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Subject: which gcc options can control layout of bit fields
From: Andrew Makhorin <mao@gnu.org>
To: gcc-help <gcc-help@gcc.gnu.org>
Date: Wed, 27 Oct 2021 02:23:48 +0300
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Hi,

Could anyone tell me which gcc options can control layout of bit
fields?

The problem I encountered is that gcc for Cygwin doesn't follow
System V ABI for i386. Namely, ABI says: 

   A bit-field must entirely reside in a storage unit appropriate for 
   its declared type. Thus a bit-field never crosses its unit boundary.

But in the code generated by gcc under Cygwin bit fields are allocated
contiguously and may cross the unit boundary. On the other hand, gcc
under Linux follows the ABI conventions.

Example:

struct { int a; char b; int c:14, d:14; int e; }
   s = {0xAAAAAAAA, 0xBB, 0xCCC, 0xDDD, 0xEEEEEEEE};

static int *p = (int *)&s;

int main(void)
{
      printf("0x%08X 0x%08X 0x%08X 0x%08X\n", p[0], p[1], p[2], p[3]);

      return 0;
}

Under Cygwin s.d is splitted between p[1] and p[2]:

0xAAAAAAAA 0x774CCCBB 0x00000003 0xEEEEEEEE

Under Linux all is okay:

0xAAAAAAAA 0x000CCCBB 0x00000DDD 0xEEEEEEEE


Thank you,

Andrew Makhorin