restore HAVE_LOG2 for cygwin blocklistingw

3 files changed, 61 insertions, 7 deletions

diff --git a/numpy/core/setup.py b/numpy/core/setup.py
index 571d067e0..9fc7c2f29 100644
--- a/numpy/core/setup.py
+++ b/numpy/core/setup.py
@@ -189,6 +189,7 @@ def check_math_capabilities(config, ext, moredefs, mathlibs):
         if config.check_decl(fname2def(f), headers=["Python.h"]):
             OPTIONAL_FILE_FUNCS.remove(f)
 
+    check_funcs(OPTIONAL_STDFUNCS)
     check_funcs(OPTIONAL_FILE_FUNCS, headers=["feature_detection_stdio.h"])
     check_funcs(OPTIONAL_MISC_FUNCS, headers=["feature_detection_misc.h"])
 
diff --git a/numpy/core/setup_common.py b/numpy/core/setup_common.py
index d618fac69..141b62e76 100644
--- a/numpy/core/setup_common.py
+++ b/numpy/core/setup_common.py
@@ -122,13 +122,16 @@ for file in [
 # Mandatory functions: if not found, fail the build
 MANDATORY_FUNCS = ["sin", "cos", "tan", "sinh", "cosh", "tanh", "fabs",
         "floor", "ceil", "sqrt", "log10", "log", "exp", "asin",
-        "acos", "atan", "fmod", 'modf', 'frexp', 'ldexp']
-
-# Some of these are C99 functions.
-MANDATORY_FUNCS += ["expm1", "log1p", "acosh", "asinh", "atanh",
-        "rint", "trunc", "exp2", "log2", "hypot", "atan2", "pow",
+        "acos", "atan", "fmod", 'modf', 'frexp', 'ldexp',
+        "expm1", "log1p", "acosh", "asinh", "atanh",
+        "rint", "trunc", "exp2", "hypot", "atan2", "pow",
         "copysign", "nextafter", "strtoll", "strtoull", "cbrt"]
 
+OPTIONAL_STDFUNCS = [
+        # cygwin
+        "log2",
+]
+
 OPTIONAL_LOCALE_FUNCS = ["strtold_l"]
 OPTIONAL_FILE_FUNCS = ["ftello", "fseeko", "fallocate"]
 OPTIONAL_MISC_FUNCS = ["backtrace", "madvise"]
diff --git a/numpy/core/src/npymath/npy_math_internal.h.src b/numpy/core/src/npymath/npy_math_internal.h.src
index 5d2f4f078..ab5576a0b 100644
--- a/numpy/core/src/npymath/npy_math_internal.h.src
+++ b/numpy/core/src/npymath/npy_math_internal.h.src
@@ -94,6 +94,41 @@ static const npy_uint64 MAGIC64[] = {0x5555555555555555ull, 0x3333333333333333ul
  * classification instead of calling npy_isinf/npy_isnan: we should have some
  * macros for this, though, instead of doing it manually
  */
+#ifndef HAVE_LOG2
+NPY_INPLACE double npy_log2(double x)
+{
+#ifdef HAVE_FREXP
+    if (!npy_isfinite(x) || x <= 0.) {
+        /* special value result */
+        return npy_log(x);
+    }
+    else {
+        /*
+         * fallback implementation copied from python3.4 math.log2
+         * provides int(log(2**i)) == i for i 1-64 in default rounding mode.
+         *
+         * We want log2(m * 2**e) == log(m) / log(2) + e.  Care is needed when
+         * x is just greater than 1.0: in that case e is 1, log(m) is negative,
+         * and we get significant cancellation error from the addition of
+         * log(m) / log(2) to e.  The slight rewrite of the expression below
+         * avoids this problem.
+         */
+        int e;
+        double m = frexp(x, &e);
+        if (x >= 1.0) {
+            return log(2.0 * m) / log(2.0) + (e - 1);
+        }
+        else {
+            return log(m) / log(2.0) + e;
+        }
+    }
+#else
+    /* does not provide int(log(2**i)) == i */
+    return NPY_LOG2E * npy_log(x);
+#endif
+}
+#endif
+
 /*
  *
  * sin, cos, tan
@@ -140,10 +175,12 @@ static const npy_uint64 MAGIC64[] = {0x5555555555555555ull, 0x3333333333333333ul
 #define WORKAROUND_APPLE_TRIG_BUG 0
 #endif
 
+/* mandatory C99 functions */
+
 /**begin repeat1
  * #kind = sin,cos,tan,sinh,cosh,tanh,fabs,floor,ceil,rint,trunc,sqrt,log10,
- *         log,exp,expm1,asin,acos,atan,asinh,acosh,atanh,log1p,exp2,log2#
- * #TRIG_WORKAROUND = WORKAROUND_APPLE_TRIG_BUG*3, 0*22#
+ *         log,exp,expm1,asin,acos,atan,asinh,acosh,atanh,log1p,exp2#
+ * #TRIG_WORKAROUND = WORKAROUND_APPLE_TRIG_BUG*3, 0*21#
  */
 NPY_INPLACE @type@ npy_@kind@@c@(@type@ x)
 {
@@ -156,6 +193,19 @@ NPY_INPLACE @type@ npy_@kind@@c@(@type@ x)
 }
 
 /**end repeat1**/
+/* Optional C99 functions */
+/**begin repeat1
+ * #kind = log2#
+ * #KIND = LOG2#
+ */
+#ifdef HAVE_@KIND@@C@
+NPY_INPLACE @type@ npy_@kind@@c@(@type@ x)
+{
+    return NPY__FP_SFX(@kind@)(x);
+}
+#endif
+
+/**end repeat1**/
 
 #undef WORKAROUND_APPLE_TRIG_BUG