diff --git a/src/cmd/compile/internal/base/flag.go b/src/cmd/compile/internal/base/flag.go
index 0889c37b0d..1ee3337088 100644
--- a/src/cmd/compile/internal/base/flag.go
+++ b/src/cmd/compile/internal/base/flag.go
@@ -184,6 +184,7 @@ func ParseFlags() {
 	Debug.SyncFrames = -1 // disable sync markers by default
 	Debug.ZeroCopy = 1
 	Debug.RangeFuncCheck = 1
+	Debug.MergeLocals = 1
 
 	Debug.Checkptr = -1 // so we can tell whether it is set explicitly