rts: Don't force debug output to stderr

Previously `+RTS -Dw -l` would emit debug output to the eventlog while
`+RTS -l -Dw` would emit it to stderr. This was because the parser for
`-D` would unconditionally override the debug output target. Now we
instead only do so if no it is currently `TRACE_NONE`.

1 files changed, 7 insertions, 6 deletions

diff --git a/rts/RtsFlags.c b/rts/RtsFlags.c
index 5f1e8d6403..be87917737 100644
--- a/rts/RtsFlags.c
+++ b/rts/RtsFlags.c
@@ -2201,13 +2201,14 @@ static void read_debug_flags(const char* arg)
     }
     // -Dx also turns on -v.  Use -l to direct trace
     // events to the .eventlog file instead.
-    RtsFlags.TraceFlags.tracing = TRACE_STDERR;
-
-   // sanity implies zero_on_gc
-   if(RtsFlags.DebugFlags.sanity){
-        RtsFlags.DebugFlags.zero_on_gc = true;
-   }
+    if (RtsFlags.TraceFlags.tracing == TRACE_NONE) {
+        RtsFlags.TraceFlags.tracing = TRACE_STDERR;
+    }
 
+    // sanity implies zero_on_gc
+    if(RtsFlags.DebugFlags.sanity){
+         RtsFlags.DebugFlags.zero_on_gc = true;
+    }
 }
 #endif