1 files changed, 169 insertions, 0 deletions
diff --git a/backend/lib/CodeGen/MachineFallthrough.cpp b/backend/lib/CodeGen/MachineFallthrough.cpp
new file mode 100644
index 0000000..06d70ef
--- /dev/null
+++ b/backend/lib/CodeGen/MachineFallthrough.cpp
@@ -0,0 +1,169 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include "rcc/CodeGen/MachineFallthrough.h"
+
+#include "rcc/CodeGen/MachineBasicBlock.h"
+#include "rcc/CodeGen/MachineFunction.h"
+#include "rcc/CodeGen/MachineFunctionCloner.h"
+#include "rcc/CodeGen/MachineLiveness.h"
+#include "rcc/CodeGen/MachineLivenessRepair.h"
+#include "rcc/CodeGen/TargetBranchInfo.h"
+#include "rcc/CodeGen/TargetDescription.h"
+
+#include <optional>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace rcc {
+namespace {
+
+[[nodiscard]] const MachinePassContract &fallthroughContract() {
+  static const MachinePassContract Contract = [] {
+    MachinePassContract Value{
+        .Name = "machine-fallthrough-normalization",
+        .Requires = {MachineFunctionProperty::Selected, MachineFunctionProperty::NoPHIs,
+                     MachineFunctionProperty::NoVRegs, MachineFunctionProperty::TracksLiveness},
+        .Sets = {},
+        .Clears = {},
+        .RequiredAnalyses = {MachineAnalysisManager::id<LiveVariablesAnalysis>()},
+        .VerificationStage = PipelineStage::RegisterAllocated,
+    };
+    Value.ThreadSafety = PassThreadSafety::ThreadSafe;
+    Value.DeclaredChanges = {MachinePassChange::Instructions, MachinePassChange::Operands};
+    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]] Expected<std::vector<MachineBasicBlock *>>
+normalizeFallthroughs(MachineFunction &Function, const TargetDescription &Target,
+                      const TargetBranchInfo &Info) {
+  std::vector<MachineBasicBlock *> ChangedBlocks;
+  for (MachineBasicBlock *Block = Function.firstBlock(); Block != nullptr; Block = Block->next()) {
+    MachineBasicBlock *LayoutSuccessor = Block->next();
+    if (LayoutSuccessor == nullptr)
+      continue;
+    auto Analysis = Info.analyzeBranch(*Block, Function, Target);
+    if (!Analysis)
+      return std::unexpected(Analysis.error());
+    if (Analysis->HasUnanalyzableTerminator)
+      continue;
+    bool Changed = false;
+    if (Analysis->Conditional != nullptr && Analysis->Unconditional != nullptr &&
+        Analysis->FalseTarget == LayoutSuccessor) {
+      auto Removed = Info.removeBranch(*Analysis->Unconditional, Function, Target);
+      if (!Removed)
+        return std::unexpected(Removed.error());
+      Changed = true;
+    } else if (Analysis->Conditional != nullptr && Analysis->Unconditional != nullptr &&
+               Analysis->TrueTarget == LayoutSuccessor && Analysis->FalseTarget != nullptr) {
+      auto Reversed = Info.reverseBranchCondition(*Analysis->Conditional, *Analysis->FalseTarget,
+                                                  Function, Target);
+      if (!Reversed)
+        return std::unexpected(Reversed.error());
+      auto Removed = Info.removeBranch(*Analysis->Unconditional, Function, Target);
+      if (!Removed)
+        return std::unexpected(Removed.error());
+      Changed = true;
+    } else if (Analysis->Conditional != nullptr && Analysis->Unconditional == nullptr &&
+               Analysis->FalseTarget == LayoutSuccessor) {
+      continue;
+    } else if (Analysis->Conditional != nullptr && Analysis->Unconditional == nullptr &&
+               Analysis->TrueTarget == LayoutSuccessor && Analysis->FalseTarget != nullptr) {
+      auto Reversed = Info.reverseBranchCondition(*Analysis->Conditional, *Analysis->FalseTarget,
+                                                  Function, Target);
+      if (!Reversed)
+        return std::unexpected(Reversed.error());
+      Changed = true;
+    } else if (Analysis->Conditional != nullptr && Analysis->Unconditional == nullptr &&
+               Analysis->FalseTarget != nullptr) {
+      auto Inserted =
+          Info.insertUnconditionalBranch(*Block, *Analysis->FalseTarget, Function, Target);
+      if (!Inserted)
+        return std::unexpected(Inserted.error());
+      Changed = true;
+    } else if (Analysis->Conditional == nullptr && Analysis->Unconditional != nullptr &&
+               Analysis->TrueTarget == LayoutSuccessor) {
+      auto Removed = Info.removeBranch(*Analysis->Unconditional, Function, Target);
+      if (!Removed)
+        return std::unexpected(Removed.error());
+      Changed = true;
+    } else if (Analysis->Conditional == nullptr && Analysis->Unconditional == nullptr &&
+               Block->successors().size() == 1 &&
+               Block->successors().front().Block != LayoutSuccessor) {
+      MachineBasicBlock *Successor = Block->successors().front().Block;
+      if (Successor == nullptr) {
+        return makeUnexpected(ErrorCode::VerificationError,
+                              "fallthrough normalization encountered a null successor");
+      }
+      auto Inserted = Info.insertUnconditionalBranch(*Block, *Successor, Function, Target);
+      if (!Inserted)
+        return std::unexpected(Inserted.error());
+      Changed = true;
+    }
+    if (Changed)
+      ChangedBlocks.push_back(Block);
+  }
+  return ChangedBlocks;
+}
+
+} // namespace
+
+const MachinePassContract &MachineFallthroughPass::contract() const noexcept {
+  return fallthroughContract();
+}
+
+Expected<MachinePassResult> MachineFallthroughPass::run(MachineFunction &Function,
+                                                        const TargetDescription &Target,
+                                                        MachineAnalysisManager &Analyses) {
+  if (!Function.properties().containsAll(contract().Requires)) {
+    return makeUnexpected(ErrorCode::VerificationError,
+                          "fallthrough normalization requires allocated tracked-liveness MIR");
+  }
+  auto Trial = cloneMachineFunction(
+      Function, Target,
+      MachineFunctionCloneOptions{.NewStableID = Function.stableID(), .NewName = std::nullopt});
+  if (!Trial)
+    return std::unexpected(prefixedError(Trial.error(), "fallthrough setup failed: "));
+  MachineAnalysisManager TrialAnalyses;
+  auto TrialLiveness =
+      TrialAnalyses.getResult<LiveVariablesAnalysis>(*Trial->ClonedFunction, Target);
+  if (!TrialLiveness)
+    return std::unexpected(TrialLiveness.error());
+  auto TrialChanged = normalizeFallthroughs(*Trial->ClonedFunction, Target, *Info);
+  if (!TrialChanged)
+    return std::unexpected(TrialChanged.error());
+  auto TrialRepair = repairPostRAMachineBlockLiveness(*TrialChanged, Target, **TrialLiveness);
+  if (!TrialRepair)
+    return std::unexpected(TrialRepair.error());
+
+  auto Liveness = Analyses.getResult<LiveVariablesAnalysis>(Function, Target);
+  if (!Liveness)
+    return std::unexpected(Liveness.error());
+  auto ChangedBlocks = normalizeFallthroughs(Function, Target, *Info);
+  if (!ChangedBlocks) {
+    return std::unexpected(prefixedError(ChangedBlocks.error(),
+                                         "fallthrough diverged after a successful preflight: "));
+  }
+  auto Repaired = repairPostRAMachineBlockLiveness(*ChangedBlocks, Target, **Liveness);
+  if (!Repaired)
+    return std::unexpected(Repaired.error());
+
+  MachinePassResult Result;
+  Result.Changed = !ChangedBlocks->empty() || *Repaired;
+  if (Result.Changed) {
+    Result.Changes = {MachinePassChange::Instructions, MachinePassChange::Operands};
+    Result.Preserved.preserve<LiveVariablesAnalysis>();
+  } else {
+    Result.Preserved = PreservedAnalyses::all();
+  }
+  return Result;
+}
+
+} // namespace rcc