From 909a321521e871254d5a1ee2d782eca48134f96a Mon Sep 17 00:00:00 2001
From: Frederic Riss <friss@apple.com>
Date: Thu, 9 Feb 2017 19:41:55 +0000
Subject: [PATCH] [dsymutil] Fix handling of empty CUs in LTO links.

r288399 introduced the DIEUnit class, and in the process broke
the corner case where dsymutil generates an empty CU during an
LTO link. This restores the logic and adds a test for the corner
case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294618 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../tools/dsymutil/X86/generate-empty-CU.test | 33 ++++++++++++++++++
 tools/dsymutil/DwarfLinker.cpp                | 34 +++++++++++--------
 2 files changed, 53 insertions(+), 14 deletions(-)
 create mode 100644 test/tools/dsymutil/X86/generate-empty-CU.test

diff --git a/test/tools/dsymutil/X86/generate-empty-CU.test b/test/tools/dsymutil/X86/generate-empty-CU.test
new file mode 100644
index 00000000000..233611460b6
--- /dev/null
+++ b/test/tools/dsymutil/X86/generate-empty-CU.test
@@ -0,0 +1,33 @@
+# RUN: llvm-dsymutil -f -o - -oso-prepend-path=%p/.. -y %s | llvm-dwarfdump - | FileCheck %s
+
+# This test on links the Dwarf for an LTO binary and on purpose doesn't retain
+# any symbol in the second CU out of 3. This is the only case where dsymutil
+# will generate an empty CU and it requires special handling.
+
+---
+triple:          'x86_64-apple-darwin'
+objects:
+  - filename:        /Inputs/basic-lto.macho.x86_64.o
+    timestamp:       1417654896
+    symbols:
+      - { sym: _main, objAddr: 0x0000000000000000, binAddr: 0x0000000100000F40, size: 0x00000010 }
+      - { sym: _bar, objAddr: 0x0000000000000050, binAddr: 0x0000000100000F90, size: 0x00000024 }
+...
+
+.debug_info contents:
+CHECK: Compile Unit: length = 0x0000007d version = 0x0002 abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x00000081)
+
+CHECK: DW_TAG_compile_unit
+CHECK:        DW_AT_name {{.*}} "basic1.c"
+CHECK:   DW_TAG_subprogram
+                DW_AT_name {{.*}} "main"
+
+CHECK: Compile Unit: length = 0x00000007 version = 0x0002 abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x0000008c)
+
+CHECK: Compile Unit: length = 0x00000089 version = 0x0002 abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x00000119)
+
+CHECK: DW_TAG_compile_unit
+CHECK:        DW_AT_name {{.*}} "basic3.c"
+
+CHECK:   DW_TAG_subprogram [7] *
+CHECK:          DW_AT_name {{.*}} = "bar"
diff --git a/tools/dsymutil/DwarfLinker.cpp b/tools/dsymutil/DwarfLinker.cpp
index 2fe4f29d408..35cefaa69e8 100644
--- a/tools/dsymutil/DwarfLinker.cpp
+++ b/tools/dsymutil/DwarfLinker.cpp
@@ -197,11 +197,8 @@ public:
 
   CompileUnit(DWARFUnit &OrigUnit, unsigned ID, bool CanUseODR,
               StringRef ClangModuleName)
-      : OrigUnit(OrigUnit), ID(ID), NewUnit(OrigUnit.getVersion(),
-                                            OrigUnit.getAddressByteSize(),
-                                            OrigUnit.getUnitDIE().getTag()),
-          LowPc(UINT64_MAX), HighPc(0), RangeAlloc(), Ranges(RangeAlloc),
-          ClangModuleName(ClangModuleName) {
+      : OrigUnit(OrigUnit), ID(ID), LowPc(UINT64_MAX), HighPc(0), RangeAlloc(),
+        Ranges(RangeAlloc), ClangModuleName(ClangModuleName) {
     Info.resize(OrigUnit.getNumDIEs());
 
     auto CUDie = OrigUnit.getUnitDIE(false);
@@ -219,8 +216,15 @@ public:
 
   unsigned getUniqueID() const { return ID; }
 
+  void createOutputDIE() {
+    NewUnit.emplace(OrigUnit.getVersion(), OrigUnit.getAddressByteSize(),
+                    OrigUnit.getUnitDIE().getTag());
+  }
+
   DIE *getOutputUnitDIE() const {
-    return &const_cast<DIEUnit &>(NewUnit).getUnitDie();
+    if (NewUnit)
+      return &const_cast<DIEUnit &>(*NewUnit).getUnitDie();
+    return nullptr;
   }
 
   bool hasODR() const { return HasODR; }
@@ -329,7 +333,7 @@ private:
   DWARFUnit &OrigUnit;
   unsigned ID;
   std::vector<DIEInfo> Info; ///< DIE info indexed by DIE index.
-  DIEUnit NewUnit;
+  Optional<DIEUnit> NewUnit;
 
   uint64_t StartOffset;
   uint64_t NextUnitOffset;
@@ -397,7 +401,8 @@ uint64_t CompileUnit::computeNextUnitOffset() {
   // The root DIE might be null, meaning that the Unit had nothing to
   // contribute to the linked output. In that case, we will emit the
   // unit header without any actual DIE.
-  NextUnitOffset += NewUnit.getUnitDie().getSize();
+  if (NewUnit)
+    NextUnitOffset += NewUnit->getUnitDie().getSize();
   return NextUnitOffset;
 }
 
@@ -3357,12 +3362,13 @@ void DwarfLinker::DIECloner::cloneAllCompileUnits(
   for (auto &CurrentUnit : CompileUnits) {
     auto InputDIE = CurrentUnit->getOrigUnit().getUnitDIE();
     CurrentUnit->setStartOffset(Linker.OutputDebugInfoSize);
-    // Clonse the InputDIE into your Unit DIE in our compile unit since it
-    // already has a DIE inside of it.
-    if (!cloneDIE(InputDIE, *CurrentUnit, 0 /* PC offset */,
-                  11 /* Unit Header size */, 0,
-                  CurrentUnit->getOutputUnitDIE()))
-      continue;
+    if (CurrentUnit->getInfo(0).Keep) {
+      // Clone the InputDIE into your Unit DIE in our compile unit since it
+      // already has a DIE inside of it.
+      CurrentUnit->createOutputDIE();
+      cloneDIE(InputDIE, *CurrentUnit, 0 /* PC offset */,
+               11 /* Unit Header size */, 0, CurrentUnit->getOutputUnitDIE());
+    }
     Linker.OutputDebugInfoSize = CurrentUnit->computeNextUnitOffset();
     if (Linker.Options.NoOutput)
       continue;
-- 
2.50.1