diff --git a/code/.gitignore b/code/.gitignore
index ff483930b8..f69a449ca2 100644
--- a/code/.gitignore
+++ b/code/.gitignore
@@ -12,6 +12,7 @@ lia6gc
 lia6ll
 lii3gc
 lii6gc
+lii6gp
 lii6ll
 w3i3mv
 w3i6mv
diff --git a/code/lii6gp.gmk b/code/lii6gp.gmk
new file mode 100644
index 0000000000..657e797d41
--- /dev/null
+++ b/code/lii6gp.gmk
@@ -0,0 +1,63 @@
+# -*- makefile -*-
+#
+# lii6gp.gmk: BUILD FOR LINUX/x64/GCC PLATFORM WITH GPROF
+#
+# $Id$
+# Copyright (c) 2001-2023 Ravenbrook Limited.  See end of file for license.
+#
+# FIXME: This file is almost identical to lii6ll.gmk.  Share common
+# stuff or work out another way to use gprof.
+#
+# Example use::
+#
+#   $ lii6gp/hot/gcbench -x 6415986 amc
+#   $ gprof lii6gp/hot/gcbench
+#
+
+PFM = lii6gp
+
+MPMPF = \
+    lockix.c \
+    prmci6.c \
+    prmcix.c \
+    prmclii6.c \
+    protix.c \
+    protsgix.c \
+    pthrdext.c \
+    span.c \
+    thix.c \
+    vmix.c
+
+LIBS = -lm -lpthread
+
+include gp.gmk
+
+
+# C. COPYRIGHT AND LICENSE
+#
+# Copyright (C) 2001-2023 Ravenbrook Limited <https://www.ravenbrook.com/>.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+#
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the
+#    distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
diff --git a/code/seg.c b/code/seg.c
index 9782e43b3c..8225987241 100644
--- a/code/seg.c
+++ b/code/seg.c
@@ -241,16 +241,19 @@ static void SegFinish(Seg seg)
 
 void SegSetGrey(Seg seg, TraceSet grey)
 {
-  AVERT(Seg, seg);
-  AVERT(TraceSet, grey);
-  AVER(grey == TraceSetEMPTY || SegRankSet(seg) != RankSetEMPTY);
+  AVERT_CRITICAL(Seg, seg);
+  AVERT_CRITICAL(TraceSet, grey);
+  AVER_CRITICAL(grey == TraceSetEMPTY || SegRankSet(seg) != RankSetEMPTY);
 
   /* Don't dispatch to the class method if there's no actual change in
      greyness, or if the segment doesn't contain any references. */
+  /* FIXME: ``Method`` includes ``MustBeA`` and not
+     ``MustBeA_CRITICAL``.  Experiment with a new ``Method_CRITICAL``
+     and measure if it's worthwhile. */
   if (grey != SegGrey(seg) && SegRankSet(seg) != RankSetEMPTY)
     Method(Seg, seg, setGrey)(seg, grey);
 
-  EVENT3(SegSetGrey, PoolArena(SegPool(seg)), seg, grey);
+  EVENT_CRITICAL3(SegSetGrey, PoolArena(SegPool(seg)), seg, grey);
 }