1 files changed, 42 insertions, 18 deletions

diff --git a/libgo/go/runtime/histogram.go b/libgo/go/runtime/histogram.go
index 4020969eb99..42baa6c5e2c 100644
--- a/libgo/go/runtime/histogram.go
+++ b/libgo/go/runtime/histogram.go
@@ -7,6 +7,7 @@ package runtime
 import (
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
+	"unsafe"
 )
 
 const (
@@ -69,17 +70,21 @@ const (
 // for concurrent use. It is also safe to read all the values
 // atomically.
 type timeHistogram struct {
-	counts   [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64
-	overflow uint64
+	counts [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64
+
+	// underflow counts all the times we got a negative duration
+	// sample. Because of how time works on some platforms, it's
+	// possible to measure negative durations. We could ignore them,
+	// but we record them anyway because it's better to have some
+	// signal that it's happening than just missing samples.
+	underflow uint64
 }
 
 // record adds the given duration to the distribution.
-//
-// Although the duration is an int64 to facilitate ease-of-use
-// with e.g. nanotime, the duration must be non-negative.
 func (h *timeHistogram) record(duration int64) {
 	if duration < 0 {
-		throw("timeHistogram encountered negative duration")
+		atomic.Xadd64(&h.underflow, 1)
+		return
 	}
 	// The index of the exponential bucket is just the index
 	// of the highest set bit adjusted for how many bits we
@@ -92,29 +97,47 @@ func (h *timeHistogram) record(duration int64) {
 		superBucket = uint(sys.Len64(uint64(duration))) - timeHistSubBucketBits
 		if superBucket*timeHistNumSubBuckets >= uint(len(h.counts)) {
 			// The bucket index we got is larger than what we support, so
-			// add into the special overflow bucket.
-			atomic.Xadd64(&h.overflow, 1)
-			return
+			// include this count in the highest bucket, which extends to
+			// infinity.
+			superBucket = timeHistNumSuperBuckets - 1
+			subBucket = timeHistNumSubBuckets - 1
+		} else {
+			// The linear subbucket index is just the timeHistSubBucketsBits
+			// bits after the top bit. To extract that value, shift down
+			// the duration such that we leave the top bit and the next bits
+			// intact, then extract the index.
+			subBucket = uint((duration >> (superBucket - 1)) % timeHistNumSubBuckets)
 		}
-		// The linear subbucket index is just the timeHistSubBucketsBits
-		// bits after the top bit. To extract that value, shift down
-		// the duration such that we leave the top bit and the next bits
-		// intact, then extract the index.
-		subBucket = uint((duration >> (superBucket - 1)) % timeHistNumSubBuckets)
 	} else {
 		subBucket = uint(duration)
 	}
 	atomic.Xadd64(&h.counts[superBucket*timeHistNumSubBuckets+subBucket], 1)
 }
 
+const (
+	fInf    = 0x7FF0000000000000
+	fNegInf = 0xFFF0000000000000
+)
+
+func float64Inf() float64 {
+	inf := uint64(fInf)
+	return *(*float64)(unsafe.Pointer(&inf))
+}
+
+func float64NegInf() float64 {
+	inf := uint64(fNegInf)
+	return *(*float64)(unsafe.Pointer(&inf))
+}
+
 // timeHistogramMetricsBuckets generates a slice of boundaries for
 // the timeHistogram. These boundaries are represented in seconds,
 // not nanoseconds like the timeHistogram represents durations.
 func timeHistogramMetricsBuckets() []float64 {
-	b := make([]float64, timeHistTotalBuckets-1)
+	b := make([]float64, timeHistTotalBuckets+1)
+	b[0] = float64NegInf()
 	for i := 0; i < timeHistNumSuperBuckets; i++ {
 		superBucketMin := uint64(0)
-		// The (inclusive) minimum for the first bucket is 0.
+		// The (inclusive) minimum for the first non-negative bucket is 0.
 		if i > 0 {
 			// The minimum for the second bucket will be
 			// 1 << timeHistSubBucketBits, indicating that all
@@ -128,7 +151,7 @@ func timeHistogramMetricsBuckets() []float64 {
 		// index to combine it with the bucketMin.
 		subBucketShift := uint(0)
 		if i > 1 {
-			// The first two buckets are exact with respect to integers,
+			// The first two super buckets are exact with respect to integers,
 			// so we'll never have to shift the sub-bucket index. Thereafter,
 			// we shift up by 1 with each subsequent bucket.
 			subBucketShift = uint(i - 2)
@@ -141,8 +164,9 @@ func timeHistogramMetricsBuckets() []float64 {
 
 			// Convert the subBucketMin which is in nanoseconds to a float64 seconds value.
 			// These values will all be exactly representable by a float64.
-			b[i*timeHistNumSubBuckets+j] = float64(subBucketMin) / 1e9
+			b[i*timeHistNumSubBuckets+j+1] = float64(subBucketMin) / 1e9
 		}
 	}
+	b[len(b)-1] = float64Inf()
 	return b
 }