YJIT: Print exit reasons on failure in test_yjit.rb

For <https://bugs.ruby-lang.org/issues/19921>, I suspect the test is
failing due to a timing related interrupt, which on paper could
happen with slow-enough GC runs.

In any case, it's helpful for debugging to have more information when
tests fail.

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>

2 files changed, 23 insertions, 9 deletions

diff --git a/test/ruby/test_yjit.rb b/test/ruby/test_yjit.rb
index fa357b4977..1fe9d68b30 100644
--- a/test/ruby/test_yjit.rb
+++ b/test/ruby/test_yjit.rb
@@ -10,6 +10,8 @@ require_relative '../lib/jit_support'
 
 return unless JITSupport.yjit_supported?
 
+require 'stringio'
+
 # Tests for YJIT with assertions on compilation and side exits
 # insipired by the RJIT tests in test/ruby/test_rjit.rb
 class TestYJIT < Test::Unit::TestCase
@@ -1470,8 +1472,15 @@ class TestYJIT < Test::Unit::TestCase
       if exits != :any &&
         exits != recorded_exits &&
         (exits.keys != recorded_exits.keys || !exits.all? { |k, v| v === recorded_exits[k] }) # triple-equal checks range membership or integer equality
-        flunk "Expected #{exits.empty? ? "no" : exits.inspect} exits" \
-          ", but got\n#{recorded_exits.inspect}"
+        stats_reasons = StringIO.new
+        ::RubyVM::YJIT.send(:_print_stats_reasons, runtime_stats, stats_reasons)
+        stats_reasons = stats_reasons.string
+        flunk <<~EOM
+          Expected #{exits.empty? ? "no" : exits.inspect} exits, but got:
+          #{recorded_exits.inspect}
+          Reasons:
+          #{stats_reasons}
+        EOM
       end
     end
 
diff --git a/yjit.rb b/yjit.rb
index 295e88ef34..c39175d12d 100644
--- a/yjit.rb
+++ b/yjit.rb
@@ -245,13 +245,8 @@ module RubyVM::YJIT
       $stderr.puts("YJIT exit locations dumped to `#{filename}`.")
     end
 
-    # Format and print out counters
-    def _print_stats(out: $stderr) # :nodoc:
-      stats = runtime_stats(context: true)
-      return unless Primitive.rb_yjit_stats_enabled_p
-
-      out.puts("***YJIT: Printing YJIT statistics on exit***")
-
+    # Print a summary of reasons for adverse performance events (e.g. exits)
+    def _print_stats_reasons(stats, out) # :nodoc:
       print_counters(stats, out: out, prefix: 'send_', prompt: 'method call fallback reasons: ')
       print_counters(stats, out: out, prefix: 'invokeblock_', prompt: 'invokeblock fallback reasons: ')
       print_counters(stats, out: out, prefix: 'invokesuper_', prompt: 'invokesuper fallback reasons: ')
@@ -286,6 +281,16 @@ module RubyVM::YJIT
       end
       print_counters(stats, out: out, prefix: 'lshift_', prompt: 'left shift (ltlt) exit reasons: ')
       print_counters(stats, out: out, prefix: 'invalidate_', prompt: 'invalidation reasons: ')
+    end
+
+    # Format and print out counters
+    def _print_stats(out: $stderr) # :nodoc:
+      stats = runtime_stats(context: true)
+      return unless Primitive.rb_yjit_stats_enabled_p
+
+      out.puts("***YJIT: Printing YJIT statistics on exit***")
+
+      _print_stats_reasons(stats, out)
 
       # Number of failed compiler invocations
       compilation_failure = stats[:compilation_failure]