8356000: C1/C2-only modes use 2 compiler threads on low CPU count machines

src/hotspot/share/compiler/compilationPolicy.cpp

@@ -440,10 +440,12 @@ void CompilationPolicy::initialize() {
       FLAG_SET_DEFAULT(CICompilerCountPerCPU, true);
     }
     if (CICompilerCountPerCPU) {
+      int min_count = (c1_only || c2_only) ? 1 : 2;
+      int active_cpus = os::active_processor_count();
       // Simple log n seems to grow too slowly for tiered, try something faster: log n * log log n
-      int log_cpu = log2i(os::active_processor_count());
+      int log_cpu = log2i(active_cpus);
       int loglog_cpu = log2i(MAX2(log_cpu, 1));
-      count = MAX2(log_cpu * loglog_cpu * 3 / 2, 2);
+      count = MAX2(log_cpu * loglog_cpu * 3 / 2, min_count);
       // Make sure there is enough space in the code cache to hold all the compiler buffers
       size_t c1_size = 0;
 #ifdef COMPILER1
@@ -453,12 +455,20 @@ void CompilationPolicy::initialize() {
 #ifdef COMPILER2
       c2_size = C2Compiler::initial_code_buffer_size();
 #endif
-      size_t buffer_size = c1_only ? c1_size : (c1_size/3 + 2*c2_size/3);
+      size_t buffer_size = 0;
+      if (c1_only) {
+        buffer_size = c1_size;
+      } else if (c2_only) {
+        buffer_size = c2_size;
+      } else {
+        buffer_size = c1_size/3 + 2*c2_size/3;
+      }
       int max_count = (ReservedCodeCacheSize - (CodeCacheMinimumUseSpace DEBUG_ONLY(* 3))) / (int)buffer_size;
       if (count > max_count) {
         // Lower the compiler count such that all buffers fit into the code cache
-        count = MAX2(max_count, c1_only ? 1 : 2);
+        count = MAX2(max_count, min_count);
       }
+      assert((!c1_only && !c2_only) || count <= active_cpus, "Too many threads: %d", count);
       FLAG_SET_ERGO(CICompilerCount, count);
     }
 #else
@@ -476,7 +486,6 @@ void CompilationPolicy::initialize() {
 #endif
 
     if (c1_only) {
-      // No C2 compiler thread required
       set_c1_count(count);
     } else if (c2_only) {
       set_c2_count(count);

test/hotspot/jtreg/compiler/arguments/TestCompilerCounts.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @library /test/lib /
+ * @bug 8356000
+ * @requires vm.flagless
+ * @requires vm.debug
+ *
+ * @summary Test compiler counts selection, verified by internal assertions
+ * @run driver compiler.arguments.TestCompilerCounts
+ */
+
+package compiler.arguments;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Arrays;
+import java.util.ArrayList;
+import jdk.test.lib.process.ProcessTools;
+import jdk.test.lib.process.OutputAnalyzer;
+
+public class TestCompilerCounts {
+
+    public static void main(String[] args) throws IOException {
+        testWith("-XX:TieredStopAtLevel=0");
+        testWith("-XX:TieredStopAtLevel=1");
+        testWith("-XX:TieredStopAtLevel=2");
+        testWith("-XX:TieredStopAtLevel=3");
+        testWith("-XX:TieredStopAtLevel=4");
+        testWith("-XX:-TieredCompilation");
+    }
+
+    public static void testWith(String mode) throws IOException {
+        for (int cpus = 1; cpus <= Runtime.getRuntime().availableProcessors(); cpus++) {
+            String[] args = new String[] {
+                mode,
+                "-XX:ActiveProcessorCount=" + cpus,
+                "-version"
+            };
+            ProcessBuilder pb = ProcessTools.createLimitedTestJavaProcessBuilder(args);
+            OutputAnalyzer output = new OutputAnalyzer(pb.start());
+            output.shouldHaveExitValue(0);
+        }
+    }
+
+}