------- Comment #9 from Zachary_Deretsky at mentor dot com 2010-03-06 00:20
-------
I was wrong, the warning is correct and there is a way to fix it.
***1. The easy recipe: For the assignment to bit-fileds use unsigned int
bit-field on the left and mask the right side with the appropriate mask.
***2. Explanation given by Alexander Smundak from Cisco:
Actually, the compiler is doing the right thing. Let's simplify it:
--------------
struct data_t {
int bit:1;
} data;
void foo() {
data.bit = 1;
}
-----------------
$ gcc.c4.4.0-p0 -fsyntax-only -Wall -Wconversion wbit.cpp
wbit.cpp: In function void foo():
wbit.cpp:6: warning: conversion to signed char:1 alters int constant value
`int' is signed, meaning that the highest bit of data.bit is to be extended
when moved to a longer integer. Which means that the values for the 1-bit sized
integer are 0 and -1. Indeed, if I change the assignment above to
data.bit = -1;
the code will compile without warnings. And indeed that's what happens.
Likewise, for the 2-bit bit fields the possible values are
0,1,-1 and -2. Etc.
***3. Here is a little code example to try with g++ -Wconversion b.cpp
-----------------------------------------------------------------
// b.cpp
#define M23 ((1 << 23) - 1)
#define M24 ((1 << 24) - 1)
class xxx {
public:
xxx():u_24(0), i_24(0) {}
unsigned int u_24:24;
int i_24:24;
};
int main(int argc, char** argv) {
xxx x;
unsigned int y = 0xffff;
x.u_24 = M24 & y;
x.i_24 = M23 & y;
x.i_24 = M24 & y; // warning: conversion to int:24 from int may alter its
value
}
---------------------------------------------------------
Thanks, Zach.
--
Zachary_Deretsky at mentor dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |Zachary_Deretsky at mentor
| |dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39170
