https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119058
Bug ID: 119058
Summary: wbN: A suffix for specifying the width of a
bit-precise integer literal
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: alx at kernel dot org
Target Milestone: ---
Currently, we have no way to specify the width of a bit-precise integer
literal. We only have the wb suffix, which say "pick the smaller width
possible".
That wb suffix is insufficient, as sometimes we need a type wider than the
value we're using, if we're going to use for example shifts.
As an example, there's no good way (and by good, I mean something that doesn't
involve casts) to create a value of width N (let's use 3 as a small example)
where all bits are 1 except for the leftmost (or rightmost) one.
alx@debian:~/tmp$ cat bw.c
int
main(void)
{
unsigned _BitInt(3) i = ~0wb >> 1;
return i;
}
alx@debian:~/tmp$ gcc -Wall -Wextra bw.c
alx@debian:~/tmp$ ./a.out; echo $?
7
alx@debian:~/tmp$ clang -Weverything -Wno-c23-extensions -Wno-bit-int-extension
bw.c
bw.c:4:31: warning: implicit conversion changes signedness: '_BitInt(2)' to
'unsigned _BitInt(3)' [-Wsign-conversion]
4 | unsigned _BitInt(3) i = ~0wb >> 1;
| ~ ~~~~~^~~~
1 warning generated.
alx@debian:~/tmp$ ./a.out; echo $?
7
It would make sense to be able to write it like this (IMO):
unsigned _BitInt(3) i = ~0wb3u >> 1; // stores the value 5 (0b011).
