
> unsigned wrapping has always been well-defined, and this is a case that is
> likely to be implemented with some operation that wraps around, right?  I

It's likely to be implemented by a shift, that might be by the number of 
bits in the argument in the overflow case, and shift by the width of the 
argument is undefined behavior; some architectures wrap the shift count 
modulo the width (do you actually want 1 or 0 as the result of 
stdc_bit_ceil?), some wrap the shift count modulo a larger value (e.g. 
treating 1 << 32 as 0 but 1 << 64 as 1) and for some architectures the 
result depends on the particular shift instruction used (meaning undefined 
behavior on the form of the result of an integer expression possibly 
comparing unequal to itself because the compiler duplicated the expression 
and then used different instructions to compute it in different places).

> Would you consider either or both of being more generous in the GNU
> implementation and guarantee wrap around, and/or suggest that the standard
> guarantees the wrap around?

The CD ballot has closed, and this doesn't appear to be one of the 338 
comments raised on that ballot, and I don't really expect time at next 
month's meeting to deal with additional technical comments not on that 
list, when we have only 15 hours to deal with 338 comments.  Once we have 
an issue tracking process for the C standard again (hopefully involving an 
issue tracker that the public can readily submit issues in), I'd suggest 
raising such concerns there (unless there's a CD2 ballot; technical 
comments are best avoided at a DIS ballot if possible).

-- 
Joseph S. Myers
joseph@codesourcery.com