Restricting the instructions to be picked

[test user]

Hi,

I have seen tutorials and articles for adding new instructions. However, what I am trying to do is "RESTRICT" the instructions compiler or assembler will pick. 

Example: Instead of using "MUL" instruction, I want the assembler to pick the "ADD" instruction as multiplication can be achieved using addition too. 

Even though my example might not sound right, I am basically trying to achieve the same effect, restriction of instructions. I am okay to recompile the gcc if needed but can someone point me to the files or config changes to be done in order to achieve this?

Thanks,

Kowshik G