From 651128f557229e79598e22102edb7fad3bf288c0 Mon Sep 17 00:00:00 2001
From: Amy Huang <akhuang@google.com>
Date: Fri, 3 Jan 2020 16:28:48 -0800
Subject: [PATCH] [DebugInfo] Add option to clang to limit debug info that is
 emitted for classes.

Summary:
This patch adds an option to limit debug info by only emitting complete class
type information when its constructor is emitted. This applies to classes
that have nontrivial user defined constructors.

I implemented the option by adding another level to `DebugInfoKind`, and
a flag `-flimit-debug-info-constructor`.

Total object file size on Windows, compiling with RelWithDebInfo:
  before: 4,257,448 kb
  after:  2,104,963 kb

And on Linux
  before: 9,225,140 kb
  after:  4,387,464 kb

According to the Windows clang.pdb files, here is a list of types that are no
longer complete with this option enabled: https://reviews.llvm.org/P8182

Reviewers: rnk, dblaikie

Subscribers: aprantl, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72427
---
 clang/lib/CodeGen/CGDebugInfo.cpp             | 17 +++++++++++
 .../CodeGenCXX/debug-info-limited-ctor.cpp    | 30 +++++++++++++++++++
 2 files changed, 47 insertions(+)
 create mode 100644 clang/test/CodeGenCXX/debug-info-limited-ctor.cpp

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 3672490fafd5..cbd524eda9d0 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -1648,6 +1648,12 @@ llvm::DISubprogram *CGDebugInfo::CreateCXXMemberFunction(
   if (CGM.getLangOpts().Optimize)
     SPFlags |= llvm::DISubprogram::SPFlagOptimized;
 
+  // In this debug mode, emit type info for a class when its constructor type
+  // info is emitted.
+  if (DebugKind == codegenoptions::DebugInfoConstructor)
+    if (const CXXConstructorDecl *CD = dyn_cast<CXXConstructorDecl>(Method))
+      completeClass(CD->getParent());
+
   llvm::DINodeArray TParamsArray = CollectFunctionTemplateParams(Method, Unit);
   llvm::DISubprogram *SP = DBuilder.createMethod(
       RecordTy, MethodName, MethodLinkageName, MethodDefUnit, MethodLine,
@@ -2211,6 +2217,17 @@ static bool shouldOmitDefinition(codegenoptions::DebugInfoKind DebugKind,
       !isClassOrMethodDLLImport(CXXDecl))
     return true;
 
+  // In constructor debug mode, only emit debug info for a class when its
+  // constructor is emitted. Skip this optimization if the class or any of
+  // its methods are marked dllimport.
+  if (DebugKind == codegenoptions::DebugInfoConstructor &&
+      !CXXDecl->isLambda() && !isClassOrMethodDLLImport(CXXDecl)) {
+    for (const auto *Ctor : CXXDecl->ctors()) {
+      if (Ctor->isUserProvided())
+        return true;
+    }
+  }
+
   TemplateSpecializationKind Spec = TSK_Undeclared;
   if (const auto *SD = dyn_cast<ClassTemplateSpecializationDecl>(RD))
     Spec = SD->getSpecializationKind();
diff --git a/clang/test/CodeGenCXX/debug-info-limited-ctor.cpp b/clang/test/CodeGenCXX/debug-info-limited-ctor.cpp
new file mode 100644
index 000000000000..321f1736391f
--- /dev/null
+++ b/clang/test/CodeGenCXX/debug-info-limited-ctor.cpp
@@ -0,0 +1,30 @@
+// RUN: %clang -cc1 -debug-info-kind=constructor -emit-llvm %s -o - | FileCheck %s
+
+// CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "A"
+// CHECK-NOT:              DIFlagFwdDecl
+// CHECK-SAME:             ){{$}}
+struct A {};
+void TestA() { A a; }
+
+// CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "B"
+// CHECK-SAME:             flags: DIFlagFwdDecl
+struct B {
+  B();
+};
+void TestB() { B b; }
+
+// CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "C"
+// CHECK-NOT:              flags: DIFlagFwdDecl
+// CHECK-SAME:             ){{$}}
+struct C {
+  C() {}
+};
+void TestC() { C c; }
+
+// CHECK: !DICompositeType(tag: DW_TAG_structure_type, name: "D"
+// CHECK-NOT:              flags: DIFlagFwdDecl
+// CHECK-SAME:             ){{$}}
+struct D {
+  D();
+};
+D::D() {}