[Bug c/39170] cannot silence -Wconversion warnings for bit-fields

[Zachary_Deretsky at mentor dot com]

------- 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