[CodeGen] Add a pass to fold null checks into nearby memory operations.

Summary:
This change adds an "ImplicitNullChecks" target dependent pass.  This
pass folds null checks into memory operation using the FAULTING_LOAD
pseudo-op introduced in previous patches.

Depends on D10197
Depends on D10199
Depends on D10200

Reviewers: reames, rnk, pgavlin, JosephTremoulet, atrick

Reviewed By: atrick

Subscribers: ab, JosephTremoulet, llvm-commits

Differential Revision: http://reviews.llvm.org/D10201

llvm-svn: 239743

include/llvm/CodeGen/Passes.h

@@ -552,6 +552,10 @@ namespace llvm {
   /// MachineCSE - This pass performs global CSE on machine instructions.
   extern char &MachineCSEID;
 
+  /// ImplicitNullChecks - This pass folds null pointer checks into nearby
+  /// memory operations.
+  extern char &ImplicitNullChecksID;
+
   /// MachineLICM - This pass performs LICM on machine instructions.
   extern char &MachineLICMID;
 

include/llvm/InitializePasses.h

@@ -187,6 +187,7 @@ void initializeMachineBlockPlacementPass(PassRegistry&);
 void initializeMachineBlockPlacementStatsPass(PassRegistry&);
 void initializeMachineBranchProbabilityInfoPass(PassRegistry&);
 void initializeMachineCSEPass(PassRegistry&);
+void initializeImplicitNullChecksPass(PassRegistry&);
 void initializeMachineDominatorTreePass(PassRegistry&);
 void initializeMachineDominanceFrontierPass(PassRegistry&);
 void initializeMachinePostDominatorTreePass(PassRegistry&);

lib/CodeGen/CMakeLists.txt

@@ -27,6 +27,7 @@ add_llvm_library(LLVMCodeGen
   GCStrategy.cpp
   GlobalMerge.cpp
   IfConversion.cpp
+  ImplicitNullChecks.cpp
   InlineSpiller.cpp
   InterferenceCache.cpp
   IntrinsicLowering.cpp

lib/CodeGen/CodeGen.cpp

@@ -42,6 +42,7 @@ void llvm::initializeCodeGen(PassRegistry &Registry) {
   initializeMachineBlockPlacementPass(Registry);
   initializeMachineBlockPlacementStatsPass(Registry);
   initializeMachineCSEPass(Registry);
+  initializeImplicitNullChecksPass(Registry);
   initializeMachineCombinerPass(Registry);
   initializeMachineCopyPropagationPass(Registry);
   initializeMachineDominatorTreePass(Registry);

lib/CodeGen/ImplicitNullChecks.cpp

@@ -0,0 +1,261 @@
+//===-- 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 the pointer is non-null.
+    MachineBasicBlock *NotNullSucc;
+
+    // The block branched to if the 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;
+
+  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)

lib/CodeGen/Passes.cpp

@@ -72,6 +72,10 @@ static cl::opt<bool> DisableCopyProp("disable-copyprop", cl::Hidden,
     cl::desc("Disable Copy Propagation pass"));
 static cl::opt<bool> DisablePartialLibcallInlining("disable-partial-libcall-inlining",
     cl::Hidden, cl::desc("Disable Partial Libcall Inlining"));
+static cl::opt<bool> EnableImplicitNullChecks(
+    "enable-implicit-null-checks",
+    cl::desc("Fold null checks into faulting memory operations"),
+    cl::init(false));
 static cl::opt<bool> PrintLSR("print-lsr-output", cl::Hidden,
     cl::desc("Print LLVM IR produced by the loop-reduce pass"));
 static cl::opt<bool> PrintISelInput("print-isel-input", cl::Hidden,
@@ -543,6 +547,9 @@ void TargetPassConfig::addMachinePasses() {
   // Run pre-sched2 passes.
   addPreSched2();
 
+  if (EnableImplicitNullChecks)
+    addPass(&ImplicitNullChecksID);
+
   // Second pass scheduler.
   if (getOptLevel() != CodeGenOpt::None) {
     if (MISchedPostRA)

test/CodeGen/X86/implicit-null-check-negative.ll

@@ -0,0 +1,52 @@
+; RUN: llc -mtriple=x86_64-apple-macosx -O3 -debug-only=faultmaps -enable-implicit-null-checks < %s | FileCheck %s
+
+; List cases where we should *not* be emitting implicit null checks.
+
+; CHECK-NOT: Fault Map Output
+
+define i32 @imp_null_check_load(i32* %x, i32* %y) {
+ entry:
+  %c = icmp eq i32* %x, null
+; It isn't legal to move the load from %x from "not_null" to here --
+; the store to %y could be aliasing it.
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  store i32 0, i32* %y
+  %t = load i32, i32* %x
+  ret i32 %t
+}
+
+define i32 @imp_null_check_gep_load(i32* %x) {
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+; null + 5000 * sizeof(i32) lies outside the null page and hence the
+; load to %t cannot be assumed to be reliably faulting.
+  %x.gep = getelementptr i32, i32* %x, i32 5000
+  %t = load i32, i32* %x.gep
+  ret i32 %t
+}
+
+define i32 @imp_null_check_load_no_md(i32* %x) {
+; Everything is okay except that the !never.executed metadata is
+; missing.
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  %t = load i32, i32* %x
+  ret i32 %t
+}

test/CodeGen/X86/implicit-null-check.ll

@@ -0,0 +1,118 @@
+; RUN: llc -O3 -mtriple=x86_64-apple-macosx -enable-implicit-null-checks < %s | FileCheck %s
+
+define i32 @imp_null_check_load(i32* %x) {
+; CHECK-LABEL: _imp_null_check_load:
+; CHECK: Ltmp1:
+; CHECK: movl (%rdi), %eax
+; CHECK: retq
+; CHECK: Ltmp0:
+; CHECK: movl $42, %eax
+; CHECK: retq
+
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  %t = load i32, i32* %x
+  ret i32 %t
+}
+
+define i32 @imp_null_check_gep_load(i32* %x) {
+; CHECK-LABEL: _imp_null_check_gep_load:
+; CHECK: Ltmp3:
+; CHECK: movl 128(%rdi), %eax
+; CHECK: retq
+; CHECK: Ltmp2:
+; CHECK: movl $42, %eax
+; CHECK: retq
+
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  %x.gep = getelementptr i32, i32* %x, i32 32
+  %t = load i32, i32* %x.gep
+  ret i32 %t
+}
+
+define i32 @imp_null_check_add_result(i32* %x, i32 %p) {
+; CHECK-LABEL: _imp_null_check_add_result:
+; CHECK: Ltmp5:
+; CHECK: addl (%rdi), %esi
+; CHECK: movl %esi, %eax
+; CHECK: retq
+; CHECK: Ltmp4:
+; CHECK: movl $42, %eax
+; CHECK: retq
+
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  %t = load i32, i32* %x
+  %p1 = add i32 %t, %p
+  ret i32 %p1
+}
+
+; CHECK-LABEL: __LLVM_FaultMaps:
+
+; Version:
+; CHECK-NEXT: .byte 1
+
+; Reserved x2
+; CHECK-NEXT: .byte 0
+; CHECK-NEXT: .short 0
+
+; # functions:
+; CHECK-NEXT: .long 3
+
+; FunctionAddr:
+; CHECK-NEXT: .quad _imp_null_check_add_result
+; NumFaultingPCs
+; CHECK-NEXT: .long 1
+; Reserved:
+; CHECK-NEXT: .long 0
+; Fault[0].Type:
+; CHECK-NEXT: .long 1
+; Fault[0].FaultOffset:
+; CHECK-NEXT: .long Ltmp5-_imp_null_check_add_result
+; Fault[0].HandlerOffset:
+; CHECK-NEXT: .long Ltmp4-_imp_null_check_add_result
+
+; FunctionAddr:
+; CHECK-NEXT: .quad _imp_null_check_gep_load
+; NumFaultingPCs
+; CHECK-NEXT: .long 1
+; Reserved:
+; CHECK-NEXT: .long 0
+; Fault[0].Type:
+; CHECK-NEXT: .long 1
+; Fault[0].FaultOffset:
+; CHECK-NEXT: .long Ltmp3-_imp_null_check_gep_load
+; Fault[0].HandlerOffset:
+; CHECK-NEXT: .long Ltmp2-_imp_null_check_gep_load
+
+; FunctionAddr:
+; CHECK-NEXT: .quad _imp_null_check_load
+; NumFaultingPCs
+; CHECK-NEXT: .long 1
+; Reserved:
+; CHECK-NEXT: .long 0
+; Fault[0].Type:
+; CHECK-NEXT: .long 1
+; Fault[0].FaultOffset:
+; CHECK-NEXT: .long Ltmp1-_imp_null_check_load
+; Fault[0].HandlerOffset:
+; CHECK-NEXT: .long Ltmp0-_imp_null_check_load