lib/Analysis/CaptureTracking.cpp

   1 //===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file contains routines that help determine which pointers are captured.
  11 // A pointer value is captured if the function makes a copy of any part of the
  12 // pointer that outlives the call.  Not being captured means, more or less, that
  13 // the pointer is only dereferenced and not stored in a global.  Returning part
  14 // of the pointer as the function return value may or may not count as capturing
  15 // the pointer, depending on the context.
  16 //
  17 //===----------------------------------------------------------------------===//
  18
  19 #include "llvm/Analysis/CaptureTracking.h"
  20 #include "llvm/Instructions.h"
  21 #include "llvm/Value.h"
  22 #include "llvm/ADT/SmallSet.h"
  23 #include "llvm/ADT/SmallVector.h"
  24 #include "llvm/Support/CallSite.h"
  25 using namespace llvm;
  26
  27 /// PointerMayBeCaptured - Return true if this pointer value may be captured
  28 /// by the enclosing function (which is required to exist).  This routine can
  29 /// be expensive, so consider caching the results.  The boolean ReturnCaptures
  30 /// specifies whether returning the value (or part of it) from the function
  31 /// counts as capturing it or not.
  32 bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
  33   assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
  34   SmallVector<Use*, 16> Worklist;
  35   SmallSet<Use*, 16> Visited;
  36
  37   for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
  38        UI != UE; ++UI) {
  39     Use *U = &UI.getUse();
  40     Visited.insert(U);
  41     Worklist.push_back(U);
  42   }
  43
  44   while (!Worklist.empty()) {
  45     Use *U = Worklist.pop_back_val();
  46     Instruction *I = cast<Instruction>(U->getUser());
  47     V = U->get();
  48
  49     switch (I->getOpcode()) {
  50     case Instruction::Call:
  51     case Instruction::Invoke: {
  52       CallSite CS = CallSite::get(I);
  53       // Not captured if the callee is readonly, doesn't return a copy through
  54       // its return value and doesn't unwind (a readonly function can leak bits
  55       // by throwing an exception or not depending on the input value).
  56       if (CS.onlyReadsMemory() && CS.doesNotThrow() &&
  57           I->getType() == Type::getVoidTy(V->getContext()))
  58         break;
  59
  60       // Not captured if only passed via 'nocapture' arguments.  Note that
  61       // calling a function pointer does not in itself cause the pointer to
  62       // be captured.  This is a subtle point considering that (for example)
  63       // the callee might return its own address.  It is analogous to saying
  64       // that loading a value from a pointer does not cause the pointer to be
  65       // captured, even though the loaded value might be the pointer itself
  66       // (think of self-referential objects).
  67       CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
  68       for (CallSite::arg_iterator A = B; A != E; ++A)
  69         if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
  70           // The parameter is not marked 'nocapture' - captured.
  71           return true;
  72       // Only passed via 'nocapture' arguments, or is the called function - not
  73       // captured.
  74       break;
  75     }
  76     case Instruction::Free:
  77       // Freeing a pointer does not cause it to be captured.
  78       break;
  79     case Instruction::Load:
  80       // Loading from a pointer does not cause it to be captured.
  81       break;
  82     case Instruction::Ret:
  83       if (ReturnCaptures)
  84         return true;
  85       break;
  86     case Instruction::Store:
  87       if (V == I->getOperand(0))
  88         // Stored the pointer - it may be captured.
  89         return true;
  90       // Storing to the pointee does not cause the pointer to be captured.
  91       break;
  92     case Instruction::BitCast:
  93     case Instruction::GetElementPtr:
  94     case Instruction::PHI:
  95     case Instruction::Select:
  96       // The original value is not captured via this if the new value isn't.
  97       for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
  98            UI != UE; ++UI) {
  99         Use *U = &UI.getUse();
 100         if (Visited.insert(U))
 101           Worklist.push_back(U);
 102       }
 103       break;
 104     default:
 105       // Something else - be conservative and say it is captured.
 106       return true;
 107     }
 108   }
 109
 110   // All uses examined - not captured.
 111   return false;
 112 }