1 //===-------- X86PadShortFunction.cpp - pad short functions -----------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the pass which will pad short functions to prevent
11 // a stall if a function returns before the return address is ready. This
12 // is needed for some Intel Atom processors.
14 //===----------------------------------------------------------------------===//
19 #include "X86InstrInfo.h"
20 #include "X86Subtarget.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/CodeGen/MachineInstrBuilder.h"
24 #include "llvm/CodeGen/MachineRegisterInfo.h"
25 #include "llvm/CodeGen/Passes.h"
26 #include "llvm/IR/Function.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/raw_ostream.h"
29 #include "llvm/Target/TargetInstrInfo.h"
33 #define DEBUG_TYPE "x86-pad-short-functions"
35 STATISTIC(NumBBsPadded, "Number of basic blocks padded");
38 struct VisitedBBInfo {
39 // HasReturn - Whether the BB contains a return instruction
42 // Cycles - Number of cycles until return if HasReturn is true, otherwise
43 // number of cycles until end of the BB
46 VisitedBBInfo() : HasReturn(false), Cycles(0) {}
47 VisitedBBInfo(bool HasReturn, unsigned int Cycles)
48 : HasReturn(HasReturn), Cycles(Cycles) {}
51 struct PadShortFunc : public MachineFunctionPass {
53 PadShortFunc() : MachineFunctionPass(ID)
54 , Threshold(4), STI(nullptr), TII(nullptr) {}
56 bool runOnMachineFunction(MachineFunction &MF) override;
58 MachineFunctionProperties getRequiredProperties() const override {
59 return MachineFunctionProperties().set(
60 MachineFunctionProperties::Property::AllVRegsAllocated);
63 const char *getPassName() const override {
64 return "X86 Atom pad short functions";
68 void findReturns(MachineBasicBlock *MBB,
69 unsigned int Cycles = 0);
71 bool cyclesUntilReturn(MachineBasicBlock *MBB,
72 unsigned int &Cycles);
74 void addPadding(MachineBasicBlock *MBB,
75 MachineBasicBlock::iterator &MBBI,
76 unsigned int NOOPsToAdd);
78 const unsigned int Threshold;
80 // ReturnBBs - Maps basic blocks that return to the minimum number of
81 // cycles until the return, starting from the entry block.
82 DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
84 // VisitedBBs - Cache of previously visited BBs.
85 DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
87 const X86Subtarget *STI;
88 const TargetInstrInfo *TII;
91 char PadShortFunc::ID = 0;
94 FunctionPass *llvm::createX86PadShortFunctions() {
95 return new PadShortFunc();
98 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
99 /// NOOP instructions before early exits.
100 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
101 if (skipFunction(*MF.getFunction()))
104 if (MF.getFunction()->optForSize()) {
108 STI = &MF.getSubtarget<X86Subtarget>();
109 if (!STI->padShortFunctions())
112 TII = STI->getInstrInfo();
114 // Search through basic blocks and mark the ones that have early returns
117 findReturns(&MF.front());
119 bool MadeChange = false;
121 MachineBasicBlock *MBB;
122 unsigned int Cycles = 0;
124 // Pad the identified basic blocks with NOOPs
125 for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
126 I != ReturnBBs.end(); ++I) {
130 if (Cycles < Threshold) {
131 // BB ends in a return. Skip over any DBG_VALUE instructions
132 // trailing the terminator.
133 assert(MBB->size() > 0 &&
134 "Basic block should contain at least a RET but is empty");
135 MachineBasicBlock::iterator ReturnLoc = --MBB->end();
137 while (ReturnLoc->isDebugValue())
139 assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
140 "Basic block does not end with RET");
142 addPadding(MBB, ReturnLoc, Threshold - Cycles);
151 /// findReturn - Starting at MBB, follow control flow and add all
152 /// basic blocks that contain a return to ReturnBBs.
153 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
154 // If this BB has a return, note how many cycles it takes to get there.
155 bool hasReturn = cyclesUntilReturn(MBB, Cycles);
156 if (Cycles >= Threshold)
160 ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
164 // Follow branches in BB and look for returns
165 for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
166 I != MBB->succ_end(); ++I) {
169 findReturns(*I, Cycles);
173 /// cyclesUntilReturn - return true if the MBB has a return instruction,
174 /// and return false otherwise.
175 /// Cycles will be incremented by the number of cycles taken to reach the
176 /// return or the end of the BB, whichever occurs first.
177 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
178 unsigned int &Cycles) {
179 // Return cached result if BB was previously visited
180 DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
181 = VisitedBBs.find(MBB);
182 if (it != VisitedBBs.end()) {
183 VisitedBBInfo BBInfo = it->second;
184 Cycles += BBInfo.Cycles;
185 return BBInfo.HasReturn;
188 unsigned int CyclesToEnd = 0;
190 for (MachineInstr &MI : *MBB) {
191 // Mark basic blocks with a return instruction. Calls to other
192 // functions do not count because the called function will be padded,
194 if (MI.isReturn() && !MI.isCall()) {
195 VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
196 Cycles += CyclesToEnd;
200 CyclesToEnd += TII->getInstrLatency(STI->getInstrItineraryData(), MI);
203 VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
204 Cycles += CyclesToEnd;
208 /// addPadding - Add the given number of NOOP instructions to the function
209 /// just prior to the return at MBBI
210 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
211 MachineBasicBlock::iterator &MBBI,
212 unsigned int NOOPsToAdd) {
213 DebugLoc DL = MBBI->getDebugLoc();
215 while (NOOPsToAdd-- > 0) {
216 BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
217 BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));