LibCallAliasAnalysis.cpp revision 029840c93521f5c54380e037a66216c8227ad1e1 
1//===- LibCallAliasAnalysis.cpp - Implement AliasAnalysis for libcalls ----===//
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 implements the LibCallAliasAnalysis class.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Analysis/Passes.h"
15#include "llvm/Analysis/LibCallSemantics.h"
16#include "llvm/Function.h"
17#include "llvm/Pass.h"
18#include "llvm/Target/TargetData.h"
19using namespace llvm;
20
21namespace llvm {
22  /// LibCallAliasAnalysis - Alias analysis driven from LibCallInfo.
23  struct LibCallAliasAnalysis : public FunctionPass, AliasAnalysis {
24    static char ID; // Class identification
25
26    LibCallInfo *LCI;
27
28    LibCallAliasAnalysis(LibCallInfo *LC = 0)
29      : FunctionPass((intptr_t)&ID), LCI(LC) {
30    }
31    ~LibCallAliasAnalysis() {
32      delete LCI;
33    }
34
35    ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
36    ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
37      // TODO: Could compare two direct calls against each other if we cared to.
38      return AliasAnalysis::getModRefInfo(CS1,CS2);
39    }
40
41    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
42      AliasAnalysis::getAnalysisUsage(AU);
43      AU.addRequired<TargetData>();
44      AU.setPreservesAll();                         // Does not transform code
45    }
46
47    virtual bool runOnFunction(Function &F) {
48      InitializeAliasAnalysis(this);                 // set up super class
49      return false;
50    }
51
52    /// hasNoModRefInfoForCalls - We can provide mod/ref information against
53    /// non-escaping allocations.
54    virtual bool hasNoModRefInfoForCalls() const { return false; }
55  private:
56    ModRefResult AnalyzeLibCallDetails(const LibCallFunctionInfo *FI,
57                                       CallSite CS, Value *P, unsigned Size);
58  };
59
60  // Register this pass...
61  char LibCallAliasAnalysis::ID = 0;
62  RegisterPass<LibCallAliasAnalysis>
63  X("libcall-aa", "LibCall Alias Analysis", false, true);
64
65  // Declare that we implement the AliasAnalysis interface
66  RegisterAnalysisGroup<AliasAnalysis> Y(X);
67}  // End of llvm namespace
68
69FunctionPass *llvm::createLibCallAliasAnalysisPass(LibCallInfo *LCI) {
70  return new LibCallAliasAnalysis(LCI);
71}
72
73
74/// AnalyzeLibCallDetails - Given a call to a function with the specified
75/// LibCallFunctionInfo, see if we can improve the mod/ref footprint of the call
76/// vs the specified pointer/size.
77AliasAnalysis::ModRefResult
78LibCallAliasAnalysis::AnalyzeLibCallDetails(const LibCallFunctionInfo *FI,
79                                            CallSite CS, Value *P,
80                                            unsigned Size) {
81  // If we have a function, check to see what kind of mod/ref effects it
82  // has.  Start by including any info globally known about the function.
83  AliasAnalysis::ModRefResult MRInfo = FI->UniversalBehavior;
84  if (MRInfo == NoModRef) return MRInfo;
85
86  // If that didn't tell us that the function is 'readnone', check to see
87  // if we have detailed info and if 'P' is any of the locations we know
88  // about.
89  const LibCallFunctionInfo::LocationMRInfo *Details = FI->LocationDetails;
90  if (Details == 0)
91    return MRInfo;
92
93  // If the details array is of the 'DoesNot' kind, we only know something if
94  // the pointer is a match for one of the locations in 'Details'.  If we find a
95  // match, we can prove some interactions cannot happen.
96  //
97  if (FI->DetailsType == LibCallFunctionInfo::DoesNot) {
98    // Find out if the pointer refers to a known location.
99    for (unsigned i = 0; Details[i].LocationID != ~0U; ++i) {
100      const LibCallLocationInfo &Loc =
101      LCI->getLocationInfo(Details[i].LocationID);
102      LibCallLocationInfo::LocResult Res = Loc.isLocation(CS, P, Size);
103      if (Res != LibCallLocationInfo::Yes) continue;
104
105      // If we find a match against a location that we 'do not' interact with,
106      // learn this info into MRInfo.
107      return ModRefResult(MRInfo & ~Details[i].MRInfo);
108    }
109    return MRInfo;
110  }
111
112  // If the details are of the 'DoesOnly' sort, we know something if the pointer
113  // is a match for one of the locations in 'Details'.  Also, if we can prove
114  // that the pointers is *not* one of the locations in 'Details', we know that
115  // the call is NoModRef.
116  assert(FI->DetailsType == LibCallFunctionInfo::DoesOnly);
117
118  // Find out if the pointer refers to a known location.
119  bool NoneMatch = true;
120  for (unsigned i = 0; Details[i].LocationID != ~0U; ++i) {
121    const LibCallLocationInfo &Loc =
122    LCI->getLocationInfo(Details[i].LocationID);
123    LibCallLocationInfo::LocResult Res = Loc.isLocation(CS, P, Size);
124    if (Res == LibCallLocationInfo::No) continue;
125
126    // If we don't know if this pointer points to the location, then we have to
127    // assume it might alias in some case.
128    if (Res == LibCallLocationInfo::Unknown) {
129      NoneMatch = false;
130      continue;
131    }
132
133    // If we know that this pointer definitely is pointing into the location,
134    // merge in this information.
135    return ModRefResult(MRInfo & Details[i].MRInfo);
136  }
137
138  // If we found that the pointer is guaranteed to not match any of the
139  // locations in our 'DoesOnly' rule, then we know that the pointer must point
140  // to some other location.  Since the libcall doesn't mod/ref any other
141  // locations, return NoModRef.
142  if (NoneMatch)
143    return NoModRef;
144
145  // Otherwise, return any other info gained so far.
146  return MRInfo;
147}
148
149// getModRefInfo - Check to see if the specified callsite can clobber the
150// specified memory object.
151//
152AliasAnalysis::ModRefResult
153LibCallAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
154  ModRefResult MRInfo = ModRef;
155
156  // If this is a direct call to a function that LCI knows about, get the
157  // information about the runtime function.
158  if (Function *F = CS.getCalledFunction()) {
159    if (LCI && F->isDeclaration()) {
160      if (const LibCallFunctionInfo *FI = LCI->getFunctionInfo(F)) {
161        MRInfo = ModRefResult(MRInfo & AnalyzeLibCallDetails(FI, CS, P, Size));
162        if (MRInfo == NoModRef) return NoModRef;
163      }
164    }
165  }
166
167  // The AliasAnalysis base class has some smarts, lets use them.
168  return (ModRefResult)(MRInfo | AliasAnalysis::getModRefInfo(CS, P, Size));
169}
170