#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Instructions.h"
#include "llvm/Value.h"
+#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/CallSite.h"
using namespace llvm;
+/// As its comment mentions, PointerMayBeCaptured can be expensive.
+/// However, it's not easy for BasicAA to cache the result, because
+/// it's an ImmutablePass. To work around this, bound queries at a
+/// fixed number of uses.
+///
+/// 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;
+
/// 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. The boolean StoreCaptures specified whether
+/// storing the value (or part of it) into memory anywhere automatically
/// counts as capturing it or not.
-bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
+bool llvm::PointerMayBeCaptured(const Value *V,
+ bool ReturnCaptures, bool StoreCaptures) {
assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
- SmallVector<Use*, 16> Worklist;
- SmallSet<Use*, 16> Visited;
+ SmallVector<Use*, 20> Worklist;
+ SmallSet<Use*, 20> Visited;
+ int Count = 0;
for (Value::use_const_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 true;
+
Use *U = &UI.getUse();
Visited.insert(U);
Worklist.push_back(U);
// 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() == Type::getVoidTy(V->getContext()))
+ if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
break;
// Not captured if only passed via 'nocapture' arguments. Note that
break;
case Instruction::Store:
if (V == I->getOperand(0))
- // Stored the pointer - it may be captured.
+ // Stored the pointer - conservatively assume it may be captured.
+ // TODO: If StoreCaptures is not true, we could do Fancy analysis
+ // to determine whether this store is not actually an escape point.
+ // In that case, BasicAliasAnalysis should be updated as well to
+ // take advantage of this.
return true;
// Storing to the pointee does not cause the pointer to be captured.
break;
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.
+ return true;
default:
// Something else - be conservative and say it is captured.
return true;