Re: long long bit-fields

[Segher Boessenkool <segher at koffie dot nl>]

Falk Hueffner wrote:
> Hi,
>
> PR c/6346, middle-end/7018 and target/9620 deal with long long
> bit-fields. I guess part of the confusion comes from the fact that the
> C standard doesn't specify how they should behave (at least I couldn't
> find it). So what is the following program supposed to print?
>
> #include <stdio.h>
>
> int
> main(int argc, char *argv[]){
>     struct {
> 	unsigned long long ull31a: 31;
> 	unsigned long long ull31b: 31;
>     } x;
>     x.ull31a = -1;
>     x.ull31b = 1;
>
>     printf("%016llx %016llx %016llx\n",
> 	   (x.ull31a << 48) >> 48,
> 	   (x.ull31a << 16),
> 	   (x.ull31b << 48));
> }
>
> Candidates are:
>
> ffffffffffffffff ffffffffffff0000 0000000000010000 (gcc)

Correct (the integer promotions promote an unsigned integer
bit-field to a signed integer, if it fits).

> ffffffffffffffff 00007fffffff0000 0001000000000000 (Compaq C)

Not correct.

> 000000000000ffff 00007fffffff0000 0001000000000000 (g++)
> 0000000000000000 ffffffffffff0000 0000000000000000 (Compaq C++)

Don't know about C++, sorry.


Segher