[TargetInstrInfo] Add new hook: AnalyzeBranchPredicate.

Summary:
NFC: no one uses AnalyzeBranchPredicate yet.

Add TargetInstrInfo::AnalyzeBranchPredicate and implement for x86.  A
later change adding support for page-fault based implicit null checks
depends on this.

Reviewers: reames, ab, atrick

Reviewed By: atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10200

llvm-svn: 239742

llvm/include/llvm/Target/TargetInstrInfo.h

@@ -387,6 +387,51 @@ public:
     return true;
   }
 
+  /// Represents a predicate at the MachineFunction level.  The control flow a
+  /// MachineBranchPredicate represents is:
+  ///
+  ///  Reg <def>= LHS `Predicate` RHS         == ConditionDef
+  ///  if Reg then goto TrueDest else goto FalseDest
+  ///
+  struct MachineBranchPredicate {
+    enum ComparePredicate {
+      PRED_EQ,     // True if two values are equal
+      PRED_NE,     // True if two values are not equal
+      PRED_INVALID // Sentinel value
+    };
+
+    ComparePredicate Predicate;
+    MachineOperand LHS;
+    MachineOperand RHS;
+    MachineBasicBlock *TrueDest;
+    MachineBasicBlock *FalseDest;
+    MachineInstr *ConditionDef;
+
+    /// SingleUseCondition is true if ConditionDef is dead except for the
+    /// branch(es) at the end of the basic block.
+    ///
+    bool SingleUseCondition;
+
+    explicit MachineBranchPredicate()
+        : Predicate(PRED_INVALID), LHS(MachineOperand::CreateImm(0)),
+          RHS(MachineOperand::CreateImm(0)), TrueDest(nullptr),
+          FalseDest(nullptr), ConditionDef(nullptr), SingleUseCondition(false) {
+    }
+  };
+
+  /// Analyze the branching code at the end of MBB and parse it into the
+  /// MachineBranchPredicate structure if possible.  Returns false on success
+  /// and true on failure.
+  ///
+  /// If AllowModify is true, then this routine is allowed to modify the basic
+  /// block (e.g. delete instructions after the unconditional branch).
+  ///
+  virtual bool AnalyzeBranchPredicate(MachineBasicBlock &MBB,
+                                      MachineBranchPredicate &MBP,
+                                      bool AllowModify = false) const {
+    return true;
+  }
+
   /// Remove the branching code at the end of the specific MBB.
   /// This is only invoked in cases where AnalyzeBranch returns success. It
   /// returns the number of instructions that were removed.

llvm/lib/Target/X86/X86InstrInfo.cpp

@@ -3456,11 +3456,11 @@ bool X86InstrInfo::isUnpredicatedTerminator(const MachineInstr *MI) const {
   return !isPredicated(MI);
 }
 
-bool X86InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
-                                 MachineBasicBlock *&TBB,
-                                 MachineBasicBlock *&FBB,
-                                 SmallVectorImpl<MachineOperand> &Cond,
-                                 bool AllowModify) const {
+bool X86InstrInfo::AnalyzeBranchImpl(
+    MachineBasicBlock &MBB, MachineBasicBlock *&TBB, MachineBasicBlock *&FBB,
+    SmallVectorImpl<MachineOperand> &Cond,
+    SmallVectorImpl<MachineInstr *> &CondBranches, bool AllowModify) const {
+
   // Start from the bottom of the block and work up, examining the
   // terminator instructions.
   MachineBasicBlock::iterator I = MBB.end();
@@ -3558,6 +3558,7 @@ bool X86InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
       FBB = TBB;
       TBB = I->getOperand(0).getMBB();
       Cond.push_back(MachineOperand::CreateImm(BranchCode));
+      CondBranches.push_back(I);
       continue;
     }
 
@@ -3595,11 +3596,90 @@ bool X86InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
 
     // Update the MachineOperand.
     Cond[0].setImm(BranchCode);
+    CondBranches.push_back(I);
   }
 
   return false;
 }
 
+bool X86InstrInfo::AnalyzeBranch(MachineBasicBlock &MBB,
+                                 MachineBasicBlock *&TBB,
+                                 MachineBasicBlock *&FBB,
+                                 SmallVectorImpl<MachineOperand> &Cond,
+                                 bool AllowModify) const {
+  SmallVector<MachineInstr *, 4> CondBranches;
+  return AnalyzeBranchImpl(MBB, TBB, FBB, Cond, CondBranches, AllowModify);
+}
+
+bool X86InstrInfo::AnalyzeBranchPredicate(MachineBasicBlock &MBB,
+                                          MachineBranchPredicate &MBP,
+                                          bool AllowModify) const {
+  using namespace std::placeholders;
+
+  SmallVector<MachineOperand, 4> Cond;
+  SmallVector<MachineInstr *, 4> CondBranches;
+  if (AnalyzeBranchImpl(MBB, MBP.TrueDest, MBP.FalseDest, Cond, CondBranches,
+                        AllowModify))
+    return true;
+
+  if (Cond.size() != 1)
+    return true;
+
+  assert(MBP.TrueDest && "expected!");
+
+  if (!MBP.FalseDest)
+    MBP.FalseDest = MBB.getNextNode();
+
+  const TargetRegisterInfo *TRI = &getRegisterInfo();
+
+  MachineInstr *ConditionDef = nullptr;
+  bool SingleUseCondition = true;
+
+  for (auto I = std::next(MBB.rbegin()), E = MBB.rend(); I != E; ++I) {
+    if (I->modifiesRegister(X86::EFLAGS, TRI)) {
+      ConditionDef = &*I;
+      break;
+    }
+
+    if (I->readsRegister(X86::EFLAGS, TRI))
+      SingleUseCondition = false;
+  }
+
+  if (!ConditionDef)
+    return true;
+
+  if (SingleUseCondition) {
+    for (auto *Succ : MBB.successors())
+      if (Succ->isLiveIn(X86::EFLAGS))
+        SingleUseCondition = false;
+  }
+
+  MBP.ConditionDef = ConditionDef;
+  MBP.SingleUseCondition = SingleUseCondition;
+
+  // Currently we only recognize the simple pattern:
+  //
+  //   test %reg, %reg
+  //   je %label
+  //
+  const unsigned TestOpcode =
+      Subtarget.is64Bit() ? X86::TEST64rr : X86::TEST32rr;
+
+  if (ConditionDef->getOpcode() == TestOpcode &&
+      ConditionDef->getNumOperands() == 3 &&
+      ConditionDef->getOperand(0).isIdenticalTo(ConditionDef->getOperand(1)) &&
+      (Cond[0].getImm() == X86::COND_NE || Cond[0].getImm() == X86::COND_E)) {
+    MBP.LHS = ConditionDef->getOperand(0);
+    MBP.RHS = MachineOperand::CreateImm(0);
+    MBP.Predicate = Cond[0].getImm() == X86::COND_NE
+                        ? MachineBranchPredicate::PRED_NE
+                        : MachineBranchPredicate::PRED_EQ;
+    return false;
+  }
+
+  return true;
+}
+
 unsigned X86InstrInfo::RemoveBranch(MachineBasicBlock &MBB) const {
   MachineBasicBlock::iterator I = MBB.end();
   unsigned Count = 0;

llvm/lib/Target/X86/X86InstrInfo.h

@@ -179,6 +179,12 @@ class X86InstrInfo final : public X86GenInstrInfo {
 
   virtual void anchor();
 
+  bool AnalyzeBranchImpl(MachineBasicBlock &MBB, MachineBasicBlock *&TBB,
+                         MachineBasicBlock *&FBB,
+                         SmallVectorImpl<MachineOperand> &Cond,
+                         SmallVectorImpl<MachineInstr *> &CondBranches,
+                         bool AllowModify) const;
+
 public:
   explicit X86InstrInfo(X86Subtarget &STI);
 
@@ -271,6 +277,10 @@ public:
   bool getMemOpBaseRegImmOfs(MachineInstr *LdSt, unsigned &BaseReg,
                              unsigned &Offset,
                              const TargetRegisterInfo *TRI) const override;
+  bool AnalyzeBranchPredicate(MachineBasicBlock &MBB,
+                              TargetInstrInfo::MachineBranchPredicate &MBP,
+                              bool AllowModify = false) const override;
+
   unsigned RemoveBranch(MachineBasicBlock &MBB) const override;
   unsigned InsertBranch(MachineBasicBlock &MBB, MachineBasicBlock *TBB,
                         MachineBasicBlock *FBB, ArrayRef<MachineOperand> Cond,