[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226592 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/IR/Module.h b/include/llvm/IR/Module.h
index b24023b05e4fad4cd032b043ccc2dfb6fc0142ab..62f41943be833c8ce2bcdf36ab4e8d1541ea4fa7 100644 (file)
--- a/include/llvm/IR/Module.h
+++ b/include/llvm/IR/Module.h
@@ -630,6 +630,15 @@ public:
                                                          named_metadata_end());
   }
 
+  /// Destroy ConstantArrays in LLVMContext if they are not used.
+  /// ConstantArrays constructed during linking can cause quadratic memory
+  /// explosion. Releasing all unused constants can cause a 20% LTO compile-time
+  /// slowdown for a large application.
+  ///
+  /// NOTE: Constants are currently owned by LLVMContext. This can then only
+  /// be called where all uses of the LLVMContext are understood.
+  void dropTriviallyDeadConstantArrays();
+
 /// @}
 /// @name Utility functions for printing and dumping Module objects
 /// @{

diff --git a/lib/IR/LLVMContextImpl.cpp b/lib/IR/LLVMContextImpl.cpp
index aa7242af997d7170db8d94b51c57f81ad9d8d62f..880e1c1bcad249219d79d6870d58be92dc9c6e25 100644 (file)
--- a/lib/IR/LLVMContextImpl.cpp
+++ b/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/lib/IR/LLVMContextImpl.h b/lib/IR/LLVMContextImpl.h
index 4604d9babb54024a324cd5ef41b8e7a86e13ecf2..3b3be0f9982dd3b68198c139e487411c43d398b5 100644 (file)
--- a/lib/IR/LLVMContextImpl.h
+++ b/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/lib/Linker/LinkModules.cpp b/lib/Linker/LinkModules.cpp
index 767d465d1bee6da2d78e2e17cce041231bb53ddd..dc002d8a5766fbcb6e028834bb32a39465b4628c 100644 (file)
--- a/lib/Linker/LinkModules.cpp
+++ b/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;
 }
 
 //===----------------------------------------------------------------------===//