WebAssemblyPrepareForLiveIntervals.cpp revision 314564 
1//===- WebAssemblyPrepareForLiveIntervals.cpp - Prepare for LiveIntervals -===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9///
10/// \file
11/// \brief Fix up code to meet LiveInterval's requirements.
12///
13/// Some CodeGen passes don't preserve LiveInterval's requirements, because
14/// they run after register allocation and it isn't important. However,
15/// WebAssembly runs LiveIntervals in a late pass. This pass transforms code
16/// to meet LiveIntervals' requirements; primarily, it ensures that all
17/// virtual register uses have definitions (IMPLICIT_DEF definitions if
18/// nothing else).
19///
20//===----------------------------------------------------------------------===//
21
22#include "WebAssembly.h"
23#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
24#include "WebAssemblyMachineFunctionInfo.h"
25#include "WebAssemblySubtarget.h"
26#include "WebAssemblyUtilities.h"
27#include "llvm/CodeGen/MachineFunctionPass.h"
28#include "llvm/CodeGen/MachineInstrBuilder.h"
29#include "llvm/CodeGen/MachineRegisterInfo.h"
30#include "llvm/CodeGen/Passes.h"
31#include "llvm/Support/Debug.h"
32#include "llvm/Support/raw_ostream.h"
33using namespace llvm;
34
35#define DEBUG_TYPE "wasm-prepare-for-live-intervals"
36
37namespace {
38class WebAssemblyPrepareForLiveIntervals final : public MachineFunctionPass {
39public:
40  static char ID; // Pass identification, replacement for typeid
41  WebAssemblyPrepareForLiveIntervals() : MachineFunctionPass(ID) {}
42
43private:
44  StringRef getPassName() const override {
45    return "WebAssembly Prepare For LiveIntervals";
46  }
47
48  void getAnalysisUsage(AnalysisUsage &AU) const override {
49    AU.setPreservesCFG();
50    MachineFunctionPass::getAnalysisUsage(AU);
51  }
52
53  bool runOnMachineFunction(MachineFunction &MF) override;
54};
55} // end anonymous namespace
56
57char WebAssemblyPrepareForLiveIntervals::ID = 0;
58FunctionPass *llvm::createWebAssemblyPrepareForLiveIntervals() {
59  return new WebAssemblyPrepareForLiveIntervals();
60}
61
62// Test whether the given register has an ARGUMENT def.
63static bool HasArgumentDef(unsigned Reg, const MachineRegisterInfo &MRI) {
64  for (const auto &Def : MRI.def_instructions(Reg))
65    if (WebAssembly::isArgument(Def))
66      return true;
67  return false;
68}
69
70bool WebAssemblyPrepareForLiveIntervals::runOnMachineFunction(MachineFunction &MF) {
71  DEBUG({
72    dbgs() << "********** Prepare For LiveIntervals **********\n"
73           << "********** Function: " << MF.getName() << '\n';
74  });
75
76  bool Changed = false;
77  MachineRegisterInfo &MRI = MF.getRegInfo();
78  const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
79  MachineBasicBlock &Entry = *MF.begin();
80
81  assert(!mustPreserveAnalysisID(LiveIntervalsID) &&
82         "LiveIntervals shouldn't be active yet!");
83
84  // We don't preserve SSA form.
85  MRI.leaveSSA();
86
87  // BranchFolding and perhaps other passes don't preserve IMPLICIT_DEF
88  // instructions. LiveIntervals requires that all paths to virtual register
89  // uses provide a definition. Insert IMPLICIT_DEFs in the entry block to
90  // conservatively satisfy this.
91  //
92  // TODO: This is fairly heavy-handed; find a better approach.
93  //
94  for (unsigned i = 0, e = MRI.getNumVirtRegs(); i < e; ++i) {
95    unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
96
97    // Skip unused registers.
98    if (MRI.use_nodbg_empty(Reg))
99      continue;
100
101    // Skip registers that have an ARGUMENT definition.
102    if (HasArgumentDef(Reg, MRI))
103      continue;
104
105    BuildMI(Entry, Entry.begin(), DebugLoc(),
106            TII.get(WebAssembly::IMPLICIT_DEF), Reg);
107    Changed = true;
108  }
109
110  // Move ARGUMENT_* instructions to the top of the entry block, so that their
111  // liveness reflects the fact that these really are live-in values.
112  for (auto MII = Entry.begin(), MIE = Entry.end(); MII != MIE; ) {
113    MachineInstr &MI = *MII++;
114    if (WebAssembly::isArgument(MI)) {
115      MI.removeFromParent();
116      Entry.insert(Entry.begin(), &MI);
117    }
118  }
119
120  // Ok, we're now ready to run LiveIntervalAnalysis again.
121  MF.getProperties().set(MachineFunctionProperties::Property::TracksLiveness);
122
123  return Changed;
124}
125