https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95213
Bug ID: 95213
Summary: GCC -Werror=conversion error when assigning to a
bitfield (when mixing constants and variables)
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: in-gcc at baka dot org
Target Milestone: ---
When trying to assign to a bitfield with -Werror=conversion, having a constant
mix with a variable causes a "conversion from 'unsigned int' to 'unsigned
int:24' may change value", where as alternate formulations with two constants
or two variables work fine. This error seems to be present on every version of
gcc ever (the ones I tried since 4.9.1, at least one in each major version, all
did the same thing).
Watch it fail on godbolt: https://gcc.godbolt.org/z/vavr-e
struct foo { unsigned int a:8; unsigned int b:24; };
void bar(struct foo num, unsigned int x) {
num.b = (5U << 1) | (1 & 1);
unsigned int z = 5U;
num.b = ((z << 1) | (x & 1)) & 0xffffffU;
num.b = ((5U << 1) | (x & 1)) & 0xffffffU;
num.a = (unsigned char)x;
}
In the above example, the last assignment of num.b fails to compile where the
other two succeed. Note in the last example, casting the bit (x&1) to _Bool
will fix the problem, but is obviously not portable to larger expressions (e.g.
x&7).
