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 "ARMInstrInfo.h"
13 #include "ARMMachineFunctionInfo.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;
26   ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
27 
28   bool runOnMachineFunction(MachineFunction &Fn) override;
29 
30   MachineFunctionProperties getRequiredProperties() const override {
31     return MachineFunctionProperties().set(
32         MachineFunctionProperties::Property::NoVRegs);
33   }
34 
35   StringRef getPassName() const override { return "optimise barriers pass"; }
36 };
37 char ARMOptimizeBarriersPass::ID = 0;
38 }
39 
40 // Returns whether the instruction can safely move past a DMB instruction
41 // The current implementation allows this iif MI does not have any possible
42 // memory access
43 static bool CanMovePastDMB(const MachineInstr *MI) {
44   return !(MI->mayLoad() ||
45           MI->mayStore() ||
46           MI->hasUnmodeledSideEffects() ||
47           MI->isCall() ||
48           MI->isReturn());
49 }
50 
51 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
52   if (skipFunction(*MF.getFunction()))
53     return false;
54 
55   // Vector to store the DMBs we will remove after the first iteration
56   std::vector<MachineInstr *> ToRemove;
57   // DMBType is the Imm value of the first operand. It determines whether it's a
58   // DMB ish, dmb sy, dmb osh, etc
59   int64_t DMBType = -1;
60 
61   // Find a dmb. If we can move it until the next dmb, tag the second one for
62   // removal
63   for (auto &MBB : MF) {
64     // Will be true when we have seen a DMB, and not seen any instruction since
65     // that cannot move past a DMB
66     bool IsRemovableNextDMB = false;
67     for (auto &MI : MBB) {
68       if (MI.getOpcode() == ARM::DMB) {
69         if (IsRemovableNextDMB) {
70           // If the Imm of this DMB is the same as that of the last DMB, we can
71           // tag this second DMB for removal
72           if (MI.getOperand(0).getImm() == DMBType) {
73             ToRemove.push_back(&MI);
74           } else {
75             // If it has a different DMBType, we cannot remove it, but will scan
76             // for the next DMB, recording this DMB's type as last seen DMB type
77             DMBType = MI.getOperand(0).getImm();
78           }
79         } else {
80           // After we see a DMB, a next one is removable
81           IsRemovableNextDMB = true;
82           DMBType = MI.getOperand(0).getImm();
83         }
84       } else if (!CanMovePastDMB(&MI)) {
85         // If we find an instruction unable to pass past a DMB, a next DMB is
86         // not removable
87         IsRemovableNextDMB = false;
88       }
89     }
90   }
91   // Remove the tagged DMB
92   for (auto MI : ToRemove) {
93     MI->eraseFromParent();
94     ++NumDMBsRemoved;
95   }
96 
97   return NumDMBsRemoved > 0;
98 }
99 
100 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
101 /// barriers
102 /// pass.
103 FunctionPass *llvm::createARMOptimizeBarriersPass() {
104   return new ARMOptimizeBarriersPass();
105 }
106