On Tue, Dec 2, 2014 at 12:11 AM, shmeel gutl
<shmeelgutl@shmuelhome.mine.nu> wrote:
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.
You can tune it with --param tree-reassoc-width=N, not sure if that
was implemented for 4.7 already.
Richard.