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(MachineFunction &MF) {
74   LLVM_DEBUG({
75     dbgs() << "********** Prepare For LiveIntervals **********\n"
76            << "********** Function: " << MF.getName() << '\n';
77   });
78 
79   bool Changed = false;
80   MachineRegisterInfo &MRI = MF.getRegInfo();
81   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
82   MachineBasicBlock &Entry = *MF.begin();
83 
84   assert(!mustPreserveAnalysisID(LiveIntervalsID) &&
85          "LiveIntervals shouldn't be active yet!");
86 
87   // We don't preserve SSA form.
88   MRI.leaveSSA();
89 
90   // BranchFolding and perhaps other passes don't preserve IMPLICIT_DEF
91   // instructions. LiveIntervals requires that all paths to virtual register
92   // uses provide a definition. Insert IMPLICIT_DEFs in the entry block to
93   // conservatively satisfy this.
94   //
95   // TODO: This is fairly heavy-handed; find a better approach.
96   //
97   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i < e; ++i) {
98     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
99 
100     // Skip unused registers.
101     if (MRI.use_nodbg_empty(Reg))
102       continue;
103 
104     // Skip registers that have an ARGUMENT definition.
105     if (HasArgumentDef(Reg, MRI))
106       continue;
107 
108     BuildMI(Entry, Entry.begin(), DebugLoc(),
109             TII.get(WebAssembly::IMPLICIT_DEF), Reg);
110     Changed = true;
111   }
112 
113   // Move ARGUMENT_* instructions to the top of the entry block, so that their
114   // liveness reflects the fact that these really are live-in values.
115   for (auto MII = Entry.begin(), MIE = Entry.end(); MII != MIE; ) {
116     MachineInstr &MI = *MII++;
117     if (WebAssembly::isArgument(MI)) {
118       MI.removeFromParent();
119       Entry.insert(Entry.begin(), &MI);
120     }
121   }
122 
123   // Ok, we're now ready to run the LiveIntervals analysis again.
124   MF.getProperties().set(MachineFunctionProperties::Property::TracksLiveness);
125 
126   return Changed;
127 }
128