MAINT: Rely on Py_NAN and Py_HUGE_VAL being correctly defined

It seems to me code all around relies on both being correct anyway.
The actual value for Py_NAN is subtly incorrect on MIPS (depending
on settings) or at least nonstandard, which seems to confuse some
builtin functions.
(Probably it is signalling, but NumPy saw this with fmin, which probably
should also ignore signalling NaNs, see also numpy/numpy#23158).

The guards about `_PY_SHORT_FLOAT_REPR` making sense are relatively
unrelated to NAN and INF being available.

Nevertheless, I currently hide the Py_NAN definition if that is not
set, since I am not sure what good alternative there is to be certain
that Py_NAN is well defined.
OTOH, I do suspect there is no platform where it is not and it should
probably be changed?!

Author: seberg

Include/internal/pycore_dtoa.h

@@ -64,8 +64,6 @@ PyAPI_FUNC(double) _Py_dg_strtod(const char *str, char **ptr);
 PyAPI_FUNC(char *) _Py_dg_dtoa(double d, int mode, int ndigits,
                         int *decpt, int *sign, char **rve);
 PyAPI_FUNC(void) _Py_dg_freedtoa(char *s);
-PyAPI_FUNC(double) _Py_dg_stdnan(int sign);
-PyAPI_FUNC(double) _Py_dg_infinity(int sign);
 
 #endif // _PY_SHORT_FLOAT_REPR == 1
 

Modules/cmathmodule.c

@@ -8,7 +8,6 @@
 
 #include "Python.h"
 #include "pycore_pymath.h"        // _PY_SHORT_FLOAT_REPR
-#include "pycore_dtoa.h"          // _Py_dg_stdnan()
 /* we need DBL_MAX, DBL_MIN, DBL_EPSILON, DBL_MANT_DIG and FLT_RADIX from
    float.h.  We assume that FLT_RADIX is either 2 or 16. */
 #include <float.h>
@@ -88,53 +87,6 @@ else {
 #endif
 #define CM_SCALE_DOWN (-(CM_SCALE_UP+1)/2)
 
-/* Constants cmath.inf, cmath.infj, cmath.nan, cmath.nanj.
-   cmath.nan and cmath.nanj are defined only when either
-   _PY_SHORT_FLOAT_REPR is 1 (which should be
-   the most common situation on machines using an IEEE 754
-   representation), or Py_NAN is defined. */
-
-static double
-m_inf(void)
-{
-#if _PY_SHORT_FLOAT_REPR == 1
-    return _Py_dg_infinity(0);
-#else
-    return Py_HUGE_VAL;
-#endif
-}
-
-static Py_complex
-c_infj(void)
-{
-    Py_complex r;
-    r.real = 0.0;
-    r.imag = m_inf();
-    return r;
-}
-
-#if _PY_SHORT_FLOAT_REPR == 1
-
-static double
-m_nan(void)
-{
-#if _PY_SHORT_FLOAT_REPR == 1
-    return _Py_dg_stdnan(0);
-#else
-    return Py_NAN;
-#endif
-}
-
-static Py_complex
-c_nanj(void)
-{
-    Py_complex r;
-    r.real = 0.0;
-    r.imag = m_nan();
-    return r;
-}
-
-#endif
 
 /* forward declarations */
 static Py_complex cmath_asinh_impl(PyObject *, Py_complex);
@@ -1274,20 +1226,26 @@ cmath_exec(PyObject *mod)
     if (PyModule_AddObject(mod, "tau", PyFloat_FromDouble(Py_MATH_TAU)) < 0) {
         return -1;
     }
-    if (PyModule_AddObject(mod, "inf", PyFloat_FromDouble(m_inf())) < 0) {
+    if (PyModule_AddObject(mod, "inf", PyFloat_FromDouble(Py_HUGE_VAL)) < 0) {
         return -1;
     }
 
+    Py_complex infj = {0.0, Py_HUGE_VAL};
     if (PyModule_AddObject(mod, "infj",
-                           PyComplex_FromCComplex(c_infj())) < 0) {
+                           PyComplex_FromCComplex(infj)) < 0) {
         return -1;
     }
 #if _PY_SHORT_FLOAT_REPR == 1
-    if (PyModule_AddObject(mod, "nan", PyFloat_FromDouble(m_nan())) < 0) {
+    /* 
+     * NaN exposure is guarded by having IEEE doubles via _PY_SHORT_FLOAT_REPR.
+     * This is probably an overly restrictive guard.
+     */
+    if (PyModule_AddObject(mod, "nan", PyFloat_FromDouble(Py_NAN)) < 0) {
         return -1;
     }
+    Py_complex nanj = {0.0, Py_NAN};
     if (PyModule_AddObject(mod, "nanj",
-                           PyComplex_FromCComplex(c_nanj())) < 0) {
+                           PyComplex_FromCComplex(nanj)) < 0) {
         return -1;
     }
 #endif

Modules/mathmodule.c

@@ -59,7 +59,6 @@ raised for division by zero and mod by zero.
 #include "Python.h"
 #include "pycore_bitutils.h"      // _Py_bit_length()
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
-#include "pycore_dtoa.h"          // _Py_dg_infinity()
 #include "pycore_long.h"          // _PyLong_GetZero()
 #include "pycore_moduleobject.h"  // _PyModule_GetState()
 #include "pycore_object.h"        // _PyObject_LookupSpecial()
@@ -389,34 +388,6 @@ lanczos_sum(double x)
     return num/den;
 }
 
-/* Constant for +infinity, generated in the same way as float('inf'). */
-
-static double
-m_inf(void)
-{
-#if _PY_SHORT_FLOAT_REPR == 1
-    return _Py_dg_infinity(0);
-#else
-    return Py_HUGE_VAL;
-#endif
-}
-
-/* Constant nan value, generated in the same way as float('nan'). */
-/* We don't currently assume that Py_NAN is defined everywhere. */
-
-#if _PY_SHORT_FLOAT_REPR == 1
-
-static double
-m_nan(void)
-{
-#if _PY_SHORT_FLOAT_REPR == 1
-    return _Py_dg_stdnan(0);
-#else
-    return Py_NAN;
-#endif
-}
-
-#endif
 
 static double
 m_tgamma(double x)
@@ -3938,7 +3909,7 @@ math_ulp_impl(PyObject *module, double x)
     if (Py_IS_INFINITY(x)) {
         return x;
     }
-    double inf = m_inf();
+    double inf = Py_HUGE_VAL;
     double x2 = nextafter(x, inf);
     if (Py_IS_INFINITY(x2)) {
         /* special case: x is the largest positive representable float */
@@ -3975,11 +3946,15 @@ math_exec(PyObject *module)
     if (PyModule_AddObject(module, "tau", PyFloat_FromDouble(Py_MATH_TAU)) < 0) {
         return -1;
     }
-    if (PyModule_AddObject(module, "inf", PyFloat_FromDouble(m_inf())) < 0) {
+    if (PyModule_AddObject(module, "inf", PyFloat_FromDouble(Py_HUGE_VAL)) < 0) {
         return -1;
     }
 #if _PY_SHORT_FLOAT_REPR == 1
-    if (PyModule_AddObject(module, "nan", PyFloat_FromDouble(m_nan())) < 0) {
+    /* 
+     * NaN exposure is guarded by having IEEE doubles via _PY_SHORT_FLOAT_REPR.
+     * This is probably an overly restrictive guard.
+     */
+    if (PyModule_AddObject(module, "nan", PyFloat_FromDouble(Py_NAN)) < 0) {
         return -1;
     }
 #endif

Objects/floatobject.c

@@ -2424,7 +2424,6 @@ PyFloat_Unpack2(const char *data, int le)
     f |= *p;
 
     if (e == 0x1f) {
-#if _PY_SHORT_FLOAT_REPR == 0
         if (f == 0) {
             /* Infinity */
             return sign ? -Py_HUGE_VAL : Py_HUGE_VAL;
@@ -2433,16 +2432,6 @@ PyFloat_Unpack2(const char *data, int le)
             /* NaN */
             return sign ? -Py_NAN : Py_NAN;
         }
-#else  // _PY_SHORT_FLOAT_REPR == 1
-        if (f == 0) {
-            /* Infinity */
-            return _Py_dg_infinity(sign);
-        }
-        else {
-            /* NaN */
-            return _Py_dg_stdnan(sign);
-        }
-#endif  // _PY_SHORT_FLOAT_REPR == 1
     }
 
     x = (double)f / 1024.0;

Python/dtoa.c

@@ -273,11 +273,6 @@ typedef union { double d; ULong L[2]; } U;
 #define Big0 (Frac_mask1 | Exp_msk1*(DBL_MAX_EXP+Bias-1))
 #define Big1 0xffffffff
 
-/* Standard NaN used by _Py_dg_stdnan. */
-
-#define NAN_WORD0 0x7ff80000
-#define NAN_WORD1 0
-
 /* Bits of the representation of positive infinity. */
 
 #define POSINF_WORD0 0x7ff00000
@@ -1399,35 +1394,6 @@ bigcomp(U *rv, const char *s0, BCinfo *bc)
     return 0;
 }
 
-/* Return a 'standard' NaN value.
-
-   There are exactly two quiet NaNs that don't arise by 'quieting' signaling
-   NaNs (see IEEE 754-2008, section 6.2.1).  If sign == 0, return the one whose
-   sign bit is cleared.  Otherwise, return the one whose sign bit is set.
-*/
-
-double
-_Py_dg_stdnan(int sign)
-{
-    U rv;
-    word0(&rv) = NAN_WORD0;
-    word1(&rv) = NAN_WORD1;
-    if (sign)
-        word0(&rv) |= Sign_bit;
-    return dval(&rv);
-}
-
-/* Return positive or negative infinity, according to the given sign (0 for
- * positive infinity, 1 for negative infinity). */
-
-double
-_Py_dg_infinity(int sign)
-{
-    U rv;
-    word0(&rv) = POSINF_WORD0;
-    word1(&rv) = POSINF_WORD1;
-    return sign ? -dval(&rv) : dval(&rv);
-}
 
 double
 _Py_dg_strtod(const char *s00, char **se)

Python/pystrtod.c

@@ -23,44 +23,6 @@ case_insensitive_match(const char *s, const char *t)
    return the NaN or Infinity as a double and set *endptr to point just beyond
    the successfully parsed portion of the string.  On failure, return -1.0 and
    set *endptr to point to the start of the string. */
-
-#if _PY_SHORT_FLOAT_REPR == 1
-
-double
-_Py_parse_inf_or_nan(const char *p, char **endptr)
-{
-    double retval;
-    const char *s;
-    int negate = 0;
-
-    s = p;
-    if (*s == '-') {
-        negate = 1;
-        s++;
-    }
-    else if (*s == '+') {
-        s++;
-    }
-    if (case_insensitive_match(s, "inf")) {
-        s += 3;
-        if (case_insensitive_match(s, "inity"))
-            s += 5;
-        retval = _Py_dg_infinity(negate);
-    }
-    else if (case_insensitive_match(s, "nan")) {
-        s += 3;
-        retval = _Py_dg_stdnan(negate);
-    }
-    else {
-        s = p;
-        retval = -1.0;
-    }
-    *endptr = (char *)s;
-    return retval;
-}
-
-#else
-
 double
 _Py_parse_inf_or_nan(const char *p, char **endptr)
 {
@@ -94,7 +56,6 @@ _Py_parse_inf_or_nan(const char *p, char **endptr)
     return retval;
 }
 
-#endif
 
 /**
  * _PyOS_ascii_strtod: