build: fix fpclassify -Wfloat-conversion warning #15570

Work around a glibc bug where it truncates the argument to fpclassify()
from double to float by implementing fpclassify() ourselves.

Correctness test (Note that the FP_SUBNORMAL test depends on an atof() that
knows how to parse subnormals. Glibc does, not sure about other libcs.):

    #include <math.h>
    #include <stdint.h>
    #include <string.h>

    int xfpclassify(double d)
    {
      uint64_t m;
      int e;

      memcpy(&m, &d, sizeof(m));
      e = 0x7ff & (m >> 52);
      m = 0xfffffffffffffULL & m;

      switch (e) {
        default: return FP_NORMAL;
        case 0x000: return m ? FP_SUBNORMAL : FP_ZERO;
        case 0x7ff: return m ? FP_NAN : FP_INFINITE;
      }
    }

    #include <assert.h>
    #include <stdlib.h>

    int main(void)
    {
      assert(FP_ZERO == xfpclassify(atof("0.0")));
      assert(FP_ZERO == xfpclassify(atof("-0.0")));
      assert(FP_NORMAL == xfpclassify(atof("1.0")));
      assert(FP_NORMAL == xfpclassify(atof("-1.0")));
      assert(FP_INFINITE == xfpclassify(atof("inf")));
      assert(FP_INFINITE == xfpclassify(atof("-inf")));
      assert(FP_NAN == xfpclassify(atof("nan")));
      assert(FP_NAN == xfpclassify(atof("-nan")));
      assert(FP_SUBNORMAL == xfpclassify(atof("1.8011670033376514e-308")));
      return 0;
    }

1 files changed, 16 insertions, 22 deletions

diff --git a/src/nvim/math.c b/src/nvim/math.c
index b51f335ed7..63309b6f7a 100644
--- a/src/nvim/math.c
+++ b/src/nvim/math.c
@@ -2,6 +2,8 @@
 // it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
 
 #include <math.h>
+#include <stdint.h>
+#include <string.h>
 
 #include "nvim/math.h"
 
@@ -9,34 +11,26 @@
 # include "math.c.generated.h"
 #endif
 
-#if defined(__clang__) && __clang__ == 1 && __clang_major__ >= 6
-// Workaround glibc + Clang 6+ bug. #8274
-// https://bugzilla.redhat.com/show_bug.cgi?id=1472437
-# pragma clang diagnostic push
-# pragma clang diagnostic ignored "-Wconversion"
-#endif
 int xfpclassify(double d)
 {
-#if defined(__MINGW32__)
-  // Workaround mingw warning. #7863
-  return __fpclassify(d);
-#else
-  return fpclassify(d);
-#endif
+  uint64_t m;
+  int e;
+
+  memcpy(&m, &d, sizeof(m));
+  e = 0x7ff & (m >> 52);
+  m = 0xfffffffffffffULL & m;
+
+  switch (e) {
+    default: return FP_NORMAL;
+    case 0x000: return m ? FP_SUBNORMAL : FP_ZERO;
+    case 0x7ff: return m ? FP_NAN : FP_INFINITE;
+  }
 }
 int xisinf(double d)
 {
-  return isinf(d);
+  return FP_INFINITE == xfpclassify(d);
 }
 int xisnan(double d)
 {
-#if defined(__MINGW32__)
-  // Workaround mingw warning. #7863
-  return _isnan(d);
-#else
-  return isnan(d);
-#endif
+  return FP_NAN == xfpclassify(d);
 }
-#if defined(__clang__) && __clang__ == 1 && __clang_major__ >= 6
-# pragma clang diagnostic pop
-#endif