+/// Utility function for reservePreviousStackSlotForValue. Tries to find
+/// stack slot index to which we have spilled value for previous statepoints.
+/// LookUpDepth specifies maximum DFS depth this function is allowed to look.
+static Optional<int> findPreviousSpillSlot(const Value *Val,
+ SelectionDAGBuilder &Builder,
+ int LookUpDepth) {
+ // Can not look any further - give up now
+ if (LookUpDepth <= 0)
+ return Optional<int>();
+
+ // Spill location is known for gc relocates
+ if (isGCRelocate(Val)) {
+ GCRelocateOperands RelocOps(cast<Instruction>(Val));
+
+ FunctionLoweringInfo::StatepointSpilledValueMapTy &SpillMap =
+ Builder.FuncInfo.StatepointRelocatedValues[RelocOps.getStatepoint()];
+
+ auto It = SpillMap.find(RelocOps.getDerivedPtr());
+ if (It == SpillMap.end())
+ return Optional<int>();
+
+ return It->second;
+ }
+
+ // Look through bitcast instructions.
+ if (const BitCastInst *Cast = dyn_cast<BitCastInst>(Val)) {
+ return findPreviousSpillSlot(Cast->getOperand(0), Builder, LookUpDepth - 1);
+ }
+
+ // Look through phi nodes
+ // All incoming values should have same known stack slot, otherwise result
+ // is unknown.
+ if (const PHINode *Phi = dyn_cast<PHINode>(Val)) {
+ Optional<int> MergedResult = None;
+
+ for (auto &IncomingValue : Phi->incoming_values()) {
+ Optional<int> SpillSlot =
+ findPreviousSpillSlot(IncomingValue, Builder, LookUpDepth - 1);
+ if (!SpillSlot.hasValue())
+ return Optional<int>();
+
+ if (MergedResult.hasValue() && *MergedResult != *SpillSlot)
+ return Optional<int>();
+
+ MergedResult = SpillSlot;
+ }
+ return MergedResult;
+ }
+
+ // TODO: We can do better for PHI nodes. In cases like this:
+ // ptr = phi(relocated_pointer, not_relocated_pointer)
+ // statepoint(ptr)
+ // We will return that stack slot for ptr is unknown. And later we might
+ // assign different stack slots for ptr and relocated_pointer. This limits
+ // llvm's ability to remove redundant stores.
+ // Unfortunately it's hard to accomplish in current infrastructure.
+ // We use this function to eliminate spill store completely, while
+ // in example we still need to emit store, but instead of any location
+ // we need to use special "preferred" location.
+
+ // TODO: handle simple updates. If a value is modified and the original
+ // value is no longer live, it would be nice to put the modified value in the
+ // same slot. This allows folding of the memory accesses for some
+ // instructions types (like an increment).
+ // statepoint (i)
+ // i1 = i+1
+ // statepoint (i1)
+ // However we need to be careful for cases like this:
+ // statepoint(i)
+ // i1 = i+1
+ // statepoint(i, i1)
+ // Here we want to reserve spill slot for 'i', but not for 'i+1'. If we just
+ // put handling of simple modifications in this function like it's done
+ // for bitcasts we might end up reserving i's slot for 'i+1' because order in
+ // which we visit values is unspecified.
+
+ // Don't know any information about this instruction
+ return Optional<int>();
+}
+