Bug 176881: we should also look at the high 32 bits of the performance

counter frequency.  Ignoring the high part of the frequency will cause
our interval timer code to malfunction if the high part is not zero.
Also took the opportunity to simplify the arithmetics.

nsprpub/pr/src/md/windows/ntinrval.c

@@ -45,9 +45,8 @@
 #define QueryPerformanceCounter(x)     FALSE
 #endif
 
-PRIntn _nt_bitShift = 0;
-PRInt32 _nt_highMask = 0;
-PRInt32 _nt_ticksPerSec = -1;
+static PRIntn _nt_bitShift = 0;
+static PRInt32 _nt_ticksPerSec = -1;
 
 void
 _PR_MD_INTERVAL_INIT()
@@ -55,17 +54,33 @@ _PR_MD_INTERVAL_INIT()
     LARGE_INTEGER count;
 
     if (QueryPerformanceFrequency(&count)) {
-        PR_ASSERT(count.HighPart == 0);
+        /*
+         * HighPart is signed (LONG).  Assert that its sign bit is 0
+         * because we will be right shifting it.  LowPart is unsigned
+         * (DWORD).
+         */
+        PR_ASSERT(count.HighPart >= 0);
+        while(count.HighPart) {
+            count.LowPart = (count.HighPart << 31) + (count.LowPart >> 1);
+            count.HighPart >>= 1;
+            _nt_bitShift++;
+        }
         while(count.LowPart > PR_INTERVAL_MAX) {
             count.LowPart >>= 1;
             _nt_bitShift++;
-            _nt_highMask = (_nt_highMask << 1)+1;
         }
 
-        _nt_ticksPerSec = count.LowPart;
-        PR_ASSERT(_nt_ticksPerSec > PR_INTERVAL_MIN);
-    } else 
-        _nt_ticksPerSec = -1;
+        /*
+         * We can't use the performance counter if after
+         * normalization we are left with fewer than 32 bits.
+         */
+        if (_nt_bitShift <= 32) {
+            _nt_ticksPerSec = count.LowPart;
+            PR_ASSERT(_nt_ticksPerSec > PR_INTERVAL_MIN);
+            return;
+        }
+    }
+    _nt_ticksPerSec = -1;
 }
 
 PRIntervalTime 
@@ -77,12 +92,15 @@ _PR_MD_GET_INTERVAL()
     * to only 100000 ticks per second; QueryPerformanceCounter is too high
     * resolution...
     */
-    if (QueryPerformanceCounter(&count)) {
-        PRInt32 top = count.HighPart & _nt_highMask;
-        top = top << (32 - _nt_bitShift);
-        count.LowPart = count.LowPart >> _nt_bitShift;   
-        count.LowPart = count.LowPart + top; 
-        return (PRUint32)count.LowPart;
+    if (_nt_ticksPerSec != -1) {
+        (void)QueryPerformanceCounter(&count);
+        PR_ASSERT(_nt_bitShift <= 32);
+        if (_nt_bitShift == 32) {
+            return (PRUint32)count.HighPart;
+        } else {
+            return (PRUint32)((count.HighPart << (32 - _nt_bitShift))
+                    + (count.LowPart >> _nt_bitShift));
+        }
     } else
 #if defined(__MINGW32__)
         return time();