CaptureTracking.cpp revision 8556d2a7f155c7edfaf454a3acda8ce28863c5e4 
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"
25using 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.
32bool 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 and doesn't return a copy
54      // through its return value.
55      if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
56        break;
57
58      // Not captured if only passed via 'nocapture' arguments.  Note that
59      // calling a function pointer does not in itself cause the pointer to
60      // be captured.  This is a subtle point considering that (for example)
61      // the callee might return its own address.  It is analogous to saying
62      // that loading a value from a pointer does not cause the pointer to be
63      // captured, even though the loaded value might be the pointer itself
64      // (think of self-referential objects).
65      CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
66      for (CallSite::arg_iterator A = B; A != E; ++A)
67        if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
68          // The parameter is not marked 'nocapture' - captured.
69          return true;
70      // Only passed via 'nocapture' arguments, or is the called function - not
71      // captured.
72      break;
73    }
74    case Instruction::Free:
75      // Freeing a pointer does not cause it to be captured.
76      break;
77    case Instruction::Load:
78      // Loading from a pointer does not cause it to be captured.
79      break;
80    case Instruction::Ret:
81      if (ReturnCaptures)
82        return true;
83      break;
84    case Instruction::Store:
85      if (V == I->getOperand(0))
86        // Stored the pointer - it may be captured.
87        return true;
88      // Storing to the pointee does not cause the pointer to be captured.
89      break;
90    case Instruction::BitCast:
91    case Instruction::GetElementPtr:
92    case Instruction::PHI:
93    case Instruction::Select:
94      // The original value is not captured via this if the new value isn't.
95      for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
96           UI != UE; ++UI) {
97        Use *U = &UI.getUse();
98        if (Visited.insert(U))
99          Worklist.push_back(U);
100      }
101      break;
102    default:
103      // Something else - be conservative and say it is captured.
104      return true;
105    }
106  }
107
108  // All uses examined - not captured.
109  return false;
110}
111