[clang] Add option to disable -clear-ast-before-backend

[aeubanks]

Summary

Some downstream users have plugins that -clear-ast-before-backend may
affect. Add an option to opt out.

[dblaikie]

Sounds good - possible flag def improvement with BoolOption.

clang/include/clang/Driver/Options.td

5317–5319

Could you use/would it be handy to use a BoolOption for this?

[aeubanks]

use BoolOption
td files are very hard to use...

clang/include/clang/Driver/Options.td

def code_completion_with_fixits : Flag<["-"], "code-completion-with-fixits">,
   HelpText<"Include code completion results which require small fix-its.">,
   MarshallingInfoFlag<FrontendOpts<"CodeCompleteOpts.IncludeFixIts">>;
 def disable_free : Flag<["-"], "disable-free">,
   HelpText<"Disable freeing of memory on exit">,
   MarshallingInfoFlag<FrontendOpts<"DisableFree">>;
 def clear_ast_before_backend : Flag<["-"], "clear-ast-before-backend">,
   HelpText<"Clear the Clang AST before running backend code generation">,
   MarshallingInfoFlag<CodeGenOpts<"ClearASTBeforeBackend">>;
+def no_clear_ast_before_backend : Flag<["-"], "no-clear-ast-before-backend">,
+  HelpText<"Don't clear the Clang AST before running backend code generation">,
+  MarshallingInfoNegativeFlag<CodeGenOpts<"ClearASTBeforeBackend">>;
 def enable_noundef_analysis : Flag<["-"], "enable-noundef-analysis">, Group<f_Group>,
   HelpText<"Enable analyzing function argument and return types for mandatory definedness">,
   MarshallingInfoFlag<CodeGenOpts<"EnableNoundefAttrs">>;
 def discard_value_names : Flag<["-"], "discard-value-names">,
   HelpText<"Discard value names in LLVM IR">,
   MarshallingInfoFlag<CodeGenOpts<"DiscardValueNames">>;
 def plugin_arg : JoinedAndSeparate<["-"], "plugin-arg-">,
     MetaVarName<"<name> <arg>">,

clang/lib/CodeGen/CodeGenAction.cpp

 #include "llvm/Support/ToolOutputFile.h"
 #include "llvm/Support/YAMLTraits.h"
 #include "llvm/Transforms/IPO/Internalize.h"
 
 #include <memory>
 using namespace clang;
 using namespace llvm;
 
+#define DEBUG_TYPE "codegenaction"
+
 namespace clang {
   class BackendConsumer;
   class ClangDiagnosticHandler final : public DiagnosticHandler {
   public:
     ClangDiagnosticHandler(const CodeGenOptions &CGOpts, BackendConsumer *BCon)
         : CodeGenOpts(CGOpts), BackendCon(BCon) {}
 
     bool handleDiagnostics(const DiagnosticInfo &DI) override;
void HandleTranslationUnit(ASTContext &C) override {
           auto Loc = FD->getASTContext().getFullLoc(FD->getLocation());
           // TODO: use a fast content hash when available.
           auto NameHash = llvm::hash_value(F.getName());
           ManglingFullSourceLocs.push_back(std::make_pair(NameHash, Loc));
         }
       }
 
       if (CodeGenOpts.ClearASTBeforeBackend) {
+        LLVM_DEBUG(llvm::dbgs() << "Clearing AST...\n");
         // Access to the AST is no longer available after this.
         // Other things that the ASTContext manages are still available, e.g.
         // the SourceManager. It'd be nice if we could separate out all the
         // things in ASTContext used after this point and null out the
         // ASTContext, but too many various parts of the ASTContext are still
         // used in various parts.
         C.cleanup();
         C.getAllocator().Reset();

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

-// RUN: %clang_cc1 -clear-ast-before-backend %s -emit-obj -o /dev/null -O1
-// RUN: %clang_cc1 -clear-ast-before-backend %s -emit-obj -o /dev/null -print-stats 2>&1 | FileCheck %s
+// REQUIRES: asserts
 
-// CHECK: *** Decl Stats:
-// CHECK: {{.*}} decls total
-// CHECK: 1 Function decls
-// CHECK: Total bytes =
+// RUN: %clang_cc1 -mllvm -debug-only=codegenaction -clear-ast-before-backend %s -emit-obj -o /dev/null -O1 2>&1 | FileCheck %s --check-prefix=YES
+// RUN: %clang_cc1 -mllvm -debug-only=codegenaction -clear-ast-before-backend -no-clear-ast-before-backend %s -emit-obj -o /dev/null -O1 2>&1 | FileCheck %s --allow-empty --check-prefix=NO
+// RUN: %clang_cc1 -clear-ast-before-backend %s -emit-obj -o /dev/null -print-stats 2>&1 | FileCheck %s --check-prefix=STATS
+
+// YES: Clearing AST
+// NO-NOT: Clearing AST
+// STATS: *** Decl Stats:
+// STATS: {{.*}} decls total
+// STATS: 1 Function decls
+// STATS: Total bytes =
 
 void f() {}