Avoid overflow on Windows high resolution timer queries (#791)

* Avoid overflow on Windows high resolution timer queries once system has been up long enough

include/aws/common/clock.h

@@ -31,6 +31,15 @@ AWS_STATIC_IMPL uint64_t aws_timestamp_convert(
     enum aws_timestamp_unit convert_to,
     uint64_t *remainder);
 
+/**
+ * More general form of aws_timestamp_convert that takes arbitrary frequencies rather than the timestamp enum.
+ */
+AWS_STATIC_IMPL uint64_t aws_timestamp_convert_u64(
+    uint64_t timestamp,
+    uint64_t convert_from_frequency,
+    uint64_t convert_to_frequency,
+    uint64_t *remainder);
+
 /**
  * Get ticks in nanoseconds (usually 100 nanosecond precision) on the high resolution clock (most-likely TSC). This
  * clock has no bearing on the actual system time. On success, timestamp will be set.

include/aws/common/clock.inl

@@ -19,18 +19,18 @@ AWS_EXTERN_C_BEGIN
  * to 0 first if you care about that kind of thing. If conversion would lead to integer overflow, the timestamp
  * returned will be the highest possible time that is representable, i.e. UINT64_MAX.
  */
-AWS_STATIC_IMPL uint64_t aws_timestamp_convert(
+AWS_STATIC_IMPL uint64_t aws_timestamp_convert_u64(
     uint64_t timestamp,
-    enum aws_timestamp_unit convert_from,
-    enum aws_timestamp_unit convert_to,
+    uint64_t convert_from_frequency,
+    uint64_t convert_to_frequency,
     uint64_t *remainder) {
     uint64_t diff = 0;
 
-    if (convert_to > convert_from) {
-        diff = convert_to / convert_from;
+    if (convert_to_frequency > convert_from_frequency) {
+        diff = convert_to_frequency / convert_from_frequency;
         return aws_mul_u64_saturating(timestamp, diff);
-    } else if (convert_to < convert_from) {
-        diff = convert_from / convert_to;
+    } else if (convert_to_frequency < convert_from_frequency) {
+        diff = convert_from_frequency / convert_to_frequency;
 
         if (remainder) {
             *remainder = timestamp % diff;
@@ -42,6 +42,15 @@ AWS_STATIC_IMPL uint64_t aws_timestamp_convert(
     }
 }
 
+AWS_STATIC_IMPL uint64_t aws_timestamp_convert(
+    uint64_t timestamp,
+    enum aws_timestamp_unit convert_from,
+    enum aws_timestamp_unit convert_to,
+    uint64_t *remainder) {
+
+    return aws_timestamp_convert_u64(timestamp, convert_from, convert_to, remainder);
+}
+
 AWS_EXTERN_C_END
 
 #endif /* AWS_COMMON_CLOCK_INL */

source/windows/clock.c

@@ -51,10 +51,9 @@ int aws_high_res_clock_get_ticks(uint64_t *timestamp) {
     LARGE_INTEGER ticks, frequency;
     /* QPC runs on sub-microsecond precision, convert to nanoseconds */
     if (QueryPerformanceFrequency(&frequency) && QueryPerformanceCounter(&ticks)) {
-        *timestamp = aws_timestamp_convert((uint64_t)ticks.QuadPart, AWS_TIMESTAMP_SECS, AWS_TIMESTAMP_MICROS, NULL) /
-                     (uint64_t)frequency.QuadPart;
+        *timestamp =
+            aws_timestamp_convert((uint64_t)ticks.QuadPart, (uint64_t)frequency.QuadPart, AWS_TIMESTAMP_NANOS, NULL);
 
-        *timestamp = aws_timestamp_convert(*timestamp, AWS_TIMESTAMP_MICROS, AWS_TIMESTAMP_NANOS, NULL);
         return AWS_OP_SUCCESS;
     }