WebAssemblyArgumentMove.cpp revision 341825 
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/// 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 "WebAssemblyUtilities.h"
34#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
35#include "llvm/CodeGen/MachineRegisterInfo.h"
36#include "llvm/CodeGen/Passes.h"
37#include "llvm/Support/Debug.h"
38#include "llvm/Support/raw_ostream.h"
39using namespace llvm;
40
41#define DEBUG_TYPE "wasm-argument-move"
42
43namespace {
44class WebAssemblyArgumentMove final : public MachineFunctionPass {
45public:
46  static char ID; // Pass identification, replacement for typeid
47  WebAssemblyArgumentMove() : MachineFunctionPass(ID) {}
48
49  StringRef getPassName() const override { return "WebAssembly Argument Move"; }
50
51  void getAnalysisUsage(AnalysisUsage &AU) const override {
52    AU.setPreservesCFG();
53    AU.addPreserved<MachineBlockFrequencyInfo>();
54    AU.addPreservedID(MachineDominatorsID);
55    MachineFunctionPass::getAnalysisUsage(AU);
56  }
57
58  bool runOnMachineFunction(MachineFunction &MF) override;
59};
60} // end anonymous namespace
61
62char WebAssemblyArgumentMove::ID = 0;
63INITIALIZE_PASS(WebAssemblyArgumentMove, DEBUG_TYPE,
64                "Move ARGUMENT instructions for WebAssembly", false, false)
65
66FunctionPass *llvm::createWebAssemblyArgumentMove() {
67  return new WebAssemblyArgumentMove();
68}
69
70bool WebAssemblyArgumentMove::runOnMachineFunction(MachineFunction &MF) {
71  LLVM_DEBUG({
72    dbgs() << "********** Argument Move **********\n"
73           << "********** Function: " << MF.getName() << '\n';
74  });
75
76  bool Changed = false;
77  MachineBasicBlock &EntryMBB = MF.front();
78  MachineBasicBlock::iterator InsertPt = EntryMBB.end();
79
80  // Look for the first NonArg instruction.
81  for (MachineInstr &MI : EntryMBB) {
82    if (!WebAssembly::isArgument(MI)) {
83      InsertPt = MI;
84      break;
85    }
86  }
87
88  // Now move any argument instructions later in the block
89  // to before our first NonArg instruction.
90  for (MachineInstr &MI : llvm::make_range(InsertPt, EntryMBB.end())) {
91    if (WebAssembly::isArgument(MI)) {
92      EntryMBB.insert(InsertPt, MI.removeFromParent());
93      Changed = true;
94    }
95  }
96
97  return Changed;
98}
99