Add a basic intra-procedural escape analysis. This hasn't be extensively tested yet, but feedback is welcome.

llvm-svn: 57342

llvm/include/llvm/Analysis/EscapeAnalysis.h

@@ -0,0 +1,59 @@
+//===------------- EscapeAnalysis.h - Pointer escape analysis -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the interface for the pointer escape analysis.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ANALYSIS_LOOPVR_H
+#define LLVM_ANALYSIS_LOOPVR_H
+
+#include "llvm/Pass.h"
+#include "llvm/Instructions.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Target/TargetData.h"
+#include <set>
+#include <vector>
+
+namespace llvm {
+
+/// EscapeAnalysis - This class determines whether an allocation (a MallocInst 
+/// or an AllocaInst) can escape from the current function.  It performs some
+/// precomputation, with the rest of the work happening on-demand.
+class EscapeAnalysis : public FunctionPass {
+private:
+  std::set<Instruction*> EscapePoints;
+
+public:
+  static char ID; // Class identification, replacement for typeinfo
+
+  EscapeAnalysis() : FunctionPass(intptr_t(&ID)) {}
+
+  bool runOnFunction(Function &F);
+  
+  void releaseMemory() {
+    EscapePoints.clear();
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const {
+    AU.addRequiredTransitive<TargetData>();
+    AU.addRequiredTransitive<AliasAnalysis>();
+    AU.setPreservesAll();
+  }
+
+  //===---------------------------------------------------------------------
+  // Client API
+
+  /// escapes - returns true if the AllocationInst can escape.
+  bool escapes(AllocationInst* A);
+};
+
+} // end llvm namespace
+
+#endif

llvm/lib/Analysis/EscapeAnalysis.cpp

@@ -0,0 +1,131 @@
+//===------------- EscapeAnalysis.h - Pointer escape analysis -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides the implementation of the pointer escape analysis.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "escape-analysis"
+#include "llvm/Analysis/EscapeAnalysis.h"
+#include "llvm/Module.h"
+#include "llvm/Support/InstIterator.h"
+#include "llvm/ADT/SmallPtrSet.h"
+using namespace llvm;
+
+char EscapeAnalysis::ID = 0;
+static RegisterPass<EscapeAnalysis> X("escape-analysis",
+                                      "Pointer Escape Analysis", true, true);
+
+
+/// runOnFunction - Precomputation for escape analysis.  This collects all know
+/// "escape points" in the def-use graph of the function.  These are 
+/// instructions which allow their inputs to escape from the current function.  
+bool EscapeAnalysis::runOnFunction(Function& F) {
+  EscapePoints.clear();
+  
+  TargetData& TD = getAnalysis<TargetData>();
+  AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
+  Module* M = F.getParent();
+  
+  // Walk through all instructions in the function, identifying those that
+  // may allow their inputs to escape.
+  for(inst_iterator II = inst_begin(F), IE = inst_end(F); II != IE; ++II) {
+    Instruction* I = &*II;
+    
+    // The most obvious case is stores.  Any store that may write to global
+    // memory or to a function argument potentially allows its input to escape.
+    if (StoreInst* S = dyn_cast<StoreInst>(I)) {
+      const Type* StoreType = S->getOperand(0)->getType();
+      unsigned StoreSize = TD.getTypeStoreSize(StoreType);
+      Value* Pointer = S->getPointerOperand();
+      
+      bool inserted = false;
+      for (Function::arg_iterator AI = F.arg_begin(), AE = F.arg_end();
+           AI != AE; ++AI) {
+        AliasAnalysis::AliasResult R = AA.alias(Pointer, StoreSize, AI, ~0UL);
+        if (R != AliasAnalysis::NoAlias) {
+          EscapePoints.insert(S);
+          inserted = true;
+          break;
+        }
+      }
+      
+      if (inserted)
+        continue;
+      
+      for (Module::global_iterator GI = M->global_begin(), GE = M->global_end();
+           GI != GE; ++GI) {
+        AliasAnalysis::AliasResult R = AA.alias(Pointer, StoreSize, GI, ~0UL);
+        if (R != AliasAnalysis::NoAlias) {
+          EscapePoints.insert(S);
+          break;
+        }
+      }
+    
+    // Calls and invokes potentially allow their parameters to escape.
+    // FIXME: This can and should be refined.  Intrinsics have known escape
+    // behavior, and alias analysis may be able to tell us more about callees.
+    } else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
+      EscapePoints.insert(I);
+    
+    // Returns allow the return value to escape.  This is mostly important
+    // for malloc to alloca promotion.
+    } else if (isa<ReturnInst>(I)) {
+      EscapePoints.insert(I);
+    }
+    
+    // FIXME: Are there any other possible escape points?
+  }
+  
+  return false;
+}
+
+/// escapes - Determines whether the passed allocation can escape from the 
+/// current function.  It does this by using a simple worklist algorithm to
+/// search for a path in the def-use graph from the allocation to an
+/// escape point.
+/// FIXME: Once we've discovered a path, it would be a good idea to memoize it,
+/// and all of its subpaths, to amortize the cost of future queries.
+bool EscapeAnalysis::escapes(AllocationInst* A) {
+  std::vector<Value*> worklist;
+  worklist.push_back(A);
+  
+  SmallPtrSet<Value*, 8> visited;
+  while (!worklist.empty()) {
+    Value* curr = worklist.back();
+    worklist.pop_back();
+    
+    visited.insert(curr);
+    
+    if (Instruction* CurrInst = dyn_cast<Instruction>(curr))
+      if (EscapePoints.count(CurrInst))
+        return true;
+    
+    for (Instruction::use_iterator UI = curr->use_begin(), UE = curr->use_end();
+         UI != UE; ++UI)
+      if (Instruction* U = dyn_cast<Instruction>(UI))
+        if (!visited.count(U))
+          if (StoreInst* S = dyn_cast<StoreInst>(U)) {
+            // We know this must be an instruction, because constant gep's would
+            // have been found to alias a global, so stores to them would have
+            // been in EscapePoints.
+            worklist.push_back(cast<Instruction>(S->getPointerOperand()));
+          } else if (isa<BranchInst>(U) || isa<SwitchInst>(U)) {
+            // Because branches on the pointer value can hide data dependencies,
+            // we need to track values that were generated by branching on the
+            // pointer (or some derived value).  To do that, we push the block,
+            // whose uses will be the PHINodes that generate information based
+            // one it.
+            worklist.push_back(U->getParent());
+          } else
+            worklist.push_back(U);
+  }
+  
+  return false;
+}