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Diffstat (limited to 'backend/lib/CodeGen/MachineBranchOptimization.cpp')
| -rw-r--r-- | backend/lib/CodeGen/MachineBranchOptimization.cpp | 361 |
1 files changed, 361 insertions, 0 deletions
diff --git a/backend/lib/CodeGen/MachineBranchOptimization.cpp b/backend/lib/CodeGen/MachineBranchOptimization.cpp new file mode 100644 index 0000000..6c6ac51 --- /dev/null +++ b/backend/lib/CodeGen/MachineBranchOptimization.cpp @@ -0,0 +1,361 @@ +// SPDX-License-Identifier: GPL-3.0-or-later + +#include "rcc/CodeGen/MachineBranchOptimization.h" + +#include "rcc/CodeGen/InstructionDescriptor.h" +#include "rcc/CodeGen/MachineBasicBlock.h" +#include "rcc/CodeGen/MachineBasicBlockAttributes.h" +#include "rcc/CodeGen/MachineCFGUpdater.h" +#include "rcc/CodeGen/MachineFunction.h" +#include "rcc/CodeGen/MachineFunctionCloner.h" +#include "rcc/CodeGen/MachineFunctionRecords.h" +#include "rcc/CodeGen/MachineInstr.h" +#include "rcc/CodeGen/MachineJumpTableInfo.h" +#include "rcc/CodeGen/MachineLiveness.h" +#include "rcc/CodeGen/TargetBranchInfo.h" +#include "rcc/CodeGen/TargetDescription.h" + +#include <algorithm> +#include <cstddef> +#include <optional> +#include <ranges> +#include <string> +#include <unordered_set> +#include <utility> +#include <vector> + +namespace rcc { +namespace { + +[[nodiscard]] const MachinePassContract &branchOptimizationContract() { + static const MachinePassContract Contract = [] { + MachinePassContract Value{ + .Name = "machine-branch-optimization", + .Requires = {MachineFunctionProperty::Selected, MachineFunctionProperty::NoPHIs}, + .Sets = {}, + .Clears = {}, + .RequiredAnalyses = {}, + .VerificationStage = PipelineStage::Selected, + }; + Value.ThreadSafety = PassThreadSafety::ThreadSafe; + Value.DeclaredChanges = {MachinePassChange::Instructions, MachinePassChange::Operands, + MachinePassChange::CFG, MachinePassChange::BlockLayout}; + return Value; + }(); + return Contract; +} + +[[nodiscard]] Error prefixedError(const Error &Failure, std::string Prefix) { + Prefix.append(Failure.message()); + return Error(Failure.code(), std::move(Prefix)); +} + +[[nodiscard]] bool hasProtectedAttachment(const MachineInstr &Instruction) noexcept { + return Instruction.debugInstrNumber() != 0 || Instruction.preInstrSymbol().has_value() || + Instruction.postInstrSymbol().has_value() || Instruction.heapAllocMarker().has_value() || + Instruction.pcSections().has_value() || Instruction.mmra().has_value() || + Instruction.cfiType().has_value() || Instruction.deactivationSymbol().has_value(); +} + +[[nodiscard]] bool isJumpTableTarget(const MachineBasicBlock &Block, + const MachineFunction &Function) noexcept { + for (const MachineJumpTableEntry &Entry : Function.jumpTableInfo().tables()) { + const MachineJumpTableDesc *Description = Entry.description(); + if (Description != nullptr && + std::ranges::find(Description->Targets, &Block) != Description->Targets.end()) { + return true; + } + } + return false; +} + +[[nodiscard]] bool hasInstructionRecord(const MachineInstr &Instruction, + const MachineFunction &Function) noexcept { + const MachineInstrStableID ID = Instruction.stableID(); + return std::ranges::any_of(Function.records().callSites(), + [ID](const auto &Record) { return Record.CallInstruction == ID; }) || + std::ranges::any_of(Function.records().calledGlobals(), + [ID](const auto &Record) { return Record.CallInstruction == ID; }); +} + +[[nodiscard]] bool hasObservableBlockIdentity(const MachineBasicBlock &Block, + const MachineFunction &Function) noexcept { + const MachineBasicBlockAttributes &Attributes = Block.attributes(); + return Attributes.rawFlags() != 0 || !Attributes.sectionID().isDefault() || + Attributes.profileID().has_value() || isJumpTableTarget(Block, Function); +} + +[[nodiscard]] bool canDeleteBlock(const MachineBasicBlock &Block, + const MachineFunction &Function) noexcept { + if (hasObservableBlockIdentity(Block, Function)) + return false; + for (const MachineInstr *Instruction = Block.firstInstruction(); Instruction != nullptr; + Instruction = Instruction->next()) { + if (hasProtectedAttachment(*Instruction) || hasInstructionRecord(*Instruction, Function)) { + return false; + } + } + return true; +} + +[[nodiscard]] bool hasSuccessor(const MachineBasicBlock &Block, + const MachineBasicBlock &Target) noexcept { + return std::ranges::any_of(Block.successors(), [&Target](const MachineSuccessor &Successor) { + return Successor.Block == &Target; + }); +} + +[[nodiscard]] Expected<bool> foldConstantBranches(MachineFunction &Function, + const TargetDescription &Target, + const TargetBranchInfo &Info) { + bool Changed = false; + MachineCFGUpdater CFG = Function.cfgUpdater(); + for (MachineBasicBlock *Block = Function.firstBlock(); Block != nullptr; Block = Block->next()) { + auto Analysis = Info.analyzeBranch(*Block, Function, Target); + if (!Analysis) + return std::unexpected(Analysis.error()); + if (Analysis->HasUnanalyzableTerminator || Analysis->Conditional == nullptr || + hasProtectedAttachment(*Analysis->Conditional)) { + continue; + } + auto Condition = Info.evaluateBranchCondition(*Analysis->Conditional, Function, Target); + if (!Condition) + return std::unexpected(Condition.error()); + if (!Condition->has_value()) + continue; + + MachineBasicBlock *Selected = **Condition ? Analysis->TrueTarget : Analysis->FalseTarget; + MachineBasicBlock *Rejected = **Condition ? Analysis->FalseTarget : Analysis->TrueTarget; + if (Selected == nullptr || !hasSuccessor(*Block, *Selected)) { + return makeUnexpected(ErrorCode::CFGOptimizationFailed, + "constant branch has no selected CFG successor"); + } + if (Analysis->Unconditional != nullptr && hasProtectedAttachment(*Analysis->Unconditional)) { + continue; + } + + auto RemovedConditional = Info.removeBranch(*Analysis->Conditional, Function, Target); + if (!RemovedConditional) + return std::unexpected(RemovedConditional.error()); + if (Analysis->Unconditional != nullptr) { + auto RemovedUnconditional = Info.removeBranch(*Analysis->Unconditional, Function, Target); + if (!RemovedUnconditional) + return std::unexpected(RemovedUnconditional.error()); + } + auto Inserted = Info.insertUnconditionalBranch(*Block, *Selected, Function, Target); + if (!Inserted) + return std::unexpected(Inserted.error()); + + if (Rejected != nullptr && Rejected != Selected && hasSuccessor(*Block, *Rejected)) { + auto RemovedEdge = CFG.removeSuccessor(*Block, *Rejected); + if (!RemovedEdge) + return std::unexpected(RemovedEdge.error()); + } + auto Probability = CFG.updateBranchProbability(*Block, *Selected, BranchProbability::one()); + if (!Probability) + return std::unexpected(Probability.error()); + Changed = true; + } + return Changed; +} + +[[nodiscard]] std::unordered_set<MachineBasicBlock *> reachableBlocks(MachineFunction &Function) { + std::unordered_set<MachineBasicBlock *> Reachable; + std::vector<MachineBasicBlock *> Worklist; + if (Function.firstBlock() != nullptr) + Worklist.push_back(Function.firstBlock()); + for (MachineBasicBlock *Block = Function.firstBlock(); Block != nullptr; Block = Block->next()) { + if (!canDeleteBlock(*Block, Function)) + Worklist.push_back(Block); + } + while (!Worklist.empty()) { + MachineBasicBlock *Block = Worklist.back(); + Worklist.pop_back(); + if (Block == nullptr || !Reachable.insert(Block).second) + continue; + for (const MachineSuccessor &Successor : Block->successors()) + Worklist.push_back(Successor.Block); + } + return Reachable; +} + +[[nodiscard]] Expected<bool> removeUnreachableBlocks(MachineFunction &Function) { + const std::unordered_set<MachineBasicBlock *> Reachable = reachableBlocks(Function); + MachineCFGUpdater CFG = Function.cfgUpdater(); + bool Changed = false; + for (MachineBasicBlock *Block = Function.firstBlock(); Block != nullptr;) { + MachineBasicBlock *Next = Block->next(); + if (!Reachable.contains(Block) && canDeleteBlock(*Block, Function)) { + auto Deleted = CFG.deleteBlock(*Block); + if (!Deleted) + return std::unexpected(Deleted.error()); + Changed = true; + } + Block = Next; + } + return Changed; +} + +[[nodiscard]] Expected<bool> isForwardingBlock(MachineBasicBlock &Block, MachineFunction &Function, + const TargetDescription &Target, + const TargetBranchInfo &Info) { + if (&Block == Function.firstBlock() || Block.successors().size() != 1 || + Block.successors().front().Block == nullptr || Block.successors().front().Block == &Block || + hasObservableBlockIdentity(Block, Function)) { + return false; + } + auto Analysis = Info.analyzeBranch(Block, Function, Target); + if (!Analysis) + return std::unexpected(Analysis.error()); + if (Analysis->HasUnanalyzableTerminator || Analysis->Conditional != nullptr) + return false; + MachineBasicBlock *Successor = Block.successors().front().Block; + if (Analysis->Unconditional == nullptr) { + return Block.firstInstruction() == nullptr && Block.next() == Successor; + } + if (Analysis->TrueTarget != Successor || hasProtectedAttachment(*Analysis->Unconditional)) { + return false; + } + return Block.firstInstruction() == Analysis->Unconditional && + Analysis->Unconditional->next() == nullptr; +} + +[[nodiscard]] Expected<bool> threadForwardingBlocks(MachineFunction &Function, + const TargetDescription &Target, + const TargetBranchInfo &Info, + std::size_t MaximumEdges) { + MachineCFGUpdater CFG = Function.cfgUpdater(); + std::size_t ThreadedEdges = 0; + bool AnyChange = false; + bool LocalChange = true; + while (LocalChange && ThreadedEdges < MaximumEdges) { + LocalChange = false; + for (MachineBasicBlock *Block = Function.firstBlock(); Block != nullptr; + Block = Block->next()) { + auto Forwarding = isForwardingBlock(*Block, Function, Target, Info); + if (!Forwarding) + return std::unexpected(Forwarding.error()); + if (!*Forwarding) + continue; + MachineBasicBlock *Successor = Block->successors().front().Block; + const std::vector<MachineBasicBlock *> Predecessors(Block->predecessors().begin(), + Block->predecessors().end()); + for (MachineBasicBlock *Predecessor : Predecessors) { + if (ThreadedEdges == MaximumEdges) + break; + if (Predecessor == nullptr || Predecessor->successors().size() != 1 || + Predecessor->successors().front().Block != Block) { + continue; + } + auto PredAnalysis = Info.analyzeBranch(*Predecessor, Function, Target); + if (!PredAnalysis) + return std::unexpected(PredAnalysis.error()); + if (PredAnalysis->HasUnanalyzableTerminator || PredAnalysis->Conditional != nullptr) + continue; + + if (PredAnalysis->Unconditional != nullptr) { + if (PredAnalysis->TrueTarget != Block || + hasProtectedAttachment(*PredAnalysis->Unconditional)) { + continue; + } + auto Rewritten = + Info.rewriteBranchTarget(*Predecessor, *Block, *Successor, Function, Target); + if (!Rewritten) + return std::unexpected(Rewritten.error()); + if (!*Rewritten) { + return makeUnexpected(ErrorCode::CFGOptimizationFailed, + "branch threading did not rewrite an explicit branch"); + } + } else { + if (Predecessor->next() != Block) + continue; + auto Inserted = + Info.insertUnconditionalBranch(*Predecessor, *Successor, Function, Target); + if (!Inserted) + return std::unexpected(Inserted.error()); + } + auto Redirected = CFG.replaceSuccessor(*Predecessor, *Block, *Successor); + if (!Redirected) + return std::unexpected(Redirected.error()); + ++ThreadedEdges; + AnyChange = true; + LocalChange = true; + } + if (Block->predecessors().empty() && canDeleteBlock(*Block, Function)) { + auto Deleted = CFG.deleteBlock(*Block); + if (!Deleted) + return std::unexpected(Deleted.error()); + AnyChange = true; + LocalChange = true; + break; + } + } + } + return AnyChange; +} + +} // namespace + +const MachinePassContract &MachineBranchOptimizationPass::contract() const noexcept { + return branchOptimizationContract(); +} + +Expected<MachinePassResult> MachineBranchOptimizationPass::run(MachineFunction &Function, + const TargetDescription &Target, + MachineAnalysisManager &) { + if (!Function.properties().containsAll(contract().Requires)) { + return makeUnexpected(ErrorCode::VerificationError, + "branch optimization requires selected PHI-free MachineIR"); + } + auto Trial = cloneMachineFunction( + Function, Target, + MachineFunctionCloneOptions{.NewStableID = Function.stableID(), .NewName = std::nullopt}); + if (!Trial) { + return std::unexpected( + prefixedError(Trial.error(), "branch optimization transaction setup failed: ")); + } + auto TrialResult = runImplementation(*Trial->ClonedFunction, Target); + if (!TrialResult) + return std::unexpected(TrialResult.error()); + auto Result = runImplementation(Function, Target); + if (!Result) { + return std::unexpected(prefixedError( + Result.error(), "branch optimization diverged after a successful transaction preflight: ")); + } + return Result; +} + +Expected<MachinePassResult> +MachineBranchOptimizationPass::runImplementation(MachineFunction &Function, + const TargetDescription &Target) const { + auto Folded = foldConstantBranches(Function, Target, *Info); + if (!Folded) + return std::unexpected(Folded.error()); + auto RemovedBeforeThreading = removeUnreachableBlocks(Function); + if (!RemovedBeforeThreading) + return std::unexpected(RemovedBeforeThreading.error()); + auto Threaded = threadForwardingBlocks(Function, Target, *Info, Options.MaximumThreadedEdges); + if (!Threaded) + return std::unexpected(Threaded.error()); + auto RemovedAfterThreading = removeUnreachableBlocks(Function); + if (!RemovedAfterThreading) + return std::unexpected(RemovedAfterThreading.error()); + + MachinePassResult Result; + Result.Changed = *Folded || *RemovedBeforeThreading || *Threaded || *RemovedAfterThreading; + if (!Result.Changed) { + Result.Preserved = PreservedAnalyses::all(); + return Result; + } + if (Function.properties().has(MachineFunctionProperty::TracksLiveness)) { + auto Rebuilt = rebuildMachineLiveness(Function, Target); + if (!Rebuilt) + return std::unexpected(Rebuilt.error()); + } + Result.Changes = {MachinePassChange::Instructions, MachinePassChange::Operands, + MachinePassChange::CFG, MachinePassChange::BlockLayout}; + Result.Preserved = PreservedAnalyses::none(); + return Result; +} + +} // namespace rcc |