Index: llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp =================================================================== --- llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp +++ llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp @@ -85,6 +85,22 @@ "Make relocations explicit at statepoints", false, false) namespace { +struct GCPtrLivenessData { + /// Values defined in this block. + DenseMap> KillSet; + /// Values used in this block (and thus live); does not included values + /// killed within this block. + DenseMap> LiveSet; + + /// Values live into this basic block (i.e. used by any + /// instruction in this basic block or ones reachable from here) + DenseMap> LiveIn; + + /// Values live out of this basic block (i.e. live into + /// any successor block) + DenseMap> LiveOut; +}; + // The type of the internal cache used inside the findBasePointers family // of functions. From the callers perspective, this is an opaque type and // should not be inspected. @@ -119,6 +135,15 @@ }; } +/// Compute the live-in set for every basic block in the function +static void computeLiveInValues(DominatorTree &DT, Function &F, + GCPtrLivenessData &Data); + +/// Given results from the dataflow liveness computation, find the set of live +/// Values at a particular instruction. +static void findLiveSetAtInst(Instruction *inst, GCPtrLivenessData &Data, + StatepointLiveSetTy &out); + // TODO: Once we can get to the GCStrategy, this becomes // Optional isGCManagedPointer(const Value *V) const override { @@ -172,113 +197,6 @@ } #endif -/// Return true if the Value is a gc reference type which is potentially used -/// after the instruction 'loc'. This is only used with the edge reachability -/// liveness code. Note: It is assumed the V dominates loc. -static bool isLiveGCReferenceAt(Value &V, Instruction *Loc, DominatorTree &DT, - LoopInfo *LI) { - if (!isHandledGCPointerType(V.getType())) - return false; - - if (V.use_empty()) - return false; - - // Given assumption that V dominates loc, this may be live - return true; -} - -// Conservatively identifies any definitions which might be live at the -// given instruction. The analysis is performed immediately before the -// given instruction. Values defined by that instruction are not considered -// live. Values used by that instruction are considered live. -// -// preconditions: valid IR graph, term is either a terminator instruction or -// a call instruction, pred is the basic block of term, DT, LI are valid -// -// side effects: none, does not mutate IR -// -// postconditions: populates liveValues as discussed above -static void findLiveGCValuesAtInst(Instruction *term, BasicBlock *pred, - DominatorTree &DT, LoopInfo *LI, - StatepointLiveSetTy &liveValues) { - liveValues.clear(); - - assert(isa(term) || isa(term) || term->isTerminator()); - - Function *F = pred->getParent(); - - auto is_live_gc_reference = - [&](Value &V) { return isLiveGCReferenceAt(V, term, DT, LI); }; - - // Are there any gc pointer arguments live over this point? This needs to be - // special cased since arguments aren't defined in basic blocks. - for (Argument &arg : F->args()) { - assert(!isUnhandledGCPointerType(arg.getType()) && - "support for FCA unimplemented"); - - if (is_live_gc_reference(arg)) { - liveValues.insert(&arg); - } - } - - // Walk through all dominating blocks - the ones which can contain - // definitions used in this block - and check to see if any of the values - // they define are used in locations potentially reachable from the - // interesting instruction. - BasicBlock *BBI = pred; - while (true) { - if (TraceLSP) { - errs() << "[LSP] Looking at dominating block " << pred->getName() << "\n"; - } - assert(DT.dominates(BBI, pred)); - assert(isPotentiallyReachable(BBI, pred, &DT) && - "dominated block must be reachable"); - - // Walk through the instructions in dominating blocks and keep any - // that have a use potentially reachable from the block we're - // considering putting the safepoint in - for (Instruction &inst : *BBI) { - if (TraceLSP) { - errs() << "[LSP] Looking at instruction "; - inst.dump(); - } - - if (pred == BBI && (&inst) == term) { - if (TraceLSP) { - errs() << "[LSP] stopped because we encountered the safepoint " - "instruction.\n"; - } - - // If we're in the block which defines the interesting instruction, - // we don't want to include any values as live which are defined - // _after_ the interesting line or as part of the line itself - // i.e. "term" is the call instruction for a call safepoint, the - // results of the call should not be considered live in that stackmap - break; - } - - assert(!isUnhandledGCPointerType(inst.getType()) && - "support for FCA unimplemented"); - - if (is_live_gc_reference(inst)) { - if (TraceLSP) { - errs() << "[LSP] found live value for this safepoint "; - inst.dump(); - term->dump(); - } - liveValues.insert(&inst); - } - } - if (!DT.getNode(BBI)->getIDom()) { - assert(BBI == &F->getEntryBlock() && - "failed to find a dominator for something other than " - "the entry block"); - break; - } - BBI = DT.getNode(BBI)->getIDom()->getBlock(); - } -} - static bool order_by_name(llvm::Value *a, llvm::Value *b) { if (a->hasName() && b->hasName()) { return -1 == a->getName().compare(b->getName()); @@ -292,16 +210,17 @@ } } -/// Find the initial live set. Note that due to base pointer -/// insertion, the live set may be incomplete. -static void -analyzeParsePointLiveness(DominatorTree &DT, const CallSite &CS, - PartiallyConstructedSafepointRecord &result) { +// Conservatively identifies any definitions which might be live at the +// given instruction. The analysis is performed immediately before the +// given instruction. Values defined by that instruction are not considered +// live. Values used by that instruction are considered live. +static void analyzeParsePointLiveness( + DominatorTree &DT, GCPtrLivenessData &OriginalLivenessData, + const CallSite &CS, PartiallyConstructedSafepointRecord &result) { Instruction *inst = CS.getInstruction(); - BasicBlock *BB = inst->getParent(); StatepointLiveSetTy liveset; - findLiveGCValuesAtInst(inst, BB, DT, nullptr, liveset); + findLiveSetAtInst(inst, OriginalLivenessData, liveset); if (PrintLiveSet) { // Note: This output is used by several of the test cases @@ -1048,56 +967,23 @@ result.NewInsertedDefs = NewInsertedDefs; } -/// Check for liveness of items in the insert defs and add them to the live -/// and base pointer sets -static void fixupLiveness(DominatorTree &DT, const CallSite &CS, - const DenseSet &allInsertedDefs, - PartiallyConstructedSafepointRecord &result) { - Instruction *inst = CS.getInstruction(); - - auto liveset = result.liveset; - auto PointerToBase = result.PointerToBase; - - auto is_live_gc_reference = - [&](Value &V) { return isLiveGCReferenceAt(V, inst, DT, nullptr); }; - - // For each new definition, check to see if a) the definition dominates the - // instruction we're interested in, and b) one of the uses of that definition - // is edge-reachable from the instruction we're interested in. This is the - // same definition of liveness we used in the intial liveness analysis - for (Value *newDef : allInsertedDefs) { - if (liveset.count(newDef)) { - // already live, no action needed - continue; - } - - // PERF: Use DT to check instruction domination might not be good for - // compilation time, and we could change to optimal solution if this - // turn to be a issue - if (!DT.dominates(cast(newDef), inst)) { - // can't possibly be live at inst - continue; - } - - if (is_live_gc_reference(*newDef)) { - // Add the live new defs into liveset and PointerToBase - liveset.insert(newDef); - PointerToBase[newDef] = newDef; - } - } - - result.liveset = liveset; - result.PointerToBase = PointerToBase; -} +/// Given an updated version of the dataflow liveness results, update the +/// liveset and base pointer maps for the call site CS. +static void recomputeLiveInValues(GCPtrLivenessData &RevisedLivenessData, + const CallSite &CS, + PartiallyConstructedSafepointRecord &result); -static void fixupLiveReferences( - Function &F, DominatorTree &DT, Pass *P, - const DenseSet &allInsertedDefs, ArrayRef toUpdate, +static void recomputeLiveInValues( + Function &F, DominatorTree &DT, Pass *P, ArrayRef toUpdate, MutableArrayRef records) { + // TODO-PERF: reuse the original liveness, then simply run the dataflow + // again. The old values are still live and will help it stablize quickly. + GCPtrLivenessData RevisedLivenessData; + computeLiveInValues(DT, F, RevisedLivenessData); for (size_t i = 0; i < records.size(); i++) { struct PartiallyConstructedSafepointRecord &info = records[i]; const CallSite &CS = toUpdate[i]; - fixupLiveness(DT, CS, allInsertedDefs, info); + recomputeLiveInValues(RevisedLivenessData, CS, info); } } @@ -1689,42 +1575,15 @@ static void findLiveReferences( Function &F, DominatorTree &DT, Pass *P, ArrayRef toUpdate, MutableArrayRef records) { + GCPtrLivenessData OriginalLivenessData; + computeLiveInValues(DT, F, OriginalLivenessData); for (size_t i = 0; i < records.size(); i++) { struct PartiallyConstructedSafepointRecord &info = records[i]; const CallSite &CS = toUpdate[i]; - analyzeParsePointLiveness(DT, CS, info); + analyzeParsePointLiveness(DT, OriginalLivenessData, CS, info); } } -static void addBasesAsLiveValues(StatepointLiveSetTy &liveset, - DenseMap &PointerToBase) { - // Identify any base pointers which are used in this safepoint, but not - // themselves relocated. We need to relocate them so that later inserted - // safepoints can get the properly relocated base register. - DenseSet missing; - for (Value *L : liveset) { - assert(PointerToBase.find(L) != PointerToBase.end()); - Value *base = PointerToBase[L]; - assert(base); - if (liveset.find(base) == liveset.end()) { - assert(PointerToBase.find(base) == PointerToBase.end()); - // uniqued by set insert - missing.insert(base); - } - } - - // Note that we want these at the end of the list, otherwise - // register placement gets screwed up once we lower to STATEPOINT - // instructions. This is an utter hack, but there doesn't seem to be a - // better one. - for (Value *base : missing) { - assert(base); - liveset.insert(base); - PointerToBase[base] = base; - } - assert(liveset.size() == PointerToBase.size()); -} - /// Remove any vector of pointers from the liveset by scalarizing them over the /// statepoint instruction. Adds the scalarized pieces to the liveset. It /// would be preferrable to include the vector in the statepoint itself, but @@ -1943,22 +1802,11 @@ insertUseHolderAfter(CS, Bases, holders); } - // Add the bases explicitly to the live vector set. This may result in a few - // extra relocations, but the base has to be available whenever a pointer - // derived from it is used. Thus, we need it to be part of the statepoint's - // gc arguments list. TODO: Introduce an explicit notion (in the following - // code) of the GC argument list as seperate from the live Values at a - // given statepoint. - for (size_t i = 0; i < records.size(); i++) { - struct PartiallyConstructedSafepointRecord &info = records[i]; - addBasesAsLiveValues(info.liveset, info.PointerToBase); - } + // By selecting base pointers, we've effectively inserted new uses. Thus, we + // need to rerun liveness. We may *also* have inserted new defs, but that's + // not the key issue. + recomputeLiveInValues(F, DT, P, toUpdate, records); - // If we inserted any new values, we need to adjust our notion of what is - // live at a particular safepoint. - if (!allInsertedDefs.empty()) { - fixupLiveReferences(F, DT, P, allInsertedDefs, toUpdate, records); - } if (PrintBasePointers) { for (size_t i = 0; i < records.size(); i++) { struct PartiallyConstructedSafepointRecord &info = records[i]; @@ -2097,3 +1945,245 @@ MadeChange |= insertParsePoints(F, DT, this, ParsePointNeeded); return MadeChange; } + +// liveness computation via standard dataflow +// ------------------------------------------------------------------- + +// TODO: Consider using bitvectors for liveness, the set of potentially +// interesting values should be small and easy to pre-compute. + +/// Is this value a constant consisting of entirely null values? +static bool isConstantNull(Value *V) { + return isa(V) && cast(V)->isNullValue(); +} + +/// Compute the live-in set for the location rbegin starting from +/// the live-out set of the basic block +static void computeLiveInValues(BasicBlock::reverse_iterator rbegin, + BasicBlock::reverse_iterator rend, + DenseSet &LiveTmp) { + + for (BasicBlock::reverse_iterator ritr = rbegin; ritr != rend; ritr++) { + Instruction *I = &*ritr; + + // KILL/Def - Remove this definition from LiveIn + LiveTmp.erase(I); + + // Don't consider *uses* in PHI nodes, we handle their contribution to + // predecessor blocks when we seed the LiveOut sets + if (isa(I)) + continue; + + // USE - Add to the LiveIn set for this instruction + for (Value *V : I->operands()) { + assert(!isUnhandledGCPointerType(V->getType()) && + "support for FCA unimplemented"); + if (isHandledGCPointerType(V->getType()) && !isConstantNull(V) && + !isa(V)) { + // The choice to exclude null and undef is arbitrary here. Reconsider? + LiveTmp.insert(V); + } + } + } +} + +static void computeLiveOutSeed(BasicBlock *BB, DenseSet &LiveTmp) { + + for (BasicBlock *Succ : successors(BB)) { + const BasicBlock::iterator E(Succ->getFirstNonPHI()); + for (BasicBlock::iterator I = Succ->begin(); I != E; I++) { + PHINode *Phi = cast(&*I); + Value *V = Phi->getIncomingValueForBlock(BB); + assert(!isUnhandledGCPointerType(V->getType()) && + "support for FCA unimplemented"); + if (isHandledGCPointerType(V->getType()) && !isConstantNull(V) && + !isa(V)) { + // The choice to exclude null and undef is arbitrary here. Reconsider? + LiveTmp.insert(V); + } + } + } +} + +static DenseSet computeKillSet(BasicBlock *BB) { + DenseSet KillSet; + for (Instruction &I : *BB) + if (isHandledGCPointerType(I.getType())) + KillSet.insert(&I); + return KillSet; +} + +/// Check that the items in 'Live' dominate 'TI'. This is used as a basic +/// sanity check for the liveness computation. +static void checkBasicSSA(DominatorTree &DT, DenseSet &Live, + TerminatorInst *TI, bool TermOkay = false) { +#ifndef NDEBUG + for (Value *V : Live) { + if (auto *I = dyn_cast(V)) { + // The terminator can be a member of the LiveOut set. LLVM's definition + // of instruction dominance states that V does not dominate itself. As + // such, we need to special case this to allow it. + if (TermOkay && TI == I) + continue; + assert(DT.dominates(I, TI) && + "basic SSA liveness expectation violated by liveness analysis"); + } + } +#endif +} + +/// Check that all the liveness sets used during the computation of liveness +/// obey basic SSA properties. This is useful for finding cases where we miss +/// a def. +static void checkBasicSSA(DominatorTree &DT, GCPtrLivenessData &Data, + BasicBlock &BB) { + checkBasicSSA(DT, Data.LiveSet[&BB], BB.getTerminator()); + checkBasicSSA(DT, Data.LiveOut[&BB], BB.getTerminator(), true); + checkBasicSSA(DT, Data.LiveIn[&BB], BB.getTerminator()); +} + +static void computeLiveInValues(DominatorTree &DT, Function &F, + GCPtrLivenessData &Data) { + + DenseSet WorklistSet; + SmallVector Worklist; + auto AddPredsToWorklist = [&](BasicBlock *BB) { + for (BasicBlock *Pred : predecessors(BB)) + if (WorklistSet.insert(Pred).second) + Worklist.push_back(Pred); + }; + auto NextItem = [&]() { + BasicBlock *BB = Worklist.back(); + Worklist.pop_back(); + WorklistSet.erase(BB); + return BB; + }; + + // Seed the liveness for each individual block + for (BasicBlock &BB : F) { + Data.KillSet[&BB] = computeKillSet(&BB); + Data.LiveSet[&BB].clear(); + computeLiveInValues(BB.rbegin(), BB.rend(), Data.LiveSet[&BB]); + +#ifndef NDEBUG + for (Value *Kill : Data.KillSet[&BB]) + assert(!Data.LiveSet[&BB].count(Kill) && "live set contains kill"); +#endif + + Data.LiveOut[&BB] = DenseSet(); + computeLiveOutSeed(&BB, Data.LiveOut[&BB]); + Data.LiveIn[&BB] = Data.LiveSet[&BB]; + set_union(Data.LiveIn[&BB], Data.LiveOut[&BB]); + set_subtract(Data.LiveIn[&BB], Data.KillSet[&BB]); + if (!Data.LiveIn[&BB].empty()) + AddPredsToWorklist(&BB); + } + + // Propagate that liveness until stable + while (!Worklist.empty()) { + BasicBlock *BB = NextItem(); + + // Compute our new liveout set, then exit early if it hasn't changed + // despite the contribution of our successor. + DenseSet LiveOut = Data.LiveOut[BB]; + const auto OldLiveOutSize = LiveOut.size(); + for (BasicBlock *Succ : successors(BB)) { + assert(Data.LiveIn.count(Succ)); + set_union(LiveOut, Data.LiveIn[Succ]); + } + // assert OutLiveOut is a subset of LiveOut + if (OldLiveOutSize == LiveOut.size()) { + // If the sets are the same size, then we didn't actually add anything + // when unioning our successors LiveIn Thus, the LiveIn of this block + // hasn't changed. + continue; + } + Data.LiveOut[BB] = LiveOut; + + // Apply the effects of this basic block + DenseSet LiveTmp = LiveOut; + set_union(LiveTmp, Data.LiveSet[BB]); + set_subtract(LiveTmp, Data.KillSet[BB]); + + assert(Data.LiveIn.count(BB)); + const DenseSet &OldLiveIn = Data.LiveIn[BB]; + // assert: OldLiveIn is a subset of LiveTmp + if (OldLiveIn.size() != LiveTmp.size()) { + Data.LiveIn[BB] = LiveTmp; + AddPredsToWorklist(BB); + } + } // while( !worklist.empty() ) + +#ifndef NDEBUG + // Sanity check our ouput against SSA properties. This helps catch any + // missing kills during the above iteration. + for (BasicBlock &BB : F) { + checkBasicSSA(DT, Data, BB); + } +#endif +} + +static void findLiveSetAtInst(Instruction *Inst, GCPtrLivenessData &Data, + StatepointLiveSetTy &Out) { + + BasicBlock *BB = Inst->getParent(); + + // Note: The copy is intentional and required + assert(Data.LiveOut.count(BB)); + DenseSet LiveOut = Data.LiveOut[BB]; + + // We want to handle the statepoint itself oddly. It's + // call result is not live (normal), nor are it's arguments + // (unless they're used again later). This adjustment is + // specifically what we need to relocate + BasicBlock::reverse_iterator rend(Inst); + computeLiveInValues(BB->rbegin(), rend, LiveOut); + LiveOut.erase(Inst); + Out.insert(LiveOut.begin(), LiveOut.end()); +} + +static void recomputeLiveInValues(GCPtrLivenessData &RevisedLivenessData, + const CallSite &CS, + PartiallyConstructedSafepointRecord &Info) { + Instruction *Inst = CS.getInstruction(); + StatepointLiveSetTy Updated; + findLiveSetAtInst(Inst, RevisedLivenessData, Updated); + +#ifndef NDEBUG + DenseSet Bases; + for (auto KVPair : Info.PointerToBase) { + Bases.insert(KVPair.second); + } +#endif + // We may have base pointers which are now live that weren't before. We need + // to update the PointerToBase structure to reflect this. + for (auto V : Updated) + if (!Info.PointerToBase.count(V)) { + assert(Bases.count(V) && "can't find base for unexpected live value"); + Info.PointerToBase[V] = V; + continue; + } + +#ifndef NDEBUG + for (auto V : Updated) { + assert(Info.PointerToBase.count(V) && + "must be able to find base for live value"); + } +#endif + + // Remove any stale base mappings - this can happen since our liveness is + // more precise then the one inherent in the base pointer analysis + DenseSet ToErase; + for (auto KVPair : Info.PointerToBase) + if (!Updated.count(KVPair.first)) + ToErase.insert(KVPair.first); + for (auto V : ToErase) + Info.PointerToBase.erase(V); + +#ifndef NDEBUG + for (auto KVPair : Info.PointerToBase) + assert(Updated.count(KVPair.first) && "record for non-live value"); +#endif + + Info.liveset = Updated; +} Index: llvm/trunk/test/Transforms/RewriteStatepointsForGC/base-pointers.ll =================================================================== --- llvm/trunk/test/Transforms/RewriteStatepointsForGC/base-pointers.ll +++ llvm/trunk/test/Transforms/RewriteStatepointsForGC/base-pointers.ll @@ -76,7 +76,9 @@ ; CHECK-LABEL: loop ; CHECK: %base_phi = phi i64 addrspace(1)* ; CHECK-DAG: [ %base_obj, %entry ] -; CHECK-DAG: [ %base_select.relocated, %loop ] +; Given the two selects are equivelent, so are their base phis - ideally, +; we'd have commoned these, but that's a missed optimization, not correctness. +; CHECK-DAG: [ [[DISCARD:%base_select.*.relocated]], %loop ] ; CHECK-NOT: base_phi2 ; CHECK: next = select ; CHECK: base_select