different rtl files

[Iman Narasamdya]

Hello,

I'd like to know why the first generated rtl files of compiling the same 
program with different optimization levels are syntactically different. 
For example, suppose my file is file.c, and I do the following compilations:
	gcc -dr file.c (a), and
	gcc -O2 -dr file.c (b).
Suppose (a) is the rtl file of the first compilation and (b) is that of 
the second compilation. According to my observation, (a) and (b) have 
the same basic-block structure, but they are syntactically different. 
Does gcc perform something else, when the option -O2 is used, before 
generating the first rtl file?

Rgds,

Iman Narasamdya