src/jdk.internal.vm.ci/share/classes/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotJVMCICompilerConfig.java

*** 44,56 ****
       * to perform a compilation. This allows the reflective parts of the JVMCI API to be used
       * without requiring a compiler implementation to be available.
       */
      private static class DummyCompilerFactory implements JVMCICompilerFactory, JVMCICompiler {
  
          @Override
          public HotSpotCompilationRequestResult compileMethod(CompilationRequest request) {
!             throw new JVMCIError("no JVMCI compiler selected");
          }
  
          @Override
          public String getCompilerName() {
              return "null";
--- 44,62 ----
       * to perform a compilation. This allows the reflective parts of the JVMCI API to be used
       * without requiring a compiler implementation to be available.
       */
      private static class DummyCompilerFactory implements JVMCICompilerFactory, JVMCICompiler {
  
+         private final String reason;
+ 
+         DummyCompilerFactory(String reason) {
+             this.reason = reason;
+         }
+ 
          @Override
          public HotSpotCompilationRequestResult compileMethod(CompilationRequest request) {
!             throw new JVMCIError("no JVMCI compiler selected: " + reason);
          }
  
          @Override
          public String getCompilerName() {
              return "null";
*** 77,88 ****
      static JVMCICompilerFactory getCompilerFactory() {
          if (compilerFactory == null) {
              JVMCICompilerFactory factory = null;
              String compilerName = Option.Compiler.getString();
              if (compilerName != null) {
!                 if (compilerName.isEmpty() || compilerName.equals("null")) {
!                     factory = new DummyCompilerFactory();
                  } else {
                      for (JVMCICompilerFactory f : getJVMCICompilerFactories()) {
                          if (f.getCompilerName().equals(compilerName)) {
                              factory = f;
                          }
--- 83,96 ----
      static JVMCICompilerFactory getCompilerFactory() {
          if (compilerFactory == null) {
              JVMCICompilerFactory factory = null;
              String compilerName = Option.Compiler.getString();
              if (compilerName != null) {
!                 if (compilerName.isEmpty()) {
!                     factory = new DummyCompilerFactory(" empty \"\" is specified");
!                 } else if (compilerName.equals("null")) {
!                     factory = new DummyCompilerFactory("\"null\" is specified");
                  } else {
                      for (JVMCICompilerFactory f : getJVMCICompilerFactories()) {
                          if (f.getCompilerName().equals(compilerName)) {
                              factory = f;
                          }
*** 91,112 ****
                          throw new JVMCIError("JVMCI compiler '%s' not found", compilerName);
                      }
                  }
              } else {
                  // Auto select a single available compiler
                  for (JVMCICompilerFactory f : getJVMCICompilerFactories()) {
                      if (factory == null) {
                          openJVMCITo(f.getClass().getModule());
                          factory = f;
                      } else {
                          // Multiple factories seen - cancel auto selection
                          factory = null;
                          break;
                      }
                  }
                  if (factory == null) {
!                     factory = new DummyCompilerFactory();
                  }
              }
              factory.onSelection();
              compilerFactory = factory;
          }
--- 99,122 ----
                          throw new JVMCIError("JVMCI compiler '%s' not found", compilerName);
                      }
                  }
              } else {
                  // Auto select a single available compiler
+                 String reason = "default compiler is not found";
                  for (JVMCICompilerFactory f : getJVMCICompilerFactories()) {
                      if (factory == null) {
                          openJVMCITo(f.getClass().getModule());
                          factory = f;
                      } else {
                          // Multiple factories seen - cancel auto selection
+                         reason = "multiple factories seen: \"" + factory.getCompilerName() + "\" and \"" + f.getCompilerName() + "\"";
                          factory = null;
                          break;
                      }
                  }
                  if (factory == null) {
!                     factory = new DummyCompilerFactory(reason);
                  }
              }
              factory.onSelection();
              compilerFactory = factory;
          }