1 files changed, 268 insertions, 0 deletions
diff --git a/contrib/llvm/lib/CodeGen/ImplicitNullChecks.cpp b/contrib/llvm/lib/CodeGen/ImplicitNullChecks.cpp
new file mode 100644
index 0000000..a02cd67
--- /dev/null
+++ b/contrib/llvm/lib/CodeGen/ImplicitNullChecks.cpp
@@ -0,0 +1,268 @@
+//===-- ImplicitNullChecks.cpp - Fold null checks into memory accesses ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass turns explicit null checks of the form
+//
+//   test %r10, %r10
+//   je throw_npe
+//   movl (%r10), %esi
+//   ...
+//
+// to
+//
+//   faulting_load_op("movl (%r10), %esi", throw_npe)
+//   ...
+//
+// With the help of a runtime that understands the .fault_maps section,
+// faulting_load_op branches to throw_npe if executing movl (%r10), %esi incurs
+// a page fault.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineOperand.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Target/TargetSubtargetInfo.h"
+#include "llvm/Target/TargetInstrInfo.h"
+
+using namespace llvm;
+
+static cl::opt<unsigned> PageSize("imp-null-check-page-size",
+                                  cl::desc("The page size of the target in "
+                                           "bytes"),
+                                  cl::init(4096));
+
+namespace {
+
+class ImplicitNullChecks : public MachineFunctionPass {
+  /// Represents one null check that can be made implicit.
+  struct NullCheck {
+    // The memory operation the null check can be folded into.
+    MachineInstr *MemOperation;
+
+    // The instruction actually doing the null check (Ptr != 0).
+    MachineInstr *CheckOperation;
+
+    // The block the check resides in.
+    MachineBasicBlock *CheckBlock;
+
+    // The block branched to if the pointer is non-null.
+    MachineBasicBlock *NotNullSucc;
+
+    // The block branched to if the pointer is null.
+    MachineBasicBlock *NullSucc;
+
+    NullCheck()
+        : MemOperation(), CheckOperation(), CheckBlock(), NotNullSucc(),
+          NullSucc() {}
+
+    explicit NullCheck(MachineInstr *memOperation, MachineInstr *checkOperation,
+                       MachineBasicBlock *checkBlock,
+                       MachineBasicBlock *notNullSucc,
+                       MachineBasicBlock *nullSucc)
+        : MemOperation(memOperation), CheckOperation(checkOperation),
+          CheckBlock(checkBlock), NotNullSucc(notNullSucc), NullSucc(nullSucc) {
+    }
+  };
+
+  const TargetInstrInfo *TII = nullptr;
+  const TargetRegisterInfo *TRI = nullptr;
+  MachineModuleInfo *MMI = nullptr;
+
+  bool analyzeBlockForNullChecks(MachineBasicBlock &MBB,
+                                 SmallVectorImpl<NullCheck> &NullCheckList);
+  MachineInstr *insertFaultingLoad(MachineInstr *LoadMI, MachineBasicBlock *MBB,
+                                   MCSymbol *HandlerLabel);
+  void rewriteNullChecks(ArrayRef<NullCheck> NullCheckList);
+
+public:
+  static char ID;
+
+  ImplicitNullChecks() : MachineFunctionPass(ID) {
+    initializeImplicitNullChecksPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+};
+}
+
+bool ImplicitNullChecks::runOnMachineFunction(MachineFunction &MF) {
+  TII = MF.getSubtarget().getInstrInfo();
+  TRI = MF.getRegInfo().getTargetRegisterInfo();
+  MMI = &MF.getMMI();
+
+  SmallVector<NullCheck, 16> NullCheckList;
+
+  for (auto &MBB : MF)
+    analyzeBlockForNullChecks(MBB, NullCheckList);
+
+  if (!NullCheckList.empty())
+    rewriteNullChecks(NullCheckList);
+
+  return !NullCheckList.empty();
+}
+
+/// Analyze MBB to check if its terminating branch can be turned into an
+/// implicit null check.  If yes, append a description of the said null check to
+/// NullCheckList and return true, else return false.
+bool ImplicitNullChecks::analyzeBlockForNullChecks(
+    MachineBasicBlock &MBB, SmallVectorImpl<NullCheck> &NullCheckList) {
+  typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate;
+
+  MDNode *BranchMD =
+      MBB.getBasicBlock()
+          ? MBB.getBasicBlock()->getTerminator()->getMetadata("make.implicit")
+          : nullptr;
+  if (!BranchMD)
+    return false;
+
+  MachineBranchPredicate MBP;
+
+  if (TII->AnalyzeBranchPredicate(MBB, MBP, true))
+    return false;
+
+  // Is the predicate comparing an integer to zero?
+  if (!(MBP.LHS.isReg() && MBP.RHS.isImm() && MBP.RHS.getImm() == 0 &&
+        (MBP.Predicate == MachineBranchPredicate::PRED_NE ||
+         MBP.Predicate == MachineBranchPredicate::PRED_EQ)))
+    return false;
+
+  // If we cannot erase the test instruction itself, then making the null check
+  // implicit does not buy us much.
+  if (!MBP.SingleUseCondition)
+    return false;
+
+  MachineBasicBlock *NotNullSucc, *NullSucc;
+
+  if (MBP.Predicate == MachineBranchPredicate::PRED_NE) {
+    NotNullSucc = MBP.TrueDest;
+    NullSucc = MBP.FalseDest;
+  } else {
+    NotNullSucc = MBP.FalseDest;
+    NullSucc = MBP.TrueDest;
+  }
+
+  // We handle the simplest case for now.  We can potentially do better by using
+  // the machine dominator tree.
+  if (NotNullSucc->pred_size() != 1)
+    return false;
+
+  // Starting with a code fragment like:
+  //
+  //   test %RAX, %RAX
+  //   jne LblNotNull
+  //
+  //  LblNull:
+  //   callq throw_NullPointerException
+  //
+  //  LblNotNull:
+  //   Def = Load (%RAX + <offset>)
+  //   ...
+  //
+  //
+  // we want to end up with
+  //
+  //   Def = TrappingLoad (%RAX + <offset>), LblNull
+  //   jmp LblNotNull ;; explicit or fallthrough
+  //
+  //  LblNotNull:
+  //   ...
+  //
+  //  LblNull:
+  //   callq throw_NullPointerException
+  //
+
+  unsigned PointerReg = MBP.LHS.getReg();
+  MachineInstr *MemOp = &*NotNullSucc->begin();
+  unsigned BaseReg, Offset;
+  if (TII->getMemOpBaseRegImmOfs(MemOp, BaseReg, Offset, TRI))
+    if (MemOp->mayLoad() && !MemOp->isPredicable() && BaseReg == PointerReg &&
+        Offset < PageSize && MemOp->getDesc().getNumDefs() == 1) {
+      NullCheckList.emplace_back(MemOp, MBP.ConditionDef, &MBB, NotNullSucc,
+                                 NullSucc);
+      return true;
+    }
+
+  return false;
+}
+
+/// Wrap a machine load instruction, LoadMI, into a FAULTING_LOAD_OP machine
+/// instruction.  The FAULTING_LOAD_OP instruction does the same load as LoadMI
+/// (defining the same register), and branches to HandlerLabel if the load
+/// faults.  The FAULTING_LOAD_OP instruction is inserted at the end of MBB.
+MachineInstr *ImplicitNullChecks::insertFaultingLoad(MachineInstr *LoadMI,
+                                                     MachineBasicBlock *MBB,
+                                                     MCSymbol *HandlerLabel) {
+  DebugLoc DL;
+  unsigned NumDefs = LoadMI->getDesc().getNumDefs();
+  assert(NumDefs == 1 && "other cases unhandled!");
+  (void)NumDefs;
+
+  unsigned DefReg = LoadMI->defs().begin()->getReg();
+  assert(std::distance(LoadMI->defs().begin(), LoadMI->defs().end()) == 1 &&
+         "expected exactly one def!");
+
+  auto MIB = BuildMI(MBB, DL, TII->get(TargetOpcode::FAULTING_LOAD_OP), DefReg)
+                 .addSym(HandlerLabel)
+                 .addImm(LoadMI->getOpcode());
+
+  for (auto &MO : LoadMI->uses())
+    MIB.addOperand(MO);
+
+  MIB.setMemRefs(LoadMI->memoperands_begin(), LoadMI->memoperands_end());
+
+  return MIB;
+}
+
+/// Rewrite the null checks in NullCheckList into implicit null checks.
+void ImplicitNullChecks::rewriteNullChecks(
+    ArrayRef<ImplicitNullChecks::NullCheck> NullCheckList) {
+  DebugLoc DL;
+
+  for (auto &NC : NullCheckList) {
+    MCSymbol *HandlerLabel = MMI->getContext().createTempSymbol();
+
+    // Remove the conditional branch dependent on the null check.
+    unsigned BranchesRemoved = TII->RemoveBranch(*NC.CheckBlock);
+    (void)BranchesRemoved;
+    assert(BranchesRemoved > 0 && "expected at least one branch!");
+
+    // Insert a faulting load where the conditional branch was originally.  We
+    // check earlier ensures that this bit of code motion is legal.  We do not
+    // touch the successors list for any basic block since we haven't changed
+    // control flow, we've just made it implicit.
+    insertFaultingLoad(NC.MemOperation, NC.CheckBlock, HandlerLabel);
+    NC.MemOperation->removeFromParent();
+    NC.CheckOperation->eraseFromParent();
+
+    // Insert an *unconditional* branch to not-null successor.
+    TII->InsertBranch(*NC.CheckBlock, NC.NotNullSucc, nullptr, /*Cond=*/None,
+                      DL);
+
+    // Emit the HandlerLabel as an EH_LABEL.
+    BuildMI(*NC.NullSucc, NC.NullSucc->begin(), DL,
+            TII->get(TargetOpcode::EH_LABEL)).addSym(HandlerLabel);
+  }
+}
+
+char ImplicitNullChecks::ID = 0;
+char &llvm::ImplicitNullChecksID = ImplicitNullChecks::ID;
+INITIALIZE_PASS_BEGIN(ImplicitNullChecks, "implicit-null-checks",
+                      "Implicit null checks", false, false)
+INITIALIZE_PASS_END(ImplicitNullChecks, "implicit-null-checks",
+                    "Implicit null checks", false, false)