https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83122
Bug ID: 83122
Summary: -Wconversion and shifting bitwise
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
Minimal testcase:
```c
// gcc -c test.c -Wall -Wextra -pedantic -pedantic-errors -std=c99 -Wconversion
typedef __UINT64_TYPE__ uint64_t;
struct foo {
uint64_t a : 3;
uint64_t b : 61;
};
void set_a(struct foo *ptr, uint64_t val){
ptr->b = val >> 3;
}
```
The program above results in a warning about loss of precision:
```
test.c: In function 'set_a':
test.c:11:11: warning: conversion to 'long long unsigned int:61' from 'uint64_t
{aka long long unsigned int}' may alter its value [-Wconversion]
ptr->b = val >> 3;
^~~
```
This is a false positive since the 3 MSBs of `val` are filled with zeroes.
In reality, the warning persists even after I replace `val >> 3` with `val >>
63`.
