The Linux ABI requires that you set control register 6 before calling a variadic
function with a float parameter. Similarly, it recommends that you clear it
before calling a variadic function without one.

A simple example is:

  #include <stdio.h>
  int main(void) {
    printf("%f\n", 1.2);
    return 0;
  }

which will print "0.0" on PPC Linux. In the assembly, right before "bl printf",
add "creqv 6,6,6" and it will work properly. This fix will require changes to
the PPCISelLowering.cpp:LowerCALL function.