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::VoidTy)
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 }
113