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::AllVRegsAllocated);
33   }
34 
35   const char *getPassName() const override {
36     return "optimise barriers pass";
37   }
38 };
39 char ARMOptimizeBarriersPass::ID = 0;
40 }
41 
42 // Returns whether the instruction can safely move past a DMB instruction
43 // The current implementation allows this iif MI does not have any possible
44 // memory access
45 static bool CanMovePastDMB(const MachineInstr *MI) {
46   return !(MI->mayLoad() ||
47           MI->mayStore() ||
48           MI->hasUnmodeledSideEffects() ||
49           MI->isCall() ||
50           MI->isReturn());
51 }
52 
53 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
54   if (skipFunction(*MF.getFunction()))
55     return false;
56 
57   // Vector to store the DMBs we will remove after the first iteration
58   std::vector<MachineInstr *> ToRemove;
59   // DMBType is the Imm value of the first operand. It determines whether it's a
60   // DMB ish, dmb sy, dmb osh, etc
61   int64_t DMBType = -1;
62 
63   // Find a dmb. If we can move it until the next dmb, tag the second one for
64   // removal
65   for (auto &MBB : MF) {
66     // Will be true when we have seen a DMB, and not seen any instruction since
67     // that cannot move past a DMB
68     bool IsRemovableNextDMB = false;
69     for (auto &MI : MBB) {
70       if (MI.getOpcode() == ARM::DMB) {
71         if (IsRemovableNextDMB) {
72           // If the Imm of this DMB is the same as that of the last DMB, we can
73           // tag this second DMB for removal
74           if (MI.getOperand(0).getImm() == DMBType) {
75             ToRemove.push_back(&MI);
76           } else {
77             // If it has a different DMBType, we cannot remove it, but will scan
78             // for the next DMB, recording this DMB's type as last seen DMB type
79             DMBType = MI.getOperand(0).getImm();
80           }
81         } else {
82           // After we see a DMB, a next one is removable
83           IsRemovableNextDMB = true;
84           DMBType = MI.getOperand(0).getImm();
85         }
86       } else if (!CanMovePastDMB(&MI)) {
87         // If we find an instruction unable to pass past a DMB, a next DMB is
88         // not removable
89         IsRemovableNextDMB = false;
90       }
91     }
92   }
93   // Remove the tagged DMB
94   for (auto MI : ToRemove) {
95     MI->eraseFromParent();
96     ++NumDMBsRemoved;
97   }
98 
99   return NumDMBsRemoved > 0;
100 }
101 
102 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
103 /// barriers
104 /// pass.
105 FunctionPass *llvm::createARMOptimizeBarriersPass() {
106   return new ARMOptimizeBarriersPass();
107 }
108