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
41   WebAssemblyPrepareForLiveIntervals() : MachineFunctionPass(ID) {}
42 
43 private:
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 
57 char WebAssemblyPrepareForLiveIntervals::ID = 0;
58 INITIALIZE_PASS(WebAssemblyPrepareForLiveIntervals, DEBUG_TYPE,
59                 "Fix up code for LiveIntervals", false, false)
60 
61 FunctionPass *llvm::createWebAssemblyPrepareForLiveIntervals() {
62   return new WebAssemblyPrepareForLiveIntervals();
63 }
64 
65 // Test whether the given register has an ARGUMENT def.
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 
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