[llvm link] Destroy ConstantArrays in LLVMContext if they are not used.

ConstantArrays constructed during linking can cause quadratic memory
explosion. An example is the ConstantArrays constructed when linking in
GlobalVariables with appending linkage.

Releasing all unused constants can cause a 20% LTO compile-time
slowdown for a large application. So this commit releases unused ConstantArrays
only.

rdar://19040716. It reduces memory footprint from 20+G to 6+G.

llvm-svn: 226592

3 files changed, 28 insertions, 1 deletions

diff --git a/llvm/lib/IR/LLVMContextImpl.cpp b/llvm/lib/IR/LLVMContextImpl.cpp
index aa7242a..880e1c1 100644
--- a/llvm/lib/IR/LLVMContextImpl.cpp
+++ b/llvm/lib/IR/LLVMContextImpl.cpp
@@ -163,6 +163,28 @@ LLVMContextImpl::~LLVMContextImpl() {
   MDStringCache.clear();
 }
 
+void LLVMContextImpl::dropTriviallyDeadConstantArrays() {
+  bool Changed;
+  do {
+    Changed = false;
+
+    for (auto I = ArrayConstants.map_begin(), E = ArrayConstants.map_end();
+         I != E; ) {
+      auto *C = I->first;
+      I++;
+      if (C->use_empty()) {
+        Changed = true;
+        C->destroyConstant();
+      }
+    }
+
+  } while (Changed);
+}
+
+void Module::dropTriviallyDeadConstantArrays() {
+  Context.pImpl->dropTriviallyDeadConstantArrays();
+}
+
 namespace llvm {
 /// \brief Make MDOperand transparent for hashing.
 ///
diff --git a/llvm/lib/IR/LLVMContextImpl.h b/llvm/lib/IR/LLVMContextImpl.h
index 4604d9b..3b3be0f 100644
--- a/llvm/lib/IR/LLVMContextImpl.h
+++ b/llvm/lib/IR/LLVMContextImpl.h
@@ -474,6 +474,9 @@ public:
   
   LLVMContextImpl(LLVMContext &C);
   ~LLVMContextImpl();
+
+  /// Destroy the ConstantArrays if they are not used.
+  void dropTriviallyDeadConstantArrays();
 };
 
 }
diff --git a/llvm/lib/Linker/LinkModules.cpp b/llvm/lib/Linker/LinkModules.cpp
index 767d465..dc002d8 100644
--- a/llvm/lib/Linker/LinkModules.cpp
+++ b/llvm/lib/Linker/LinkModules.cpp
@@ -1721,7 +1721,9 @@ void Linker::deleteModule() {
 bool Linker::linkInModule(Module *Src) {
   ModuleLinker TheLinker(Composite, IdentifiedStructTypes, Src,
                          DiagnosticHandler);
-  return TheLinker.run();
+  bool RetCode = TheLinker.run();
+  Composite->dropTriviallyDeadConstantArrays();
+  return RetCode;
 }
 
 //===----------------------------------------------------------------------===//