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Re: macro BITS_BIG_ENDIAN


I tried to get the behavior from the following example.

struct str_tag
{
	int a:3 ;
} str_var = {1} ;

Note: 'int' size is 32bits

1. BYTES_BIG_ENDIAN defined to 0

1.1 BITS_BIG_ENDIAN defined to 0
Assembly output for bits little-endian:
	.size	str_var, 4
str_var:
	.byte	1
	.space	3

1.2 BITS_BIG_ENDIAN defined to 1
Assembly output for bits big-endian:
	.size	str_var, 4
str_var:
	.byte	1
	.space	3

2. BYTES_BIG_ENDIAN defined to 1

2.1 BITS_BIG_ENDIAN defined to 0
Assembly output for bits little-endian:
	.size	str_var, 4
str_var:
	.byte	32
	.space	3

2.2 BITS_BIG_ENDIAN defined to 1
Assembly output for bits big-endian:
	.size	str_var, 4
str_var:
	.byte	32
	.space	3

Is the following, the behavior of BITS_BIG_ENDIAN?

bytes little-endian and bits little-endian:
7 0 7 0 7 0 7 0 +---------+---------+---------+-----+---+
| | | | |001|
+---------+---------+---------+-----+---+
31 24 23 16 15 8 7 0


bytes little-endian and bits big-endian:
0 7 0 7 0 7 0 7 +---------+---------+---------+-----+---+
| | | | |001|
+---------+---------+---------+-----+---+
24 31 16 23 8 15 0 7


Thanks in advance,
Balaji Sivan

_James E Wilson wrote, on 9/10/2004 4:09 AM_:
Sivan Balaji wrote:

I have been through GCC internals documentation and I am not clear about BITS_BIG_ENDIAN. does BITS_BIG_ENDIAN related to the order of bit field members allocation.


There is no way to control the order of bit-field allocation. Bit-fields are always assigned to the first available bit, possibly constrained by other factors, such as alignment. That means that they start at the low order bit for little-endian, and the high order bit for big-endian. This is the "right" way to do things. It is very unusual for a compiler to do this differently.

The only thing controlled by BITS_BIG_ENDIAN is the bit numbering used for bit-field instructions. Or more specifically, it controls how the operands to ZERO_EXTRACT and SIGN_EXTRACT RTL operators are interpreted. It has no other effect.

Apparently, Red Hat has patches that allow one to switch the order of bit-field allocation, but these patches have not been contributed.


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