[clang] Don't clear AST if we have consumers running after the main action

[aeubanks]

Summary

Downstream users may have Clang plugins. By default these plugins run
after the main action if they are specified on the command line.

Since these plugins are ASTConsumers, presumably they inspect the AST.
So we shouldn't clear it if any plugins run after the main action.

[aeubanks]

update

[dblaikie]

clang/test/Misc/clear-ast-before-backend-plugins.c

4–9

Can you test that the "consumers running after the main action" are actually running here in a relatively lightweight manner? (ie: in a way that depends on as little details of their behavior as possible, just that they ran)

[hans]

lgtm

[aeubanks]

clang/test/Misc/clear-ast-before-backend-plugins.c

4–9

I don't think so, the only observable behavior is anything the plugin does, and this plugin only prints things. Clang on its own doesn't do much debug logging.

[dblaikie]

clang/test/Misc/clear-ast-before-backend-plugins.c

4–9

Sorry, I figured it'd probably be something like that - I just meant testing a fairly vague part of that plugin-specific output (seems you chose a plugin with pretty simple/stable output, so it wouldn't be high maintenance to depend on some parts of its output).

[aeubanks]

clang/test/Misc/clear-ast-before-backend-plugins.c

4–9

Ah I can do that.

clang/lib/Frontend/FrontendAction.cpp

if ((ActionType == PluginASTAction::AddBeforeMainAction ||
       } else {
         AfterConsumers.push_back(std::move(PluginConsumer));
       }
     }
   }
 
   // Add to Consumers the main consumer, then all the plugins that go after it
   Consumers.push_back(std::move(Consumer));
-  for (auto &C : AfterConsumers) {
+  if (!AfterConsumers.empty()) {
+    // If we have plugins after the main consumer, which may be the codegen
+    // action, they likely will need the ASTContext, so don't clear it in the
+    // codegen action.
+    CI.getCodeGenOpts().ClearASTBeforeBackend = false;
+    for (auto &C : AfterConsumers)
       Consumers.push_back(std::move(C));
   }
 
   return std::make_unique<MultiplexConsumer>(std::move(Consumers));
 }
 
 /// For preprocessed files, if the first line is the linemarker and specifies
 /// the original source file name, use that name as the input file name.
 /// Returns the location of the first token after the line marker directive.

clang/test/Misc/clear-ast-before-backend-plugins.c

+// REQUIRES: plugins, examples, asserts
+
+// RUN: %clang_cc1 -mllvm -debug-only=codegenaction -clear-ast-before-backend -emit-obj -o /dev/null -load %llvmshlibdir/PrintFunctionNames%pluginext %s 2>&1 | FileCheck %s --check-prefix=YES
+// YES: Clearing AST
+
+// RUN: %clang_cc1 -mllvm -debug-only=codegenaction -clear-ast-before-backend -emit-obj -o /dev/null -load %llvmshlibdir/PrintFunctionNames%pluginext -add-plugin print-fns -plugin-arg-print-fns help %s 2>&1 | FileCheck %s --check-prefix=NO
+// NO-NOT: Clearing AST
+
+void f() {}