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