Initial import

libc/math/lrint.c

@@ -0,0 +1,65 @@
+#include "libc/limits.h"
+#include "libc/math/fenv.h"
+#include "libc/math/libm.h"
+#include "libc/math/math.h"
+
+/*
+If the result cannot be represented (overflow, nan), then
+lrint raises the invalid exception.
+
+Otherwise if the input was not an integer then the inexact
+exception is raised.
+
+C99 is a bit vague about whether inexact exception is
+allowed to be raised when invalid is raised.
+(F.9 explicitly allows spurious inexact exceptions, F.9.6.5
+does not make it clear if that rule applies to lrint, but
+IEEE 754r 7.8 seems to forbid spurious inexact exception in
+the ineger conversion functions)
+
+So we try to make sure that no spurious inexact exception is
+raised in case of an overflow.
+
+If the bit size of long > precision of double, then there
+cannot be inexact rounding in case the result overflows,
+otherwise LONG_MAX and LONG_MIN can be represented exactly
+as a double.
+*/
+
+#if LONG_MAX < 1U << 53 && defined(FE_INEXACT)
+#if FLT_EVAL_METHOD == 0 || FLT_EVAL_METHOD == 1
+#define EPS DBL_EPSILON
+#elif FLT_EVAL_METHOD == 2
+#define EPS LDBL_EPSILON
+#endif
+#ifdef __GNUC__
+/* avoid stack frame in lrint */
+__attribute__((noinline))
+#endif
+static long
+lrint_slow(double x) {
+#pragma STDC FENV_ACCESS ON
+  int e;
+
+  e = fetestexcept(FE_INEXACT);
+  x = rint(x);
+  if (!e && (x > LONG_MAX || x < LONG_MIN)) feclearexcept(FE_INEXACT);
+  /* conversion */
+  return x;
+}
+
+long lrint(double x) {
+  uint32_t abstop = asuint64(x) >> 32 & 0x7fffffff;
+  uint64_t sign = asuint64(x) & (1ULL << 63);
+
+  if (abstop < 0x41dfffff) {
+    /* |x| < 0x7ffffc00, no overflow */
+    double_t toint = asdouble(asuint64(1 / EPS) | sign);
+    double_t y = x + toint - toint;
+    return (long)y;
+  }
+  return lrint_slow(x);
+}
+#else
+long lrint(double x) { return rint(x); }
+#endif