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 using namespace llvm;
21 
22 namespace {
23   struct SimpleCaptureTracker {
24     explicit SimpleCaptureTracker(bool ReturnCaptures)
25       : ReturnCaptures(ReturnCaptures), Captured(false) {}
26 
27     void tooManyUses() { Captured = true; }
28 
29     bool shouldExplore(Use *U) { return true; }
30 
31     bool captured(Instruction *I) {
32       if (isa<ReturnInst>(I) && !ReturnCaptures)
33 	return false;
34 
35       Captured = true;
36       return true;
37     }
38 
39     bool ReturnCaptures;
40 
41     bool Captured;
42   };
43 }
44 
45 /// PointerMayBeCaptured - Return true if this pointer value may be captured
46 /// by the enclosing function (which is required to exist).  This routine can
47 /// be expensive, so consider caching the results.  The boolean ReturnCaptures
48 /// specifies whether returning the value (or part of it) from the function
49 /// counts as capturing it or not.  The boolean StoreCaptures specified whether
50 /// storing the value (or part of it) into memory anywhere automatically
51 /// counts as capturing it or not.
52 bool llvm::PointerMayBeCaptured(const Value *V,
53                                 bool ReturnCaptures, bool StoreCaptures) {
54   // TODO: If StoreCaptures is not true, we could do Fancy analysis
55   // to determine whether this store is not actually an escape point.
56   // In that case, BasicAliasAnalysis should be updated as well to
57   // take advantage of this.
58   (void)StoreCaptures;
59 
60   SimpleCaptureTracker SCT(ReturnCaptures);
61   PointerMayBeCaptured(V, SCT);
62   return SCT.Captured;
63 }
64