| Issue |
129256
|
| Summary |
wbN: A suffix for specifying the width of a bit-precise integer literal
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
alejandro-colomar
|
Currently, we have no way of specifying 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.
```c
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):
```c
unsigned _BitInt(3) i = ~0wb3u >> 1; // stores the value 5 (0b011).
```
GCC bug: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119058>
Cc: @AaronBallman
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