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      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 "WebAssembly.h"
     30 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
     31 #include "WebAssemblyMachineFunctionInfo.h"
     32 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
     33 #include "llvm/CodeGen/MachineRegisterInfo.h"
     34 #include "llvm/CodeGen/Passes.h"
     35 #include "llvm/Support/Debug.h"
     36 #include "llvm/Support/raw_ostream.h"
     37 using namespace llvm;
     38 
     39 #define DEBUG_TYPE "wasm-argument-move"
     40 
     41 namespace {
     42 class WebAssemblyArgumentMove final : public MachineFunctionPass {
     43 public:
     44   static char ID; // Pass identification, replacement for typeid
     45   WebAssemblyArgumentMove() : MachineFunctionPass(ID) {}
     46 
     47   const char *getPassName() const override {
     48     return "WebAssembly Argument Move";
     49   }
     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 
     62 char WebAssemblyArgumentMove::ID = 0;
     63 FunctionPass *llvm::createWebAssemblyArgumentMove() {
     64   return new WebAssemblyArgumentMove();
     65 }
     66 
     67 /// Test whether the given instruction is an ARGUMENT.
     68 static bool IsArgument(const MachineInstr &MI) {
     69   switch (MI.getOpcode()) {
     70   case WebAssembly::ARGUMENT_I32:
     71   case WebAssembly::ARGUMENT_I64:
     72   case WebAssembly::ARGUMENT_F32:
     73   case WebAssembly::ARGUMENT_F64:
     74     return true;
     75   default:
     76     return false;
     77   }
     78 }
     79 
     80 bool WebAssemblyArgumentMove::runOnMachineFunction(MachineFunction &MF) {
     81   DEBUG({
     82     dbgs() << "********** Argument Move **********\n"
     83            << "********** Function: " << MF.getName() << '\n';
     84   });
     85 
     86   bool Changed = false;
     87   MachineBasicBlock &EntryMBB = MF.front();
     88   MachineBasicBlock::iterator InsertPt = EntryMBB.end();
     89 
     90   // Look for the first NonArg instruction.
     91   for (MachineInstr &MI : EntryMBB) {
     92     if (!IsArgument(MI)) {
     93       InsertPt = MI;
     94       break;
     95     }
     96   }
     97 
     98   // Now move any argument instructions later in the block
     99   // to before our first NonArg instruction.
    100   for (MachineInstr &MI : llvm::make_range(InsertPt, EntryMBB.end())) {
    101     if (IsArgument(MI)) {
    102       EntryMBB.insert(InsertPt, MI.removeFromParent());
    103       Changed = true;
    104     }
    105   }
    106 
    107   return Changed;
    108 }
    109