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