Less template, more virtual! Refactoring suggested by Chris in code review.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145014 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 107 insertions, 2 deletions

diff --git a/lib/Analysis/CaptureTracking.cpp b/lib/Analysis/CaptureTracking.cpp
index a84dafb5fb..9be00971dc 100644
--- a/lib/Analysis/CaptureTracking.cpp
+++ b/lib/Analysis/CaptureTracking.cpp
@@ -20,7 +20,7 @@
 using namespace llvm;
 
 namespace {
-  struct SimpleCaptureTracker {
+  struct SimpleCaptureTracker : public CaptureTracker {
     explicit SimpleCaptureTracker(bool ReturnCaptures)
       : ReturnCaptures(ReturnCaptures), Captured(false) {}
 
@@ -58,6 +58,111 @@ bool llvm::PointerMayBeCaptured(const Value *V,
   (void)StoreCaptures;
 
   SimpleCaptureTracker SCT(ReturnCaptures);
-  PointerMayBeCaptured(V, SCT);
+  PointerMayBeCaptured(V, &SCT);
   return SCT.Captured;
 }
+
+/// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
+/// a cache. Then we can move the code from BasicAliasAnalysis into
+/// that path, and remove this threshold.
+static int const Threshold = 20;
+
+void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
+  assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
+  SmallVector<Use*, Threshold> Worklist;
+  SmallSet<Use*, Threshold> Visited;
+  int Count = 0;
+
+  for (Value::const_use_iterator UI = V->use_begin(), UE = V->use_end();
+       UI != UE; ++UI) {
+    // If there are lots of uses, conservatively say that the value
+    // is captured to avoid taking too much compile time.
+    if (Count++ >= Threshold)
+      return Tracker->tooManyUses();
+
+    Use *U = &UI.getUse();
+    if (!Tracker->shouldExplore(U)) continue;
+    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(I);
+      // Not captured if the callee is readonly, doesn't return a copy through
+      // its return value and doesn't unwind (a readonly function can leak bits
+      // by throwing an exception or not depending on the input value).
+      if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
+        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.doesNotCapture(A - B))
+          // The parameter is not marked 'nocapture' - captured.
+          if (Tracker->captured(I))
+            return;
+      break;
+    }
+    case Instruction::Load:
+      // Loading from a pointer does not cause it to be captured.
+      break;
+    case Instruction::VAArg:
+      // "va-arg" from a pointer does not cause it to be captured.
+      break;
+    case Instruction::Store:
+      if (V == I->getOperand(0))
+        // Stored the pointer - conservatively assume it may be captured.
+        if (Tracker->captured(I))
+          return;
+      // 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))
+          if (Tracker->shouldExplore(U))
+            Worklist.push_back(U);
+      }
+      break;
+    case Instruction::ICmp:
+      // Don't count comparisons of a no-alias return value against null as
+      // captures. This allows us to ignore comparisons of malloc results
+      // with null, for example.
+      if (isNoAliasCall(V->stripPointerCasts()))
+        if (ConstantPointerNull *CPN =
+              dyn_cast<ConstantPointerNull>(I->getOperand(1)))
+          if (CPN->getType()->getAddressSpace() == 0)
+            break;
+      // Otherwise, be conservative. There are crazy ways to capture pointers
+      // using comparisons.
+      if (Tracker->captured(I))
+        return;
+      break;
+    default:
+      // Something else - be conservative and say it is captured.
+      if (Tracker->captured(I))
+        return;
+      break;
+    }
+  }
+
+  // All uses examined.
+}