--- a/jvmci/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotJVMCICompilerConfig.java	Thu Jun 15 01:01:04 2017 +0200
+++ b/jvmci/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotJVMCICompilerConfig.java	Thu Jun 15 12:04:08 2017 +0200
@@ -22,6 +22,11 @@
  */
 package jdk.vm.ci.hotspot;
 
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.ArrayList;
+import java.util.List;
+
 import jdk.vm.ci.code.CompilationRequest;
 import jdk.vm.ci.code.CompilationRequestResult;
 import jdk.vm.ci.common.JVMCIError;
@@ -29,8 +34,9 @@
 import jdk.vm.ci.runtime.JVMCICompiler;
 import jdk.vm.ci.runtime.JVMCICompilerFactory;
 import jdk.vm.ci.runtime.JVMCIRuntime;
+import jdk.vm.ci.services.JVMCIPermission;
 import jdk.vm.ci.services.JVMCIServiceLocator;
-import jdk.vm.ci.services.JVMCIPermission;
+import sun.misc.VM;
 
 final class HotSpotJVMCICompilerConfig {
 
@@ -41,8 +47,14 @@
      */
     private static class DummyCompilerFactory implements JVMCICompilerFactory, JVMCICompiler {
 
+        private final String reason;
+
+        DummyCompilerFactory(String reason) {
+            this.reason = reason;
+        }
+
         public CompilationRequestResult compileMethod(CompilationRequest request) {
-            throw new JVMCIError("no JVMCI compiler selected");
+            throw new JVMCIError("No JVMCI compiler selected. " + reason);
         }
 
         @Override
@@ -74,7 +86,8 @@
             String compilerName = Option.Compiler.getString();
             if (compilerName != null) {
                 if (compilerName.isEmpty() || compilerName.equals("null")) {
-                    factory = new DummyCompilerFactory();
+                    factory = new DummyCompilerFactory("Value of " + Option.Compiler.getPropertyName() + " property is \"" +
+                                    compilerName + "\" which denotes the null JVMCI compiler.");
                 } else {
                     for (JVMCICompilerFactory f : JVMCIServiceLocator.getProviders(JVMCICompilerFactory.class)) {
                         if (f.getCompilerName().equals(compilerName)) {
@@ -82,22 +95,33 @@
                         }
                     }
                     if (factory == null) {
-                        throw new JVMCIError("JVMCI compiler '%s' not found", compilerName);
+                        throw new JVMCIError("JVMCI compiler \"%s\" not found", compilerName);
                     }
                 }
             } else {
                 // Auto select a single available compiler
+                List<String> multiple = null;
                 for (JVMCICompilerFactory f : JVMCIServiceLocator.getProviders(JVMCICompilerFactory.class)) {
-                    if (factory == null) {
+                    if (multiple != null) {
+                        multiple.add(f.getCompilerName());
+                    } else if (factory == null) {
                         factory = f;
                     } else {
-                        // Multiple factories seen - cancel auto selection
+                        multiple = new ArrayList<>();
+                        multiple.add(f.getCompilerName());
+                        multiple.add(factory.getCompilerName());
                         factory = null;
-                        break;
                     }
                 }
-                if (factory == null) {
-                    factory = new DummyCompilerFactory();
+                if (multiple != null) {
+                    factory = new DummyCompilerFactory("Multiple providers of " + JVMCICompilerFactory.class + " available: " +
+                                    String.join(", ", multiple) +
+                                    ". You can select one of these with the " + Option.Compiler.getPropertyName() + " property " +
+                                    "(e.g., -D" + Option.Compiler.getPropertyName() + "=" + multiple.get(0) + ").");
+                } else if (factory == null) {
+                    Path jvmciDir = Paths.get(VM.getSavedProperty("java.home"), "lib", "jvmci");
+                    factory = new DummyCompilerFactory("No providers of " + JVMCICompilerFactory.class + " found in " + jvmciDir +
+                                    " or on the class path specified by the jvmci.class.path.append property.");
                 }
             }
             factory.onSelection();

--- a/src/share/vm/jvmci/jvmci_globals.cpp	Thu Jun 15 01:01:04 2017 +0200
+++ b/src/share/vm/jvmci/jvmci_globals.cpp	Thu Jun 15 12:04:08 2017 +0200
@@ -118,35 +118,12 @@
 
 void JVMCIGlobals::set_jvmci_specific_flags() {
   if (FLAG_IS_DEFAULT(UseJVMCICompiler) && !UseJVMCICompiler) {
-    if (Arguments::get_property("jvmci.class.path.append") != NULL) {
-      // If jvmci.class.path.append is defined then
-      // assume there is a JVMCI compiler.
+    char filename[JVM_MAXPATHLEN];
+    const char* fileSep = os::file_separator();
+    jio_snprintf(filename, sizeof(filename), "%s%slib%suse-jvmci-compiler-by-default", Arguments::get_java_home(), fileSep, fileSep);
+    struct stat statbuf;
+    if (os::stat(filename, &statbuf) == 0) {
       FLAG_SET_ERGO(bool, UseJVMCICompiler, true);
-    } else {
-      // If lib/jvmci contains at least one jar apart from
-      // jvmci-api.jar and jvmci-hotspot.jar, then
-      // assume there is a JVMCI compiler.
-      char jvmciDir[JVM_MAXPATHLEN];
-      const char* fileSep = os::file_separator();
-      jio_snprintf(jvmciDir, sizeof(jvmciDir), "%s%slib%sjvmci", Arguments::get_java_home(), fileSep, fileSep);
-      DIR* dir = os::opendir(jvmciDir);
-      if (dir != NULL) {
-        /* Scan the directory for jars */
-        struct dirent *entry;
-        char *dbuf = NEW_C_HEAP_ARRAY(char, os::readdir_buf_size(jvmciDir), mtInternal);
-        while ((entry = os::readdir(dir, (dirent *) dbuf)) != NULL) {
-          const char* name = entry->d_name;
-          const char* ext = name + strlen(name) - 4;
-          if (ext > name && os::file_name_strcmp(ext, ".jar") == 0) {
-            if (strcmp(name, "jvmci-api.jar") != 0 && strcmp(name, "jvmci-hotspot.jar") != 0) {
-              FLAG_SET_ERGO(bool, UseJVMCICompiler, true);
-              break;
-            }
-          }
-        }
-        FREE_C_HEAP_ARRAY(char, dbuf, mtInternal);
-        os::closedir(dir);
-      }
     }
   }
 

--- a/src/share/vm/jvmci/jvmci_globals.hpp	Thu Jun 15 01:01:04 2017 +0200
+++ b/src/share/vm/jvmci/jvmci_globals.hpp	Thu Jun 15 12:04:08 2017 +0200
@@ -50,9 +50,8 @@
           "Enable JVMCI")                                                   \
                                                                             \
   product(bool, UseJVMCICompiler, false,                                    \
-          "Use JVMCI as the default compiler. Ergonomically set to "        \
-          "true if non-core JVMCI jars are in jre/lib/jvmci/ or the "       \
-          "jvmci.class.path.append system property is defined.")            \
+          "Use JVMCI as the default compiler. Will be true by default "     \
+          "if <java.home>/lib/use-jvmci-compiler-by-default exists.")       \
                                                                             \
   product(bool, JVMCIPrintProperties, false,                                \
           "Prints properties used by the JVMCI compiler and exits")         \