lib/Target/ARM/ARMOptimizeBarriersPass.cpp

   1 //===-- ARMOptimizeBarriersPass - two DMBs without a memory access in between,
   2 //removed one -===//
   3 //
   4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   5 // See https://llvm.org/LICENSE.txt for license information.
   6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   7 //
   8 //===------------------------------------------------------------------------------------------===//
   9
  10 #include "ARM.h"
  11 #include "ARMInstrInfo.h"
  12 #include "ARMMachineFunctionInfo.h"
  13 #include "llvm/ADT/Statistic.h"
  14 #include "llvm/CodeGen/MachineFunctionPass.h"
  15 using namespace llvm;
  16
  17 #define DEBUG_TYPE "double barriers"
  18
  19 STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
  20
  21 namespace {
  22 class ARMOptimizeBarriersPass : public MachineFunctionPass {
  23 public:
  24   static char ID;
  25   ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
  26
  27   bool runOnMachineFunction(MachineFunction &Fn) override;
  28
  29   MachineFunctionProperties getRequiredProperties() const override {
  30     return MachineFunctionProperties().set(
  31         MachineFunctionProperties::Property::NoVRegs);
  32   }
  33
  34   StringRef getPassName() const override { return "optimise barriers pass"; }
  35 };
  36 char ARMOptimizeBarriersPass::ID = 0;
  37 }
  38
  39 // Returns whether the instruction can safely move past a DMB instruction
  40 // The current implementation allows this iif MI does not have any possible
  41 // memory access
  42 static bool CanMovePastDMB(const MachineInstr *MI) {
  43   return !(MI->mayLoad() ||
  44           MI->mayStore() ||
  45           MI->hasUnmodeledSideEffects() ||
  46           MI->isCall() ||
  47           MI->isReturn());
  48 }
  49
  50 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
  51   if (skipFunction(MF.getFunction()))
  52     return false;
  53
  54   // Vector to store the DMBs we will remove after the first iteration
  55   std::vector<MachineInstr *> ToRemove;
  56   // DMBType is the Imm value of the first operand. It determines whether it's a
  57   // DMB ish, dmb sy, dmb osh, etc
  58   int64_t DMBType = -1;
  59
  60   // Find a dmb. If we can move it until the next dmb, tag the second one for
  61   // removal
  62   for (auto &MBB : MF) {
  63     // Will be true when we have seen a DMB, and not seen any instruction since
  64     // that cannot move past a DMB
  65     bool IsRemovableNextDMB = false;
  66     for (auto &MI : MBB) {
  67       if (MI.getOpcode() == ARM::DMB) {
  68         if (IsRemovableNextDMB) {
  69           // If the Imm of this DMB is the same as that of the last DMB, we can
  70           // tag this second DMB for removal
  71           if (MI.getOperand(0).getImm() == DMBType) {
  72             ToRemove.push_back(&MI);
  73           } else {
  74             // If it has a different DMBType, we cannot remove it, but will scan
  75             // for the next DMB, recording this DMB's type as last seen DMB type
  76             DMBType = MI.getOperand(0).getImm();
  77           }
  78         } else {
  79           // After we see a DMB, a next one is removable
  80           IsRemovableNextDMB = true;
  81           DMBType = MI.getOperand(0).getImm();
  82         }
  83       } else if (!CanMovePastDMB(&MI)) {
  84         // If we find an instruction unable to pass past a DMB, a next DMB is
  85         // not removable
  86         IsRemovableNextDMB = false;
  87       }
  88     }
  89   }
  90   bool Changed = false;
  91   // Remove the tagged DMB
  92   for (auto MI : ToRemove) {
  93     MI->eraseFromParent();
  94     ++NumDMBsRemoved;
  95     Changed = true;
  96   }
  97
  98   return Changed;
  99 }
 100
 101 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
 102 /// barriers
 103 /// pass.
 104 FunctionPass *llvm::createARMOptimizeBarriersPass() {
 105   return new ARMOptimizeBarriersPass();
 106 }