bit-fields integral promotion issues

[He Leon via Gcc-help <gcc-help@xxxxxxxxxxx>]

Hi all,

I meet an issue of integral promotion. I have small APP as appended.

The bit-fields integral promotion(***shift = fl_a.e << 28***) result is different between Linux and windows.
a) Linux gcc:  fl_a.e is promoted as integer, and shift result is "0xffffffffe0000000",
b) windows: fl_a.e is promoted as unsigned integer, and shift result is "0x7e0000000"

Per C++ spec, seems Linux behavior is correct.

chapter 4.6, Working Draft, Standard for Programming Language C++
"A prvalue for an integral bit-field (9.6) can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field. If the bit-field is larger yet, no integral promotion applies to it. If the bit-field has an enumerated type, it is treated as any other value of that type for promotion purposes."

1) can you confirm window's behavior is not right?
2) Is there any way to make the behaviors are the same over Linux and Window without change code?
     Since we have so much such coding in our project.
     It'll be much better if there is a building option to handle this.

Thanks a lot.
Leon

===================================================================


#include <stdio.h>

typedef union
{
   struct {
      unsigned long long m : 28;
      unsigned long long e : 8;
      unsigned long long s : 1;
   };
   unsigned long long hex : 37;
} fl;

int main()
{
   unsigned long long shift = 0;
   unsigned long long cast_shift = 0;
   fl fl_a;

   fl_a.m = 0;
   fl_a.e = 0x7e;
   fl_a.s = 0;

   shift = fl_a.e << 28;
   cast_shift = ((unsigned long long)fl_a.e) << 28;

   printf("%s() (a<<28)-0x%llx, ((uint64)a<<28)-0x%llx\n", __func__, shift, cast_shift);

   return 0;
}