2 files changed, 44 insertions, 4 deletions
diff --git a/testsuite/tests/perf/compiler/T12545.measure.sh b/testsuite/tests/perf/compiler/T12545.measure.sh
new file mode 100755
index 0000000000..e578c7197a
--- /dev/null
+++ b/testsuite/tests/perf/compiler/T12545.measure.sh
@@ -0,0 +1,39 @@
+#!/usr/bin/env sh
+
+cat <<EOF
+This script is for painlessly finding min and max allocations of T12545 based on
+playing with -dunique-increment (see #19414) to diagnose if a metric increase is
+a false positive.
+A ratio of about 4.8% (48 per mille) is expected.
+EOF
+
+# https://stackoverflow.com/a/4774063/388010
+TOP="$( cd -- "$(dirname "$0")/../../../../" >/dev/null 2>&1 ; pwd -P )"
+GHC=${GHC:-$TOP/_validate/stage1/bin/ghc}
+
+echo "Using GHC=$GHC. Feel free to override via env var"
+
+function measure() {
+  $GHC -fforce-recomp -v0 -dunique-increment=$1 T12545.hs +RTS -t 2>&1 | cut -f1 -d',' | grep -o -P '\d+'
+}
+
+min=999999999999
+max=-999999999999
+while true; do
+  inc=$((1 + $RANDOM % 1000000))
+  n=$(measure $inc)
+  any_change=false
+  if [ $n -lt $min ]; then
+    min=$n
+    any_change=true
+    echo "New min: $min (on $inc)"
+  fi
+  if [ $n -gt $max ]; then
+    max=$n
+    any_change=true
+    echo "New max: $max (on $inc)"
+  fi
+  if [ "$any_change" = true ]; then
+    echo "New ratio: $(($max*1000/$min - 1000)) per mille"
+  fi
+done
diff --git a/testsuite/tests/perf/compiler/all.T b/testsuite/tests/perf/compiler/all.T
index eadf2ff920..ee764f15cb 100644
--- a/testsuite/tests/perf/compiler/all.T
+++ b/testsuite/tests/perf/compiler/all.T
@@ -211,12 +211,13 @@ test('T12234',
      compile,
      [''])
 
-# T12545 is sensitive to -dunique-increments changes, see #19414.
-# I've seen variations of 3% by playing with that parameter, so that's the
-# current acceptance window.
+# T12545 is sensitive to -dunique-increments changes, see #19414. I've seen
+# variations of as much as 4.8% by playing with that parameter, but I think
+# it's better to check with T12545.measure.sh that lower and upper bounds of
+# allocations indeed haven't changed and then simply accept the metric increase.
 test('T12545',
      [ only_ways(['normal']),
-       collect_compiler_stats('bytes allocated', 3),
+       collect_compiler_stats('bytes allocated', 3), # Don't increase it, run T12545.measure.sh and leave a comment on #19414!
        extra_clean(['T12545a.hi', 'T12545a.o'])
      ],
      multimod_compile,