Does the ARM Compiler treat a "plain" int bitfield as a signed int bitfield or as an unsigned int bitfield?

Applies to: DS-5, RealView Development Suite (RVDS)

Scenario

Consider the following structure that contains a single bit-field:

struct bitfield
{
 int m_offset:31;

}data;

char isSet()
{
  return (data.m_offset != -1);
}

Building the C program as follows:

armcc -c test.c (defaulting to optimization level -O2) 

... results in the following code (translated by using fromelf -c test.o):

    isSet
        0x00000000:    e3a00001    ....    MOV      r0,#1
        0x00000004:    e12fff1e    ../.    BX       lr

The test that data.m_offset is not equal to minus one always holds true (#1) because the bit-field is treated as unsigned by default.

If the code is changed so that the number that data.m_offset is tested against is a positive number, for example: 

char isSet2()
{
  return (data.m_offset != 0x7fffffff);
}

... the resultant disassembly listing is as one might expect: 

    isSet2
        0x00000008:    e59f0010    ....    LDR      r0,[pc,#16] ; [0x20] = 0
        0x0000000c:    e3e01102    ....    MVN      r1,#0x80000000
        0x00000010:    e5900000    ....    LDR      r0,[r0,#0]
        0x00000014:    e1d10000    ....    BICS     r0,r1,r0
        0x00000018:    13a00001    ....    MOVNE    r0,#1
        0x0000001c:    e12fff1e    ../.    BX       lr
    $d
        0x00000020:    00000000    ....    DCD    0

Answer

The C Standard specifies that if the type specifier used in declaring a bit-field is either int, then whether the bit-field is signed or unsigned is dependent on the implementation.

In the ARM Compiler toolchain, a plain bit-field declared without the signed or unsigned qualifiers, is treated as unsigned.  In the example above, the bit-field int m_offset: 31 is an unsigned int bit-field by default.

If you require integer bit-fields to be treated as signed int bit-fields, two possible solutions are given below:

• Use the signed qualifier

  struct bitfield
{  
  signed int m_offset:31;
};

• Use the compiler option --signed_bitfields or --unsigned_bitfields to make bitfields of type signed int or unsigned int.  For further information, please see the ARM Compiler toolchain - Compiler Reference.