diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 4ef577f82f1..3a004ec4a45 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2018-09-14  Richard Biener  <rguenther@suse.de>
+
+	PR middle-end/63155
+	* tree-ssa-coalesce.c (ssa_conflicts_merge): Remove conflict
+	bits for the merged partition.
+
 2018-09-13  Martin Sebor  <msebor@redhat.com>
 	    Bernd Edlinger  <bernd.edlinger@hotmail.de>
 
diff --git a/gcc/tree-ssa-coalesce.c b/gcc/tree-ssa-coalesce.c
index e4f576fbc19..750fd835041 100644
--- a/gcc/tree-ssa-coalesce.c
+++ b/gcc/tree-ssa-coalesce.c
@@ -620,7 +620,11 @@ ssa_conflicts_merge (ssa_conflicts *ptr, unsigned x, unsigned y)
     {
       bitmap bz = ptr->conflicts[z];
       if (bz)
-	bitmap_set_bit (bz, x);
+	{
+	  bool was_there = bitmap_clear_bit (bz, y);
+	  gcc_checking_assert (was_there);
+	  bitmap_set_bit (bz, x);
+	}
     }
 
   if (bx)