From 6acec6bf34b19821da7a4605586d0c1a5f340454 Mon Sep 17 00:00:00 2001
From: John MacFarlane <jgm@berkeley.edu>
Date: Mon, 23 Apr 2018 12:02:59 -0700
Subject: [PATCH] CONTRIBUTING.md: added note about --trace.

---
 CONTRIBUTING.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 1145dd835..11a0c9c94 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -205,6 +205,10 @@ placed in the source directory):
 Profiling
 ---------
 
+To diagnose a performance issue with parsing, first try using
+the `--trace` option.  This will give you a record of when block
+parsers succeed, so you can spot backtracking issues.
+
 To use the GHC profiler with cabal:
 
     cabal clean