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 /// 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 "MCTargetDesc/WebAssemblyMCTargetDesc.h"
23 #include "WebAssembly.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"
33 using namespace llvm;
34
35 #define DEBUG_TYPE "wasm-prepare-for-live-intervals"
36
37 namespace {
38 class WebAssemblyPrepareForLiveIntervals final : public MachineFunctionPass {
39 public:
40 static char ID; // Pass identification, replacement for typeid
WebAssemblyPrepareForLiveIntervals()41 WebAssemblyPrepareForLiveIntervals() : MachineFunctionPass(ID) {}
42
43 private:
getPassName() const44 StringRef getPassName() const override {
45 return "WebAssembly Prepare For LiveIntervals";
46 }
47
getAnalysisUsage(AnalysisUsage & AU) const48 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
57 char WebAssemblyPrepareForLiveIntervals::ID = 0;
58 INITIALIZE_PASS(WebAssemblyPrepareForLiveIntervals, DEBUG_TYPE,
59 "Fix up code for LiveIntervals", false, false)
60
createWebAssemblyPrepareForLiveIntervals()61 FunctionPass *llvm::createWebAssemblyPrepareForLiveIntervals() {
62 return new WebAssemblyPrepareForLiveIntervals();
63 }
64
65 // Test whether the given register has an ARGUMENT def.
HasArgumentDef(unsigned Reg,const MachineRegisterInfo & MRI)66 static bool HasArgumentDef(unsigned Reg, const MachineRegisterInfo &MRI) {
67 for (const auto &Def : MRI.def_instructions(Reg))
68 if (WebAssembly::isArgument(Def))
69 return true;
70 return false;
71 }
72
runOnMachineFunction(MachineFunction & MF)73 bool WebAssemblyPrepareForLiveIntervals::runOnMachineFunction(
74 MachineFunction &MF) {
75 LLVM_DEBUG({
76 dbgs() << "********** Prepare For LiveIntervals **********\n"
77 << "********** Function: " << MF.getName() << '\n';
78 });
79
80 bool Changed = false;
81 MachineRegisterInfo &MRI = MF.getRegInfo();
82 const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
83 MachineBasicBlock &Entry = *MF.begin();
84
85 assert(!mustPreserveAnalysisID(LiveIntervalsID) &&
86 "LiveIntervals shouldn't be active yet!");
87
88 // We don't preserve SSA form.
89 MRI.leaveSSA();
90
91 // BranchFolding and perhaps other passes don't preserve IMPLICIT_DEF
92 // instructions. LiveIntervals requires that all paths to virtual register
93 // uses provide a definition. Insert IMPLICIT_DEFs in the entry block to
94 // conservatively satisfy this.
95 //
96 // TODO: This is fairly heavy-handed; find a better approach.
97 //
98 for (unsigned i = 0, e = MRI.getNumVirtRegs(); i < e; ++i) {
99 unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
100
101 // Skip unused registers.
102 if (MRI.use_nodbg_empty(Reg))
103 continue;
104
105 // Skip registers that have an ARGUMENT definition.
106 if (HasArgumentDef(Reg, MRI))
107 continue;
108
109 BuildMI(Entry, Entry.begin(), DebugLoc(),
110 TII.get(WebAssembly::IMPLICIT_DEF), Reg);
111 Changed = true;
112 }
113
114 // Move ARGUMENT_* instructions to the top of the entry block, so that their
115 // liveness reflects the fact that these really are live-in values.
116 for (auto MII = Entry.begin(), MIE = Entry.end(); MII != MIE;) {
117 MachineInstr &MI = *MII++;
118 if (WebAssembly::isArgument(MI)) {
119 MI.removeFromParent();
120 Entry.insert(Entry.begin(), &MI);
121 }
122 }
123
124 // Ok, we're now ready to run the LiveIntervals analysis again.
125 MF.getProperties().set(MachineFunctionProperties::Property::TracksLiveness);
126
127 return Changed;
128 }
129