Refactored out pass ObjCARCAPElim from ObjCARCOpts.cpp => ObjCARCAPElim.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173654 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 179 insertions, 0 deletions

diff --git a/lib/Transforms/ObjCARC/ObjCARCAPElim.cpp b/lib/Transforms/ObjCARC/ObjCARCAPElim.cpp
new file mode 100644
index 0000000000..c849bcd686
--- /dev/null
+++ b/lib/Transforms/ObjCARC/ObjCARCAPElim.cpp
@@ -0,0 +1,179 @@
+//===- ObjCARCOpts.cpp - ObjC ARC Optimization ----------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+/// \file
+/// This file defines ObjC ARC optimizations. ARC stands for Automatic
+/// Reference Counting and is a system for managing reference counts for objects
+/// in Objective C.
+///
+/// The optimizations performed include elimination of redundant, partially
+/// redundant, and inconsequential reference count operations, elimination of
+/// redundant weak pointer operations, pattern-matching and replacement of
+/// low-level operations into higher-level operations, and numerous minor
+/// simplifications.
+///
+/// This file also defines a simple ARC-aware AliasAnalysis.
+///
+/// WARNING: This file knows about certain library functions. It recognizes them
+/// by name, and hardwires knowledge of their semantics.
+///
+/// WARNING: This file knows about how certain Objective-C library functions are
+/// used. Naive LLVM IR transformations which would otherwise be
+/// behavior-preserving may break these assumptions.
+///
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "objc-arc-ap-elim"
+#include "ObjCARC.h"
+
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/IR/Constants.h"
+
+using namespace llvm;
+using namespace llvm::objcarc;
+
+namespace {
+  /// \brief Autorelease pool elimination.
+  class ObjCARCAPElim : public ModulePass {
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
+    virtual bool runOnModule(Module &M);
+
+    static bool MayAutorelease(ImmutableCallSite CS, unsigned Depth = 0);
+    static bool OptimizeBB(BasicBlock *BB);
+
+  public:
+    static char ID;
+    ObjCARCAPElim() : ModulePass(ID) {
+      initializeObjCARCAPElimPass(*PassRegistry::getPassRegistry());
+    }
+  };
+}
+
+char ObjCARCAPElim::ID = 0;
+INITIALIZE_PASS(ObjCARCAPElim,
+                "objc-arc-apelim",
+                "ObjC ARC autorelease pool elimination",
+                false, false)
+
+Pass *llvm::createObjCARCAPElimPass() {
+  return new ObjCARCAPElim();
+}
+
+void ObjCARCAPElim::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.setPreservesCFG();
+}
+
+/// Interprocedurally determine if calls made by the given call site can
+/// possibly produce autoreleases.
+bool ObjCARCAPElim::MayAutorelease(ImmutableCallSite CS, unsigned Depth) {
+  if (const Function *Callee = CS.getCalledFunction()) {
+    if (Callee->isDeclaration() || Callee->mayBeOverridden())
+      return true;
+    for (Function::const_iterator I = Callee->begin(), E = Callee->end();
+         I != E; ++I) {
+      const BasicBlock *BB = I;
+      for (BasicBlock::const_iterator J = BB->begin(), F = BB->end();
+           J != F; ++J)
+        if (ImmutableCallSite JCS = ImmutableCallSite(J))
+          // This recursion depth limit is arbitrary. It's just great
+          // enough to cover known interesting testcases.
+          if (Depth < 3 &&
+              !JCS.onlyReadsMemory() &&
+              MayAutorelease(JCS, Depth + 1))
+            return true;
+    }
+    return false;
+  }
+
+  return true;
+}
+
+bool ObjCARCAPElim::OptimizeBB(BasicBlock *BB) {
+  bool Changed = false;
+
+  Instruction *Push = 0;
+  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
+    Instruction *Inst = I++;
+    switch (GetBasicInstructionClass(Inst)) {
+    case IC_AutoreleasepoolPush:
+      Push = Inst;
+      break;
+    case IC_AutoreleasepoolPop:
+      // If this pop matches a push and nothing in between can autorelease,
+      // zap the pair.
+      if (Push && cast<CallInst>(Inst)->getArgOperand(0) == Push) {
+        Changed = true;
+        DEBUG(dbgs() << "ObjCARCAPElim::OptimizeBB: Zapping push pop "
+                        "autorelease pair:\n"
+                        "                           Pop: " << *Inst << "\n"
+                     << "                           Push: " << *Push << "\n");
+        Inst->eraseFromParent();
+        Push->eraseFromParent();
+      }
+      Push = 0;
+      break;
+    case IC_CallOrUser:
+      if (MayAutorelease(ImmutableCallSite(Inst)))
+        Push = 0;
+      break;
+    default:
+      break;
+    }
+  }
+
+  return Changed;
+}
+
+bool ObjCARCAPElim::runOnModule(Module &M) {
+  if (!EnableARCOpts)
+    return false;
+
+  // If nothing in the Module uses ARC, don't do anything.
+  if (!ModuleHasARC(M))
+    return false;
+
+  // Find the llvm.global_ctors variable, as the first step in
+  // identifying the global constructors. In theory, unnecessary autorelease
+  // pools could occur anywhere, but in practice it's pretty rare. Global
+  // ctors are a place where autorelease pools get inserted automatically,
+  // so it's pretty common for them to be unnecessary, and it's pretty
+  // profitable to eliminate them.
+  GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");
+  if (!GV)
+    return false;
+
+  assert(GV->hasDefinitiveInitializer() &&
+         "llvm.global_ctors is uncooperative!");
+
+  bool Changed = false;
+
+  // Dig the constructor functions out of GV's initializer.
+  ConstantArray *Init = cast<ConstantArray>(GV->getInitializer());
+  for (User::op_iterator OI = Init->op_begin(), OE = Init->op_end();
+       OI != OE; ++OI) {
+    Value *Op = *OI;
+    // llvm.global_ctors is an array of pairs where the second members
+    // are constructor functions.
+    Function *F = dyn_cast<Function>(cast<ConstantStruct>(Op)->getOperand(1));
+    // If the user used a constructor function with the wrong signature and
+    // it got bitcasted or whatever, look the other way.
+    if (!F)
+      continue;
+    // Only look at function definitions.
+    if (F->isDeclaration())
+      continue;
+    // Only look at functions with one basic block.
+    if (llvm::next(F->begin()) != F->end())
+      continue;
+    // Ok, a single-block constructor function definition. Try to optimize it.
+    Changed |= OptimizeBB(F->begin());
+  }
+
+  return Changed;
+}
+