1 //===-- ARMOptimizeBarriersPass - two DMBs without a memory access in between,
4 // The LLVM Compiler Infrastructure
6 // This file is distributed under the University of Illinois Open Source
7 // License. See LICENSE.TXT for details.
9 //===------------------------------------------------------------------------------------------===//
12 #include "ARMInstrInfo.h"
13 #include "ARMMachineFunctionInfo.h"
14 #include "llvm/ADT/Statistic.h"
15 #include "llvm/CodeGen/MachineFunctionPass.h"
18 #define DEBUG_TYPE "double barriers"
20 STATISTIC(NumDMBsRemoved, "Number of DMBs removed");
23 class ARMOptimizeBarriersPass : public MachineFunctionPass {
26 ARMOptimizeBarriersPass() : MachineFunctionPass(ID) {}
28 bool runOnMachineFunction(MachineFunction &Fn) override;
30 MachineFunctionProperties getRequiredProperties() const override {
31 return MachineFunctionProperties().set(
32 MachineFunctionProperties::Property::AllVRegsAllocated);
35 const char *getPassName() const override {
36 return "optimise barriers pass";
39 char ARMOptimizeBarriersPass::ID = 0;
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
45 static bool CanMovePastDMB(const MachineInstr *MI) {
46 return !(MI->mayLoad() ||
48 MI->hasUnmodeledSideEffects() ||
53 bool ARMOptimizeBarriersPass::runOnMachineFunction(MachineFunction &MF) {
54 if (skipFunction(*MF.getFunction()))
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
63 // Find a dmb. If we can move it until the next dmb, tag the second one for
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);
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();
82 // After we see a DMB, a next one is removable
83 IsRemovableNextDMB = true;
84 DMBType = MI.getOperand(0).getImm();
86 } else if (!CanMovePastDMB(&MI)) {
87 // If we find an instruction unable to pass past a DMB, a next DMB is
89 IsRemovableNextDMB = false;
93 // Remove the tagged DMB
94 for (auto MI : ToRemove) {
95 MI->eraseFromParent();
99 return NumDMBsRemoved > 0;
102 /// createARMOptimizeBarriersPass - Returns an instance of the remove double
105 FunctionPass *llvm::createARMOptimizeBarriersPass() {
106 return new ARMOptimizeBarriersPass();