mingw-w64-crt/math/signbitl.c

#define __FP_SIGNBIT  0x0200

int __signbitl (long double x) {
  unsigned short sw;
  __asm__ ("fxam; fstsw %%ax;"
	   : "=a" (sw)
	   : "t" (x) );
  return (sw & __FP_SIGNBIT) != 0;
}

int __attribute__ ((alias ("__signbitl"))) signbitl (long double);