BasicAliasAnalysis and FunctionAttrs were both

doing very similar pointer capture analysis.
Factor out the common logic.  The new version
is from FunctionAttrs since it does a better
job than the version in BasicAliasAnalysis


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62461 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 113 insertions, 52 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 9cefdcf40d..82514400b8 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -14,6 +14,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CaptureTracking.h"
 #include "llvm/Analysis/Passes.h"
 #include "llvm/Constants.h"
 #include "llvm/DerivedTypes.h"
@@ -35,56 +36,6 @@ using namespace llvm;
 // Useful predicates
 //===----------------------------------------------------------------------===//
 
-// Determine if a value escapes from the function it is contained in (being
-// returned by the function does not count as escaping here).  If a value local
-// to the function does not escape, there is no way another function can mod/ref
-// it.  We do this by looking at its uses and determining if they can escape
-// (recursively).
-static bool AddressMightEscape(const Value *V) {
-  for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end();
-       UI != E; ++UI) {
-    const Instruction *I = cast<Instruction>(*UI);
-    switch (I->getOpcode()) {
-    case Instruction::Load: 
-      break; //next use.
-    case Instruction::Store:
-      if (I->getOperand(0) == V)
-        return true; // Escapes if the pointer is stored.
-      break; // next use.
-    case Instruction::GetElementPtr:
-      if (AddressMightEscape(I))
-        return true;
-      break; // next use.
-    case Instruction::BitCast:
-      if (AddressMightEscape(I))
-        return true;
-      break; // next use
-    case Instruction::Ret:
-      // If returned, the address will escape to calling functions, but no
-      // callees could modify it.
-      break; // next use
-    case Instruction::Call:
-      // If the argument to the call has the nocapture attribute, then the call
-      // may store or load to the pointer, but it cannot escape.
-      if (cast<CallInst>(I)->paramHasAttr(UI.getOperandNo(), 
-                                          Attribute::NoCapture))
-        continue;
-      return true;
-    case Instruction::Invoke:
-      // If the argument to the call has the nocapture attribute, then the call
-      // may store or load to the pointer, but it cannot escape.
-      // Do compensate for the two BB operands, i.e. Arg1 is at index 3!
-      if (cast<InvokeInst>(I)->paramHasAttr(UI.getOperandNo()-2,
-                                            Attribute::NoCapture))
-        continue;
-      return true;
-    default:
-      return true;
-    }
-  }
-  return false;
-}
-
 static const User *isGEP(const Value *V) {
   if (isa<GetElementPtrInst>(V) ||
       (isa<ConstantExpr>(V) &&
@@ -158,7 +109,7 @@ static bool isKnownNonNull(const Value *V) {
 static bool isNonEscapingLocalObject(const Value *V) {
   // If this is a local allocation, check to see if it escapes.
   if (isa<AllocationInst>(V) || isNoAliasCall(V))
-    return !AddressMightEscape(V);
+    return !PointerMayBeCaptured(V, false);
 
   // If this is an argument that corresponds to a byval or noalias argument,
   // then it has not escaped before entering the function.  Check if it escapes
@@ -168,7 +119,7 @@ static bool isNonEscapingLocalObject(const Value *V) {
       // Don't bother analyzing arguments already known not to escape.
       if (A->hasNoCaptureAttr())
         return true;
-      return !AddressMightEscape(V);
+      return !PointerMayBeCaptured(V, false);
     }
   return false;
 }
diff --git a/lib/Analysis/CaptureTracking.cpp b/lib/Analysis/CaptureTracking.cpp
new file mode 100644
index 0000000000..ceb964619c
--- /dev/null
+++ b/lib/Analysis/CaptureTracking.cpp
@@ -0,0 +1,110 @@
+//===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains routines that help determine which pointers are captured.
+// A pointer value is captured if the function makes a copy of any part of the
+// pointer that outlives the call.  Not being captured means, more or less, that
+// the pointer is only dereferenced and not stored in a global.  Returning part
+// of the pointer as the function return value may or may not count as capturing
+// the pointer, depending on the context.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Instructions.h"
+#include "llvm/Value.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/CallSite.h"
+using namespace llvm;
+
+/// PointerMayBeCaptured - Return true if this pointer value may be captured
+/// by the enclosing function (which is required to exist).  This routine can
+/// be expensive, so consider caching the results.  The boolean ReturnCaptures
+/// specifies whether returning the value (or part of it) from the function
+/// counts as capturing it or not.
+bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
+  assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
+  SmallVector<Use*, 16> Worklist;
+  SmallSet<Use*, 16> Visited;
+
+  for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
+       UI != UE; ++UI) {
+    Use *U = &UI.getUse();
+    Visited.insert(U);
+    Worklist.push_back(U);
+  }
+
+  while (!Worklist.empty()) {
+    Use *U = Worklist.pop_back_val();
+    Instruction *I = cast<Instruction>(U->getUser());
+    V = U->get();
+
+    switch (I->getOpcode()) {
+    case Instruction::Call:
+    case Instruction::Invoke: {
+      CallSite CS = CallSite::get(I);
+      // Not captured if the callee is readonly and doesn't return a copy
+      // through its return value.
+      if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
+        break;
+
+      // Not captured if only passed via 'nocapture' arguments.  Note that
+      // calling a function pointer does not in itself cause the pointer to
+      // be captured.  This is a subtle point considering that (for example)
+      // the callee might return its own address.  It is analogous to saying
+      // that loading a value from a pointer does not cause the pointer to be
+      // captured, even though the loaded value might be the pointer itself
+      // (think of self-referential objects).
+      CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
+      for (CallSite::arg_iterator A = B; A != E; ++A)
+        if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
+          // The parameter is not marked 'nocapture' - captured.
+          return true;
+      // Only passed via 'nocapture' arguments, or is the called function - not
+      // captured.
+      break;
+    }
+    case Instruction::Free:
+      // Freeing a pointer does not cause it to be captured.
+      break;
+    case Instruction::Load:
+      // Loading from a pointer does not cause it to be captured.
+      break;
+    case Instruction::Ret:
+      if (ReturnCaptures)
+        return true;
+      break;
+    case Instruction::Store:
+      if (V == I->getOperand(0))
+        // Stored the pointer - it may be captured.
+        return true;
+      // Storing to the pointee does not cause the pointer to be captured.
+      break;
+    case Instruction::BitCast:
+    case Instruction::GetElementPtr:
+    case Instruction::PHI:
+    case Instruction::Select:
+      // The original value is not captured via this if the new value isn't.
+      for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
+           UI != UE; ++UI) {
+        Use *U = &UI.getUse();
+        if (Visited.insert(U))
+          Worklist.push_back(U);
+      }
+      break;
+    default:
+      // Something else - be conservative and say it is captured.
+      return true;
+    }
+  }
+
+  // All uses examined - not captured.
+  return false;
+}