llvm-gcc currently miscompiles this on darwin:

typedef struct {
  double re,im;
} DComplex;
void bar(DComplex *P, void (*FP)(DComplex)) {
  FP(*P, *P);
}

The prototype it emits for "FP" is:

void (double, double)* %FP

Unfortunately, darwin requires structs to be passed in integer regs regardless of whether the incoming 
values are FP or not.  This means the prototype should be:

void (long, long)* %FP

or 

void (int, int, int, int)* %FP

... yuck.

-Chris