bench: Fix subtle counting issue when rescaling iteration count

laanwj · luke-jr · commit eebc699d3070 · 2016-12-02T07:52:12.000Z
Make sure that the count is a zero modulo the new mask before scaling, otherwise the next time until a measure triggers will take only 1/2 as long as accounted for. This caused the 'min time' to be potentially off by as much as 100%. Github-Pull: #9200 Rebased-From: e0a9cb2
diff --git a/src/bench/bench.cpp b/src/bench/bench.cpp
@@ -64,8 +64,11 @@ bool State::KeepRunning()
           return true;
         }
         if (elapsed*16 < maxElapsed) {
-          countMask = ((countMask<<1)|1) & ((1LL<<60)-1);
-          countMaskInv = 1./(countMask+1);
+          uint64_t newCountMask = ((countMask<<1)|1) & ((1LL<<60)-1);
+          if ((count & newCountMask)==0) {
+              countMask = newCountMask;
+              countMaskInv = 1./(countMask+1);
+          }
         }
     }
     lastTime = now;

Original file line number	Diff line number	Diff line change
`@@ -64,8 +64,11 @@ bool State::KeepRunning()`
`64`	`64`	`return true;`
`65`	`65`	`}`
`66`	`66`	`if (elapsed*16 < maxElapsed) {`
`67`		`- countMask = ((countMask<<1)\|1) & ((1LL<<60)-1);`
`68`		`- countMaskInv = 1./(countMask+1);`
	`67`	`+ uint64_t newCountMask = ((countMask<<1)\|1) & ((1LL<<60)-1);`
	`68`	`+ if ((count & newCountMask)==0) {`
	`69`	`+ countMask = newCountMask;`
	`70`	`+ countMaskInv = 1./(countMask+1);`
	`71`	`+ }`
`69`	`72`	`}`
`70`	`73`	`}`
`71`	`74`	`lastTime = now;`