[MC] Adding code padding for performance stability - infrastructure. …

…NFC.

Infrastructure designed for padding code with nop instructions in key places such that preformance improvement will be achieved.
The infrastructure is implemented such that the padding is done in the Assembler after the layout is done and all IPs and alignments are known.
This patch by itself in a NFC. Future patches will make use of this infrastructure to implement required policies for code padding.

Reviewers:
aaboud
zvi
craig.topper
gadi.haber

Differential revision: https://reviews.llvm.org/D34393

Change-Id: I92110d0c0a757080a8405636914a93ef6f8ad00e
llvm-svn: 316413

llvm/include/llvm/CodeGen/AsmPrinter.h

@@ -58,6 +58,7 @@ class MachineModuleInfo;
 class MachineOptimizationRemarkEmitter;
 class MCAsmInfo;
 class MCCFIInstruction;
+struct MCCodePaddingContext;
 class MCContext;
 class MCExpr;
 class MCInst;
@@ -377,7 +378,7 @@ class AsmPrinter : public MachineFunctionPass {
   virtual void EmitBasicBlockStart(const MachineBasicBlock &MBB) const;
 
   /// Targets can override this to emit stuff at the end of a basic block.
-  virtual void EmitBasicBlockEnd(const MachineBasicBlock &MBB) {}
+  virtual void EmitBasicBlockEnd(const MachineBasicBlock &MBB);
 
   /// Targets should implement this to emit instructions.
   virtual void EmitInstruction(const MachineInstr *) {
@@ -626,6 +627,8 @@ class AsmPrinter : public MachineFunctionPass {
   /// Emit GlobalAlias or GlobalIFunc.
   void emitGlobalIndirectSymbol(Module &M,
                                 const GlobalIndirectSymbol& GIS);
+  void setupCodePaddingContext(const MachineBasicBlock &MBB,
+                               MCCodePaddingContext &Context) const;
 };
 
 } // end namespace llvm

llvm/include/llvm/MC/MCAsmBackend.h

@@ -15,26 +15,34 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/MC/MCDirectives.h"
 #include "llvm/MC/MCFixup.h"
+#include "llvm/MC/MCFragment.h"
 #include <cstdint>
+#include <memory>
 
 namespace llvm {
 
 class MCAsmLayout;
 class MCAssembler;
 class MCCFIInstruction;
+class MCCodePadder;
 struct MCFixupKindInfo;
 class MCFragment;
 class MCInst;
+class MCObjectStreamer;
 class MCObjectWriter;
+struct MCCodePaddingContext;
 class MCRelaxableFragment;
 class MCSubtargetInfo;
 class MCValue;
 class raw_pwrite_stream;
 
 /// Generic interface to target specific assembler backends.
 class MCAsmBackend {
+  std::unique_ptr<MCCodePadder> CodePadder;
+
 protected: // Can only create subclasses.
   MCAsmBackend();
+  MCAsmBackend(std::unique_ptr<MCCodePadder> TargetCodePadder);
 
 public:
   MCAsmBackend(const MCAsmBackend &) = delete;
@@ -133,6 +141,40 @@ class MCAsmBackend {
       generateCompactUnwindEncoding(ArrayRef<MCCFIInstruction>) const {
     return 0;
   }
+
+  /// Handles all target related code padding when starting to write a new
+  /// basic block to an object file.
+  ///
+  /// \param OS The streamer used for writing the padding data and function.
+  /// \param Context the context of the padding, Embeds the basic block's
+  /// parameters.
+  void handleCodePaddingBasicBlockStart(MCObjectStreamer *OS,
+                                        const MCCodePaddingContext &Context);
+  /// Handles all target related code padding after writing a block to an object
+  /// file.
+  ///
+  /// \param Context the context of the padding, Embeds the basic block's
+  /// parameters.
+  void handleCodePaddingBasicBlockEnd(const MCCodePaddingContext &Context);
+  /// Handles all target related code padding before writing a new instruction
+  /// to an object file.
+  ///
+  /// \param Inst the instruction.
+  void handleCodePaddingInstructionBegin(const MCInst &Inst);
+  /// Handles all target related code padding after writing an instruction to an
+  /// object file.
+  ///
+  /// \param Inst the instruction.
+  void handleCodePaddingInstructionEnd(const MCInst &Inst);
+
+  /// Relaxes a fragment (changes the size of the padding) according to target
+  /// requirements. The new size computation is done w.r.t a layout.
+  ///
+  /// \param Fragment The fragment to relax.
+  /// \param Layout Code layout information.
+  ///
+  /// \returns true iff any relaxation occured.
+  bool relaxFragment(MCPaddingFragment *PF, MCAsmLayout &Layout);
 };
 
 } // end namespace llvm

llvm/include/llvm/MC/MCAssembler.h

@@ -183,6 +183,8 @@ class MCAssembler {
 
   bool relaxInstruction(MCAsmLayout &Layout, MCRelaxableFragment &IF);
 
+  bool relaxPaddingFragment(MCAsmLayout &Layout, MCPaddingFragment &PF);
+
   bool relaxLEB(MCAsmLayout &Layout, MCLEBFragment &IF);
 
   bool relaxDwarfLineAddr(MCAsmLayout &Layout, MCDwarfLineAddrFragment &DF);

llvm/include/llvm/MC/MCCodePadder.h

@@ -0,0 +1,243 @@
+//===- llvm/MC/CodePadder.h - MC Code Padder --------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_MC_MCCODEPADDER_H
+#define LLVM_MC_MCCODEPADDER_H
+
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+
+namespace llvm {
+
+class MCAsmLayout;
+class MCCodePaddingPolicy;
+class MCFragment;
+class MCInst;
+class MCObjectStreamer;
+class MCPaddingFragment;
+class MCSection;
+
+typedef SmallVector<const MCPaddingFragment *, 8> MCPFRange;
+
+struct MCCodePaddingContext {
+  bool IsPaddingActive;
+  bool IsBasicBlockInsideInnermostLoop;
+  bool IsBasicBlockReachableViaFallthrough;
+  bool IsBasicBlockReachableViaBranch;
+};
+
+/// Target-independent base class incharge of all code padding decisions for a
+/// target. During encoding it determines if and where MCPaddingFragments will
+/// be located, as later on, when layout information is available, it determines
+/// their sizes.
+class MCCodePadder {
+  MCCodePadder(const MCCodePadder &) = delete;
+  void operator=(const MCCodePadder &) = delete;
+
+  /// Determines if the MCCodePaddingPolicies are active.
+  bool ArePoliciesActive;
+
+  /// All the supported MCCodePaddingPolicies.
+  SmallPtrSet<MCCodePaddingPolicy *, 4> CodePaddingPolicies;
+
+  /// A pointer to the fragment of the instruction whose padding is currently
+  /// done for.
+  MCPaddingFragment *CurrHandledInstFragment;
+
+  /// A map holding the jurisdiction for each padding fragment. Key: padding
+  /// fragment. Value: The fragment's jurisdiction. A jurisdiction is a vector
+  /// of padding fragments whose conditions are being controlled by another
+  /// fragment, the key fragment.
+  DenseMap<MCPaddingFragment *, MCPFRange> FragmentToJurisdiction;
+  MCPFRange &getJurisdiction(MCPaddingFragment *Fragment, MCAsmLayout &Layout);
+
+  /// A map holding the maximal instruction window size relevant for a padding
+  /// fragment.
+  DenseMap<MCPaddingFragment *, uint64_t> FragmentToMaxWindowSize;
+  uint64_t getMaxWindowSize(MCPaddingFragment *Fragment, MCAsmLayout &Layout);
+
+protected:
+  /// The current streamer, used to stream code padding.
+  MCObjectStreamer *OS;
+
+  bool addPolicy(MCCodePaddingPolicy *Policy);
+
+  virtual bool
+  basicBlockRequiresInsertionPoint(const MCCodePaddingContext &Context) {
+    return false;
+  }
+
+  virtual bool instructionRequiresInsertionPoint(const MCInst &Inst) {
+    return false;
+  }
+
+  virtual bool usePoliciesForBasicBlock(const MCCodePaddingContext &Context) {
+    return Context.IsPaddingActive;
+  }
+
+public:
+  MCCodePadder()
+      : ArePoliciesActive(false), CurrHandledInstFragment(nullptr),
+        OS(nullptr) {}
+  virtual ~MCCodePadder();
+
+  /// Handles all target related code padding when starting to write a new
+  /// basic block to an object file.
+  ///
+  /// \param OS The streamer used for writing the padding data and function.
+  /// \param Context the context of the padding, Embeds the basic block's
+  /// parameters.
+  void handleBasicBlockStart(MCObjectStreamer *OS,
+                             const MCCodePaddingContext &Context);
+  /// Handles all target related code padding when done writing a block to an
+  /// object file.
+  ///
+  /// \param Context the context of the padding, Embeds the basic block's
+  /// parameters.
+  void handleBasicBlockEnd(const MCCodePaddingContext &Context);
+  /// Handles all target related code padding before writing a new instruction
+  /// to an object file.
+  ///
+  /// \param Inst the instruction.
+  void handleInstructionBegin(const MCInst &Inst);
+  /// Handles all target related code padding after writing an instruction to an
+  /// object file.
+  ///
+  /// \param Inst the instruction.
+  void handleInstructionEnd(const MCInst &Inst);
+
+  /// Relaxes a fragment (changes the size of the padding) according to target
+  /// requirements. The new size computation is done w.r.t a layout.
+  ///
+  /// \param Fragment The fragment to relax.
+  /// \param Layout Code layout information.
+  ///
+  /// \returns true iff any relaxation occured.
+  bool relaxFragment(MCPaddingFragment *Fragment, MCAsmLayout &Layout);
+};
+
+/// The base class for all padding policies, i.e. a rule or set of rules to pad
+/// the generated code.
+class MCCodePaddingPolicy {
+  MCCodePaddingPolicy() = delete;
+  MCCodePaddingPolicy(const MCCodePaddingPolicy &) = delete;
+  void operator=(const MCCodePaddingPolicy &) = delete;
+
+protected:
+  /// A mask holding the kind of this policy, i.e. only the i'th bit will be set
+  /// where i is the kind number.
+  const uint64_t KindMask;
+  /// Instruction window size relevant to this policy.
+  const uint64_t WindowSize;
+  /// A boolean indicating which byte of the instruction determies its
+  /// instruction window. If true - the last byte of the instructions, o.w. -
+  /// the first byte of the instruction.
+  const bool InstByteIsLastByte;
+
+  MCCodePaddingPolicy(uint64_t Kind, uint64_t WindowSize,
+                      bool InstByteIsLastByte)
+      : KindMask(UINT64_C(1) << Kind), WindowSize(WindowSize),
+        InstByteIsLastByte(InstByteIsLastByte) {}
+
+  /// Computes and returns the offset of the consecutive fragment of a given
+  /// fragment.
+  ///
+  /// \param Fragment The fragment whose consecutive offset will be computed.
+  /// \param Layout Code layout information.
+  ///
+  /// \returns the offset of the consecutive fragment of \p Fragment.
+  static uint64_t getNextFragmentOffset(const MCFragment *Fragment,
+                                        const MCAsmLayout &Layout);
+  /// Returns the instruction byte of an instruction pointed by a given
+  /// MCPaddingFragment. An instruction byte is the address of the byte of an
+  /// instruction which determines its instruction window.
+  ///
+  /// \param Fragment The fragment pointing to the instruction.
+  /// \param Layout Code layout information.
+  ///
+  /// \returns the instruction byte of an instruction pointed by \p Fragment.
+  uint64_t getFragmentInstByte(const MCPaddingFragment *Fragment,
+                               MCAsmLayout &Layout) const;
+  uint64_t computeWindowEndAddress(const MCPaddingFragment *Fragment,
+                                   uint64_t Offset, MCAsmLayout &Layout) const;
+
+  /// Computes and returns the penalty weight of a first instruction window in a
+  /// range. This requires a special function since the first window does not
+  /// contain all the padding fragments in that window. It only contains all the
+  /// padding fragments starting from the relevant insertion point.
+  ///
+  /// \param Window The first window.
+  /// \param Offset The offset of the parent section relative to the beginning
+  /// of the file, mod the window size.
+  /// \param Layout Code layout information.
+  ///
+  /// \returns the penalty weight of a first instruction window in a range, \p
+  /// Window.
+  double computeFirstWindowPenaltyWeight(const MCPFRange &Window,
+                                         uint64_t Offset,
+                                         MCAsmLayout &Layout) const;
+  /// Computes and returns the penalty caused by an instruction window.
+  ///
+  /// \param Window The instruction window.
+  /// \param Offset The offset of the parent section relative to the beginning
+  /// of the file, mod the window size.
+  /// \param Layout Code layout information.
+  ///
+  /// \returns the penalty caused by \p Window.
+  virtual double computeWindowPenaltyWeight(const MCPFRange &Window,
+                                            uint64_t Offset,
+                                            MCAsmLayout &Layout) const = 0;
+
+public:
+  virtual ~MCCodePaddingPolicy() {}
+
+  /// Returns the kind mask of this policy -  A mask holding the kind of this
+  /// policy, i.e. only the i'th bit will be set where i is the kind number.
+  uint64_t getKindMask() const { return KindMask; }
+  /// Returns the instruction window size relevant to this policy.
+  uint64_t getWindowSize() const { return WindowSize; }
+  /// Returns true if the last byte of an instruction determines its instruction
+  /// window, or false if the first of an instruction determines it.
+  bool isInstByteLastByte() const { return InstByteIsLastByte; }
+
+  /// Returns true iff this policy needs padding for a given basic block.
+  ///
+  /// \param Context the context of the padding, Embeds the basic block's
+  /// parameters.
+  ///
+  /// \returns true iff this policy needs padding for the basic block.
+  virtual bool
+  basicBlockRequiresPaddingFragment(const MCCodePaddingContext &Context) const {
+    return false;
+  }
+  /// Returns true iff this policy needs padding for a given instruction.
+  ///
+  /// \param Inst The given instruction.
+  ///
+  /// \returns true iff this policy needs padding for \p Inst.
+  virtual bool instructionRequiresPaddingFragment(const MCInst &Inst) const {
+    return false;
+  }
+  /// Computes and returns the penalty caused by a range of instruction windows.
+  /// The weight is computed for each window separelty and then accumulated.
+  ///
+  /// \param Range The range.
+  /// \param Offset The offset of the parent section relative to the beginning
+  /// of the file, mod the window size.
+  /// \param Layout Code layout information.
+  ///
+  /// \returns the penalty caused by \p Range.
+  double computeRangePenaltyWeight(const MCPFRange &Range, uint64_t Offset,
+                                   MCAsmLayout &Layout) const;
+};
+
+} // namespace llvm
+
+#endif // LLVM_MC_MCCODEPADDER_H