llvm/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 "llvm/ADT/Statistic.h"
  13 #include "llvm/CodeGen/MachineFunctionPass.h"
  14 using namespace llvm;
  15
  16 #define DEBUG_TYPE "double barriers"
  17
  18 STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
  19
  20 namespace {
  21 class ARMOptimizeBarriersPass : public MachineFunctionPass {
  22 public:
  23   static char ID;
  24   ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
  25
  26   bool runOnMachineFunction(MachineFunction &Fn) override;
  27
  28   MachineFunctionProperties getRequiredProperties() const override {
  29     return MachineFunctionProperties().set(
  30         MachineFunctionProperties::Property::NoVRegs);
  31   }
  32
  33   StringRef getPassName() const override { return "optimise barriers pass"; }
  34 };
  35 char ARMOptimizeBarriersPass::ID = 0;
  36 }
  37
  38 // Returns whether the instruction can safely move past a DMB instruction
  39 // The current implementation allows this iif MI does not have any possible
  40 // memory access
  41 static bool CanMovePastDMB(const MachineInstr *MI) {
  42   return !(MI->mayLoad() ||
  43           MI->mayStore() ||
  44           MI->hasUnmodeledSideEffects() ||
  45           MI->isCall() ||
  46           MI->isReturn());
  47 }
  48
  49 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
  50   if (skipFunction(MF.getFunction()))
  51     return false;
  52
  53   // Vector to store the DMBs we will remove after the first iteration
  54   std::vector<MachineInstr *> ToRemove;
  55   // DMBType is the Imm value of the first operand. It determines whether it's a
  56   // DMB ish, dmb sy, dmb osh, etc
  57   int64_t DMBType = -1;
  58
  59   // Find a dmb. If we can move it until the next dmb, tag the second one for
  60   // removal
  61   for (auto &MBB : MF) {
  62     // Will be true when we have seen a DMB, and not seen any instruction since
  63     // that cannot move past a DMB
  64     bool IsRemovableNextDMB = false;
  65     for (auto &MI : MBB) {
  66       if (MI.getOpcode() == ARM::DMB) {
  67         if (IsRemovableNextDMB) {
  68           // If the Imm of this DMB is the same as that of the last DMB, we can
  69           // tag this second DMB for removal
  70           if (MI.getOperand(0).getImm() == DMBType) {
  71             ToRemove.push_back(&MI);
  72           } else {
  73             // If it has a different DMBType, we cannot remove it, but will scan
  74             // for the next DMB, recording this DMB's type as last seen DMB type
  75             DMBType = MI.getOperand(0).getImm();
  76           }
  77         } else {
  78           // After we see a DMB, a next one is removable
  79           IsRemovableNextDMB = true;
  80           DMBType = MI.getOperand(0).getImm();
  81         }
  82       } else if (!CanMovePastDMB(&MI)) {
  83         // If we find an instruction unable to pass past a DMB, a next DMB is
  84         // not removable
  85         IsRemovableNextDMB = false;
  86       }
  87     }
  88   }
  89   bool Changed = false;
  90   // Remove the tagged DMB
  91   for (auto *MI : ToRemove) {
  92     MI->eraseFromParent();
  93     ++NumDMBsRemoved;
  94     Changed = true;
  95   }
  96
  97   return Changed;
  98 }
  99
 100 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
 101 /// barriers
 102 /// pass.
 103 FunctionPass *llvm::createARMOptimizeBarriersPass() {
 104   return new ARMOptimizeBarriersPass();
 105 }