Compiler optimization?

[Chirag Wighe <chirag dot wighe at windriver dot com>]

Hi

I am an embedded developer and I am trying to understand the endian issues 
with respect to compilers and how they access structure fields.

Consider a structure defined as follows

typedef struct _s1
{
UINT32 f;
UINT16 f1;
UINT16 f2;
} S1
;

and code something like this:

S1 s;
void foo (void)
{
s.f1 = 0x0123;
s.f2 = 0x4567;
}

Is it possible for an optimizing compiler to combine the two half word 
writes into a single word write. The compiler will form a single 32 bit 
immediate value and do a word write thus saving one write cycle. This would 
however cause problems if the memory where the code is data is being stored 
is defined with an endian attribute other than that of the CPU as it will 
lead to a reversal of the order of the structure fields. In the code 
compiled using GNU that I am using I have not seen this happen. I would 
like to know if there are compilers doing this, if not whether it is 
prohibited and how likely it is that such an optimization may be introduced 
in the future.

I am not on the email list so I would appreciate it if you could copy my 
email in any replies.

Thanks
Chirag Wighe
Software Development Engineer
Wind River Systems