cmFileTime: Fix overflow on time computation

On Windows, time starting point is Januray, 1st of 1601.
So computing number of nanoseconds from this date exceeds 64bit
capabilities.

Source/cmFileTime.cxx

@@ -24,13 +24,13 @@ bool cmFileTime::Load(std::string const& fileName)
   }
 #  if CMake_STAT_HAS_ST_MTIM
   // Nanosecond resolution
-  this->NS = fst.st_mtim.tv_sec * NsPerS + fst.st_mtim.tv_nsec;
+  this->Time = fst.st_mtim.tv_sec * UtPerS + fst.st_mtim.tv_nsec;
 #  elif CMake_STAT_HAS_ST_MTIMESPEC
   // Nanosecond resolution
-  this->NS = fst.st_mtimespec.tv_sec * NsPerS + fst.st_mtimespec.tv_nsec;
+  this->Time = fst.st_mtimespec.tv_sec * UtPerS + fst.st_mtimespec.tv_nsec;
 #  else
   // Second resolution
-  this->NS = fst.st_mtime * NsPerS;
+  this->Time = fst.st_mtime * UtPerS;
 #  endif
 #else
   // Windows version.  Get the modification time from extended file attributes.
@@ -41,10 +41,11 @@ bool cmFileTime::Load(std::string const& fileName)
   }
 
   // Copy the file time to the output location.
-  this->NS = (static_cast<NSC>(fdata.ftLastWriteTime.dwHighDateTime) << 32) |
-    static_cast<NSC>(fdata.ftLastWriteTime.dwLowDateTime);
-  // The file time resolution is 100 ns.
-  this->NS *= 100;
+  using uint64 = unsigned long long;
+
+  this->Time = static_cast<TimeType>(
+    (uint64(fdata.ftLastWriteTime.dwHighDateTime) << 32) +
+    fdata.ftLastWriteTime.dwLowDateTime);
 #endif
   return true;
 }

Source/cmFileTime.h

@@ -13,9 +13,15 @@
 class cmFileTime
 {
 public:
-  using NSC = long long;
-  static constexpr NSC NsPerS = 1000000000;
-
+  using TimeType = long long;
+  // unit time per second
+#if !defined(_WIN32) || defined(__CYGWIN__)
+  // unit time is one nanosecond
+  static constexpr TimeType UtPerS = 1000000000;
+#else
+  // unit time is 100 nanosecond
+  static constexpr TimeType UtPerS = 10000000;
+#endif
   cmFileTime() = default;
   ~cmFileTime() = default;
 
@@ -28,22 +34,28 @@ public:
   /**
    * @brief Return true if this is older than ftm
    */
-  bool Older(cmFileTime const& ftm) const { return (this->NS - ftm.NS) < 0; }
+  bool Older(cmFileTime const& ftm) const
+  {
+    return (this->Time - ftm.Time) < 0;
+  }
 
   /**
    * @brief Return true if this is newer than ftm
    */
-  bool Newer(cmFileTime const& ftm) const { return (ftm.NS - this->NS) < 0; }
+  bool Newer(cmFileTime const& ftm) const
+  {
+    return (ftm.Time - this->Time) < 0;
+  }
 
   /**
    * @brief Return true if this is the same as ftm
    */
-  bool Equal(cmFileTime const& ftm) const { return this->NS == ftm.NS; }
+  bool Equal(cmFileTime const& ftm) const { return this->Time == ftm.Time; }
 
   /**
    * @brief Return true if this is not the same as ftm
    */
-  bool Differ(cmFileTime const& ftm) const { return this->NS != ftm.NS; }
+  bool Differ(cmFileTime const& ftm) const { return this->Time != ftm.Time; }
 
   /**
    * @brief Compare file modification times.
@@ -51,7 +63,7 @@ public:
    */
   int Compare(cmFileTime const& ftm) const
   {
-    NSC const diff = this->NS - ftm.NS;
+    TimeType const diff = this->Time - ftm.Time;
     if (diff == 0) {
       return 0;
     }
@@ -65,7 +77,7 @@ public:
    */
   bool OlderS(cmFileTime const& ftm) const
   {
-    return (ftm.NS - this->NS) >= cmFileTime::NsPerS;
+    return (ftm.Time - this->Time) >= cmFileTime::UtPerS;
   }
 
   /**
@@ -73,7 +85,7 @@ public:
    */
   bool NewerS(cmFileTime const& ftm) const
   {
-    return (this->NS - ftm.NS) >= cmFileTime::NsPerS;
+    return (this->Time - ftm.Time) >= cmFileTime::UtPerS;
   }
 
   /**
@@ -81,11 +93,11 @@ public:
    */
   bool EqualS(cmFileTime const& ftm) const
   {
-    NSC diff = this->NS - ftm.NS;
+    TimeType diff = this->Time - ftm.Time;
     if (diff < 0) {
       diff = -diff;
     }
-    return (diff < cmFileTime::NsPerS);
+    return (diff < cmFileTime::UtPerS);
   }
 
   /**
@@ -93,11 +105,11 @@ public:
    */
   bool DifferS(cmFileTime const& ftm) const
   {
-    NSC diff = this->NS - ftm.NS;
+    TimeType diff = this->Time - ftm.Time;
     if (diff < 0) {
       diff = -diff;
     }
-    return (diff >= cmFileTime::NsPerS);
+    return (diff >= cmFileTime::UtPerS);
   }
 
   /**
@@ -107,21 +119,21 @@ public:
    */
   int CompareS(cmFileTime const& ftm) const
   {
-    NSC const diff = this->NS - ftm.NS;
-    if (diff <= -cmFileTime::NsPerS) {
+    TimeType const diff = this->Time - ftm.Time;
+    if (diff <= -cmFileTime::UtPerS) {
       return -1;
     }
-    if (diff >= cmFileTime::NsPerS) {
+    if (diff >= cmFileTime::UtPerS) {
       return 1;
     }
     return 0;
   }
 
   /**
-   * @brief The file modification time in nanoseconds
+   * @brief The file modification time in unit time per second
    */
-  NSC GetNS() const { return this->NS; }
+  TimeType GetTime() const { return this->Time; }
 
 private:
-  NSC NS = 0;
+  TimeType Time = 0;
 };