Add an MCID::Select flag and TII hooks for optimizing selects.

Select instructions pick one of two virtual registers based on a
condition, like x86 cmov. On targets like ARM that support predication,
selects can sometimes be eliminated by predicating the instruction
defining one of the operands.

Teach PeepholeOptimizer to recognize select instructions, and ask the
target to optimize them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162059 91177308-0d34-0410-b5e6-96231b3b80d8

4 files changed, 59 insertions, 0 deletions

diff --git a/include/llvm/CodeGen/MachineInstr.h b/include/llvm/CodeGen/MachineInstr.h
index 4c5eb8b36f..27756abf3f 100644
--- a/include/llvm/CodeGen/MachineInstr.h
+++ b/include/llvm/CodeGen/MachineInstr.h
@@ -420,6 +420,12 @@ public:
     return hasProperty(MCID::Bitcast, Type);
   }
 
+  /// isSelect - Return true if this instruction is a select instruction.
+  ///
+  bool isSelect(QueryType Type = IgnoreBundle) const {
+    return hasProperty(MCID::Select, Type);
+  }
+
   /// isNotDuplicable - Return true if this instruction cannot be safely
   /// duplicated.  For example, if the instruction has a unique labels attached
   /// to it, duplicating it would cause multiple definition errors.
diff --git a/include/llvm/MC/MCInstrDesc.h b/include/llvm/MC/MCInstrDesc.h
index 186612d904..dbf16d8700 100644
--- a/include/llvm/MC/MCInstrDesc.h
+++ b/include/llvm/MC/MCInstrDesc.h
@@ -107,6 +107,7 @@ namespace MCID {
     Compare,
     MoveImm,
     Bitcast,
+    Select,
     DelaySlot,
     FoldableAsLoad,
     MayLoad,
@@ -282,6 +283,12 @@ public:
     return Flags & (1 << MCID::Bitcast);
   }
 
+  /// isSelect - Return true if this is a select instruction.
+  ///
+  bool isSelect() const {
+    return Flags & (1 << MCID::Select);
+  }
+
   /// isNotDuplicable - Return true if this instruction cannot be safely
   /// duplicated.  For example, if the instruction has a unique labels attached
   /// to it, duplicating it would cause multiple definition errors.
diff --git a/include/llvm/Target/Target.td b/include/llvm/Target/Target.td
index cc76714812..1816445579 100644
--- a/include/llvm/Target/Target.td
+++ b/include/llvm/Target/Target.td
@@ -339,6 +339,7 @@ class Instruction {
   bit isCompare    = 0;     // Is this instruction a comparison instruction?
   bit isMoveImm    = 0;     // Is this instruction a move immediate instruction?
   bit isBitcast    = 0;     // Is this instruction a bitcast instruction?
+  bit isSelect     = 0;     // Is this instruction a select instruction?
   bit isBarrier    = 0;     // Can control flow fall through this instruction?
   bit isCall       = 0;     // Is this instruction a call instruction?
   bit canFoldAsLoad = 0;    // Can this be folded as a simple memory operand?
diff --git a/include/llvm/Target/TargetInstrInfo.h b/include/llvm/Target/TargetInstrInfo.h
index a18b0305a9..da30ab82d6 100644
--- a/include/llvm/Target/TargetInstrInfo.h
+++ b/include/llvm/Target/TargetInstrInfo.h
@@ -413,6 +413,51 @@ public:
     llvm_unreachable("Target didn't implement TargetInstrInfo::insertSelect!");
   }
 
+  /// analyzeSelect - Analyze the given select instruction, returning true if
+  /// it cannot be understood. It is assumed that MI->isSelect() is true.
+  ///
+  /// When successful, return the controlling condition and the operands that
+  /// determine the true and false result values.
+  ///
+  ///   Result = SELECT Cond, TrueOp, FalseOp
+  ///
+  /// Some targets can optimize select instructions, for example by predicating
+  /// the instruction defining one of the operands. Such targets should set
+  /// Optimizable.
+  ///
+  /// @param         MI Select instruction to analyze.
+  /// @param Cond    Condition controlling the select.
+  /// @param TrueOp  Operand number of the value selected when Cond is true.
+  /// @param FalseOp Operand number of the value selected when Cond is false.
+  /// @param Optimizable Returned as true if MI is optimizable.
+  /// @returns False on success.
+  virtual bool analyzeSelect(const MachineInstr *MI,
+                             SmallVectorImpl<MachineOperand> &Cond,
+                             unsigned &TrueOp, unsigned &FalseOp,
+                             bool &Optimizable) const {
+    assert(MI && MI->isSelect() && "MI must be a select instruction");
+    return true;
+  }
+
+  /// optimizeSelect - Given a select instruction that was understood by
+  /// analyzeSelect and returned Optimizable = true, attempt to optimize MI by
+  /// merging it with one of its operands. Returns NULL on failure.
+  ///
+  /// When successful, returns the new select instruction. The client is
+  /// responsible for deleting MI.
+  ///
+  /// If both sides of the select can be optimized, PreferFalse is used to pick
+  /// a side.
+  ///
+  /// @param MI          Optimizable select instruction.
+  /// @param PreferFalse Try to optimize FalseOp instead of TrueOp.
+  /// @returns Optimized instruction or NULL.
+  virtual MachineInstr *optimizeSelect(MachineInstr *MI,
+                                       bool PreferFalse = false) const {
+    // This function must be implemented if Optimizable is ever set.
+    llvm_unreachable("Target must implement TargetInstrInfo::optimizeSelect!");
+  }
+
   /// copyPhysReg - Emit instructions to copy a pair of physical registers.
   virtual void copyPhysReg(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator MI, DebugLoc DL,