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

1 files changed, 110 insertions, 0 deletions

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;
+}