instruction scheduler and passing arguments in registers

[Jonah Graham]

Hello,

I am working on a port of GCC using the DFA scheduler. The processor ABI passes its first two arguments in registers. That causes a problem, the lifetime of the argument register get increased unnecessarily when a simple value is moved to it (eventually leading to a spill failure).

For example:

void simple_a (int, int);

void simple_b (int x, int y)
{
   increment x, multiply it by y, do a bunch of other
   more stuff....
   simple_a (2, x);
}

will create RTL roughly like:

   (insn 1 (set x (add x y)))
   (insn 2 (set x (mult x y)))
   ...
   (insn 45 (set r0 (const_int 2)))
   (insn 46 (set r1 x))
   (call_insn 47 (call simple_a))

When the scheduler runs it moves insn 45 to near the top. (It is actually in the initial ready list for the basic block). That causes r0 to be in use throughout the basic block (instead of for just a couple of insns before the call). Because it is used throughout, it gets marked as such and reload cannot use it. The final result is I have spill failures, something I did not use to have before I started using the instruction scheduler.

Any suggestions as to the cause or a resolution would be highly appreciated.

Thanks,
Jonah Graham