[Attributor] Function level undefined behavior attribute
_Eventually_, this attribute will be assigned to a function if it
contains undefined behavior. As a first small step, I tried to make it
loop through the load instructions in a function (eventually, the plan
is to check if a load instructions causes undefined behavior, because
e.g. dereferences a null pointer - Also eventually, this won't happen in
initialize() but in updateImpl()).
Patch By: Stefanos Baziotis (@baziotis)
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D71435
diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index cd305ee..1bb3b41 100644
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -1987,6 +1987,98 @@
void trackStatistics() const override { STATS_DECLTRACK_CS_ATTR(norecurse); }
};
+/// -------------------- Undefined-Behavior Attributes ------------------------
+
+struct AAUndefinedBehaviorImpl : public AAUndefinedBehavior {
+ AAUndefinedBehaviorImpl(const IRPosition &IRP) : AAUndefinedBehavior(IRP) {}
+
+ /// See AbstractAttribute::updateImpl(...).
+ ChangeStatus updateImpl(Attributor &A) override {
+ size_t PrevSize = NoUBLoads.size();
+
+ // TODO: We should not only check for load instructions.
+ auto InspectLoadForUB = [&](Instruction &I) {
+ // Skip instructions that are already saved.
+ if (NoUBLoads.count(&I) || UBLoads.count(&I))
+ return true;
+
+ Value *PtrOp = cast<LoadInst>(&I)->getPointerOperand();
+
+ // A load is considered UB only if it dereferences a constant
+ // null pointer.
+ if (!isa<ConstantPointerNull>(PtrOp)) {
+ NoUBLoads.insert(&I);
+ return true;
+ }
+ Type *PtrTy = PtrOp->getType();
+
+ // Because we only consider loads inside functions,
+ // assume that a parent function exists.
+ const Function *F = I.getFunction();
+
+ // A dereference on constant null is only considered UB
+ // if null dereference is _not_ defined for the target platform.
+ // TODO: Expand it to not only check constant values.
+ if (!llvm::NullPointerIsDefined(F, PtrTy->getPointerAddressSpace()))
+ UBLoads.insert(&I);
+ else
+ NoUBLoads.insert(&I);
+ return true;
+ };
+
+ A.checkForAllInstructions(InspectLoadForUB, *this, {Instruction::Load});
+ if (PrevSize != NoUBLoads.size())
+ return ChangeStatus::CHANGED;
+ return ChangeStatus::UNCHANGED;
+ }
+
+ bool isAssumedToCauseUB(Instruction *I) const override {
+ return UBLoads.count(I);
+ }
+
+ ChangeStatus manifest(Attributor &A) override {
+ if (!UBLoads.size())
+ return ChangeStatus::UNCHANGED;
+ for (Instruction *I : UBLoads)
+ changeToUnreachable(I, /* UseLLVMTrap */ false);
+ return ChangeStatus::CHANGED;
+ }
+
+ /// See AbstractAttribute::getAsStr()
+ const std::string getAsStr() const override {
+ return getAssumed() ? "undefined-behavior" : "no-ub";
+ }
+
+protected:
+ // A set of all the (live) load instructions that _are_ assumed to cause UB.
+ SmallPtrSet<Instruction *, 8> UBLoads;
+
+private:
+ // A set of all the (live) load instructions that are _not_ assumed to cause
+ // UB.
+ // Note: The correctness of the procedure depends on the fact that this
+ // set stops changing after some point. "Change" here means that the size
+ // of the set changes. The size of this set is monotonically increasing
+ // (we only add items to it) and is upper bounded by the number of load
+ // instructions in the processed function (we can never save more elements
+ // in this set than this number). Hence, the size of this set, at some
+ // point, will stop increasing, effectively reaching a fixpoint.
+ SmallPtrSet<Instruction *, 8> NoUBLoads;
+};
+
+struct AAUndefinedBehaviorFunction final : AAUndefinedBehaviorImpl {
+ AAUndefinedBehaviorFunction(const IRPosition &IRP)
+ : AAUndefinedBehaviorImpl(IRP) {}
+
+ /// See AbstractAttribute::trackStatistics()
+ void trackStatistics() const override {
+ STATS_DECL(UndefinedBehaviorInstruction, Instruction,
+ "Number of instructions known to have UB");
+ BUILD_STAT_NAME(UndefinedBehaviorInstruction, Instruction) +=
+ UBLoads.size();
+ }
+};
+
/// ------------------------ Will-Return Attributes ----------------------------
// Helper function that checks whether a function has any cycle.
@@ -5523,6 +5615,9 @@
// Every function might be "will-return".
getOrCreateAAFor<AAWillReturn>(FPos);
+ // Every function might contain instructions that cause "undefined behavior".
+ getOrCreateAAFor<AAUndefinedBehavior>(FPos);
+
// Every function can be nounwind.
getOrCreateAAFor<AANoUnwind>(FPos);
@@ -5827,6 +5922,7 @@
const char AANonNull::ID = 0;
const char AANoRecurse::ID = 0;
const char AAWillReturn::ID = 0;
+const char AAUndefinedBehavior::ID = 0;
const char AANoAlias::ID = 0;
const char AAReachability::ID = 0;
const char AANoReturn::ID = 0;
@@ -5949,6 +6045,7 @@
CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAHeapToStack)
CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAReachability)
+CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAUndefinedBehavior)
CREATE_NON_RET_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAMemoryBehavior)