Use NeverZero instead of Saturated Counters for coverage

According to
https://www.usenix.org/system/files/woot20-paper-fioraldi.pdf,
letting coverage 8-bit counters wrap from 255 to 1 on increment performs
better than having them stay at 255. In fact, the latter has been
observed to hurt overall coverage.

1 files changed, 6 insertions, 3 deletions

diff --git a/agent/src/test/java/com/code_intelligence/jazzer/instrumentor/CoverageInstrumentationTest.kt b/agent/src/test/java/com/code_intelligence/jazzer/instrumentor/CoverageInstrumentationTest.kt
index 31f40575..e52231cd 100644
--- a/agent/src/test/java/com/code_intelligence/jazzer/instrumentor/CoverageInstrumentationTest.kt
+++ b/agent/src/test/java/com/code_intelligence/jazzer/instrumentor/CoverageInstrumentationTest.kt
@@ -109,11 +109,14 @@ class CoverageInstrumentationTest {
         // Control flows through the first if branch once per run.
         val takenOnEveryRunEdge = ifFirstBranch
 
-        for (i in 1..300) {
+        var lastCounter = 0.toUByte()
+        for (i in 1..600) {
             assertSelfCheck(target)
             assertEquals(1, MockCoverageMap.mem[takenOnceEdge])
-            // Verify that the counter does not overflow.
-            val expectedCounter = i.coerceAtMost(255).toUByte()
+            // Verify that the counter increments, but is never zero.
+            val expectedCounter = (lastCounter + 1U).toUByte().takeUnless { it == 0.toUByte() }
+                ?: (lastCounter + 2U).toUByte()
+            lastCounter = expectedCounter
             val actualCounter = MockCoverageMap.mem[takenOnEveryRunEdge].toUByte()
             assertEquals(expectedCounter, actualCounter, "After $i runs:")
         }