This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Using associativity for optimization


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.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]