about unroll loops

[Ö£¬B]

   I am a new user of gcc.
   I have written a small code loop.c like this:
main()
{
	int a[16];
	int i;
	for(i=0;i<16;i++)
	   a[i]=3;
	return;
}
   I wanted to unroll the loop, so I compile it use the command 
"gcc -unroll-all-loops -S loop.c", then I look up the loop.s,
but the loop is not unroolled. Please tell me the reason.
Thank you. 


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