Print times with more sensible units

Previously the "Time" tab would always show times in unit of "seconds".
This made it impossible to use to measure short durations.

We now rather choose a suitable unit for the scale of the time being
displayed.

Author: bgamari

GUI/SummaryView.hs

@@ -186,9 +186,9 @@ setSummaryStats view SummaryStats{..} hasHeapEvents = do
 setTimeStats :: SummaryView -> TimeStats -> IO ()
 setTimeStats SummaryView{..} TimeStats{..} =
   mapM_ (\(label, text) -> set label [ labelText := text ])
-    [ (labelTimeTotal       , showFFloat (Just 2) (timeToSecondsDbl timeTotal) "s")
-    , (labelTimeMutator     , showFFloat (Just 2) (timeToSecondsDbl timeMutator) "s")
-    , (labelTimeGC          , showFFloat (Just 2) (timeToSecondsDbl timeGC) "s")
+    [ (labelTimeTotal       , showTimeWithUnit timeTotal)
+    , (labelTimeMutator     , showTimeWithUnit timeMutator)
+    , (labelTimeGC          , showTimeWithUnit timeGC)
     , (labelTimeProductivity, showFFloat (Just 1) (timeProductivity * 100) "% of mutator vs total")
     ]
 
@@ -563,6 +563,17 @@ sparkStats StatsAccum{dsparkTable} =
 
 ------------------------------------------------------------------------------
 
+showTimeWithUnit :: Integral a => a -> String
+showTimeWithUnit t =
+    showFFloat (Just 3) t'' unit
+  where
+    (t'', unit) =
+      case timeToSecondsDbl t of
+        t' | t' < 1e-6  -> (t' / 1e-9, "ns")
+           | t' < 1e-3  -> (t' / 1e-6, "μs")
+           | t' < 1     -> (t' / 1e-3, "ms")
+           | otherwise  -> (t', "s")
+
 timeToSecondsDbl :: Integral a => a -> Double
 timeToSecondsDbl t = timeToSeconds $ fromIntegral t