Using associativity for optimization

[shmeel gutl <shmeelgutl at shmuelhome dot mine dot nu>]

While testing my implementation of passing arguments in registers, I 
noticed that gcc 4.7 creates instruction dependencies when it doesn't 
have to. Consider:

int foo(int a1, int a2, int a3, int a4)
{
    return a1|a2|a3|a4;
}

gcc, even with -O2 generated code that was equivalent to

temp1 = a1 | a2;
temp2 = temp1 | a3;
temp3 = temp2 | a4;

return temp3;

This code must be executed serially.

Could I create patterns, or enable optimizations that would cause the 
compiler to generate

temp1 = a1 | a2;
temp2 = a3 | a4;
temp3 = temp1 | temp2;

Thereby allowing the scheduler to compute temp1 and temp2 in parallel.