[test] Change stress compaction flag to boolean value.

Bug: v8:6972
Change-Id: I116d48f22045cf42cf2123297458640b551d37da
Reviewed-on: https://chromium-review.googlesource.com/768868Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michał Majewski <majeski@google.com>
Cr-Commit-Position: refs/heads/master@{#49408}

src/flag-definitions.h

@@ -675,9 +675,9 @@ DEFINE_BOOL(force_marking_deque_overflows, false,
 DEFINE_BOOL(stress_compaction, false,
             "stress the GC compactor to flush out bugs (implies "
             "--force_marking_deque_overflows)")
-DEFINE_INT(stress_compaction_percentage, 0,
-           "Stress GC compaction by selecting X percent of pages as evacuation "
-           "candidates. It overrides stress_compaction.")
+DEFINE_BOOL(stress_compaction_random, false,
+            "Stress GC compaction by selecting random percent of pages as "
+            "evacuation candidates. It overrides stress_compaction.")
 DEFINE_BOOL(stress_incremental_marking, false,
             "force incremental marking for small heaps and run it more often")
 DEFINE_INT(stress_marking, 0,

src/heap/mark-compact.cc

@@ -911,19 +911,16 @@ void MarkCompactCollector::CollectEvacuationCandidates(PagedSpace* space) {
         AddEvacuationCandidate(p);
       }
     }
-  } else if (FLAG_stress_compaction_percentage > 0) {
-    size_t percent =
-        isolate()->fuzzer_rng()->NextInt(FLAG_stress_compaction_percentage + 1);
-
-    size_t pages_to_mark_count = percent * pages.size() / 100;
-    if (pages_to_mark_count) {
+  } else if (FLAG_stress_compaction_random) {
+    double fraction = isolate()->fuzzer_rng()->NextDouble();
+    size_t pages_to_mark_count =
+        static_cast<size_t>(fraction * (pages.size() + 1));
     for (uint64_t i : isolate()->fuzzer_rng()->NextSample(
              pages.size(), pages_to_mark_count)) {
       candidate_count++;
       total_live_bytes += pages[i].first;
       AddEvacuationCandidate(pages[i].second);
     }
-    }
   } else if (FLAG_stress_compaction) {
     for (size_t i = 0; i < pages.size(); i++) {
       Page* p = pages[i].second;

tools/testrunner/gc_fuzzer.py

@@ -244,9 +244,7 @@ class GCFuzzer(base_runner.BaseTestRunner):
             '--fuzzer_random_seed', str(fuzzer_seed),
           ]
           if options.stress_compaction:
-            fuzzing_flags += [
-              '--stress_compaction_percentage', '100',
-            ]
+            fuzzing_flags.append('--stress_compaction_random')
           s.tests.append(t.CopyAddingFlags(t.variant, fuzzing_flags))
       num_tests += len(s.tests)