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1 //===-- ARMOptimizeBarriersPass - two DMBs without a memory access in between,
2 //removed one -===//
3 //
4 //                     The LLVM Compiler Infrastructure
5 //
6 // This file is distributed under the University of Illinois Open Source
7 // License. See LICENSE.TXT for details.
8 //
9 //===------------------------------------------------------------------------------------------===//
10 
11 #include "ARM.h"
12 #include "ARMMachineFunctionInfo.h"
13 #include "ARMInstrInfo.h"
14 #include "llvm/ADT/Statistic.h"
15 #include "llvm/CodeGen/MachineFunctionPass.h"
16 using namespace llvm;
17 
18 #define DEBUG_TYPE "double barriers"
19 
20 STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
21 
22 namespace {
23 class ARMOptimizeBarriersPass : public MachineFunctionPass {
24 public:
25   static char ID;
ARMOptimizeBarriersPass()26   ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
27 
28   bool runOnMachineFunction(MachineFunction &Fn) override;
29 
getPassName() const30   const char *getPassName() const override {
31     return "optimise barriers pass";
32   }
33 
34 private:
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
CanMovePastDMB(const MachineInstr * MI)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 
runOnMachineFunction(MachineFunction & MF)50 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
51   // Vector to store the DMBs we will remove after the first iteration
52   std::vector<MachineInstr *> ToRemove;
53   // DMBType is the Imm value of the first operand. It determines whether it's a
54   // DMB ish, dmb sy, dmb osh, etc
55   int64_t DMBType = -1;
56 
57   // Find a dmb. If we can move it until the next dmb, tag the second one for
58   // removal
59   for (auto &MBB : MF) {
60     // Will be true when we have seen a DMB, and not seen any instruction since
61     // that cannot move past a DMB
62     bool IsRemovableNextDMB = false;
63     for (auto &MI : MBB) {
64       if (MI.getOpcode() == ARM::DMB) {
65         if (IsRemovableNextDMB) {
66           // If the Imm of this DMB is the same as that of the last DMB, we can
67           // tag this second DMB for removal
68           if (MI.getOperand(0).getImm() == DMBType) {
69             ToRemove.push_back(&MI);
70           } else {
71             // If it has a different DMBType, we cannot remove it, but will scan
72             // for the next DMB, recording this DMB's type as last seen DMB type
73             DMBType = MI.getOperand(0).getImm();
74           }
75         } else {
76           // After we see a DMB, a next one is removable
77           IsRemovableNextDMB = true;
78           DMBType = MI.getOperand(0).getImm();
79         }
80       } else if (!CanMovePastDMB(&MI)) {
81         // If we find an instruction unable to pass past a DMB, a next DMB is
82         // not removable
83         IsRemovableNextDMB = false;
84       }
85     }
86   }
87   // Remove the tagged DMB
88   for (auto MI : ToRemove) {
89     MI->eraseFromParent();
90     ++NumDMBsRemoved;
91   }
92 
93   return NumDMBsRemoved > 0;
94 }
95 
96 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
97 /// barriers
98 /// pass.
createARMOptimizeBarriersPass()99 FunctionPass *llvm::createARMOptimizeBarriersPass() {
100   return new ARMOptimizeBarriersPass();
101 }
102