1 //===-- WebAssemblyArgumentMove.cpp - Argument instruction moving ---------===//
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 This file moves ARGUMENT instructions after ScheduleDAG scheduling.
12 ///
13 /// Arguments are really live-in registers, however, since we use virtual
14 /// registers and LLVM doesn't support live-in virtual registers, we're
15 /// currently making do with ARGUMENT instructions which are placed at the top
16 /// of the entry block. The trick is to get them to *stay* at the top of the
17 /// entry block.
18 ///
19 /// The ARGUMENTS physical register keeps these instructions pinned in place
20 /// during liveness-aware CodeGen passes, however one thing which does not
21 /// respect this is the ScheduleDAG scheduler. This pass is therefore run
22 /// immediately after that.
23 ///
24 /// This is all hopefully a temporary solution until we find a better solution
25 /// for describing the live-in nature of arguments.
26 ///
27 //===----------------------------------------------------------------------===//
28 
29 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
30 #include "WebAssembly.h"
31 #include "WebAssemblyMachineFunctionInfo.h"
32 #include "WebAssemblySubtarget.h"
33 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
34 #include "llvm/CodeGen/MachineRegisterInfo.h"
35 #include "llvm/CodeGen/Passes.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/raw_ostream.h"
38 using namespace llvm;
39 
40 #define DEBUG_TYPE "wasm-argument-move"
41 
42 namespace {
43 class WebAssemblyArgumentMove final : public MachineFunctionPass {
44 public:
45   static char ID; // Pass identification, replacement for typeid
46   WebAssemblyArgumentMove() : MachineFunctionPass(ID) {}
47 
48   const char *getPassName() const override {
49     return "WebAssembly Argument Move";
50   }
51 
52   void getAnalysisUsage(AnalysisUsage &AU) const override {
53     AU.setPreservesCFG();
54     AU.addPreserved<MachineBlockFrequencyInfo>();
55     AU.addPreservedID(MachineDominatorsID);
56     MachineFunctionPass::getAnalysisUsage(AU);
57   }
58 
59   bool runOnMachineFunction(MachineFunction &MF) override;
60 };
61 } // end anonymous namespace
62 
63 char WebAssemblyArgumentMove::ID = 0;
64 FunctionPass *llvm::createWebAssemblyArgumentMove() {
65   return new WebAssemblyArgumentMove();
66 }
67 
68 /// Test whether the given instruction is an ARGUMENT.
69 static bool IsArgument(const MachineInstr &MI) {
70   switch (MI.getOpcode()) {
71   case WebAssembly::ARGUMENT_I32:
72   case WebAssembly::ARGUMENT_I64:
73   case WebAssembly::ARGUMENT_F32:
74   case WebAssembly::ARGUMENT_F64:
75   case WebAssembly::ARGUMENT_v16i8:
76   case WebAssembly::ARGUMENT_v8i16:
77   case WebAssembly::ARGUMENT_v4i32:
78   case WebAssembly::ARGUMENT_v4f32:
79     return true;
80   default:
81     return false;
82   }
83 }
84 
85 bool WebAssemblyArgumentMove::runOnMachineFunction(MachineFunction &MF) {
86   DEBUG({
87     dbgs() << "********** Argument Move **********\n"
88            << "********** Function: " << MF.getName() << '\n';
89   });
90 
91   bool Changed = false;
92   MachineBasicBlock &EntryMBB = MF.front();
93   MachineBasicBlock::iterator InsertPt = EntryMBB.end();
94 
95   // Look for the first NonArg instruction.
96   for (MachineInstr &MI : EntryMBB) {
97     if (!IsArgument(MI)) {
98       InsertPt = MI;
99       break;
100     }
101   }
102 
103   // Now move any argument instructions later in the block
104   // to before our first NonArg instruction.
105   for (MachineInstr &MI : llvm::make_range(InsertPt, EntryMBB.end())) {
106     if (IsArgument(MI)) {
107       EntryMBB.insert(InsertPt, MI.removeFromParent());
108       Changed = true;
109     }
110   }
111 
112   return Changed;
113 }
114