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      1 //===-- WebAssemblyArgumentMove.cpp - Argument instruction moving ---------===//
      2 //
      3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
      4 // See https://llvm.org/LICENSE.txt for license information.
      5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
      6 //
      7 //===----------------------------------------------------------------------===//
      8 ///
      9 /// \file
     10 /// This file moves ARGUMENT instructions after ScheduleDAG scheduling.
     11 ///
     12 /// Arguments are really live-in registers, however, since we use virtual
     13 /// registers and LLVM doesn't support live-in virtual registers, we're
     14 /// currently making do with ARGUMENT instructions which are placed at the top
     15 /// of the entry block. The trick is to get them to *stay* at the top of the
     16 /// entry block.
     17 ///
     18 /// The ARGUMENTS physical register keeps these instructions pinned in place
     19 /// during liveness-aware CodeGen passes, however one thing which does not
     20 /// respect this is the ScheduleDAG scheduler. This pass is therefore run
     21 /// immediately after that.
     22 ///
     23 /// This is all hopefully a temporary solution until we find a better solution
     24 /// for describing the live-in nature of arguments.
     25 ///
     26 //===----------------------------------------------------------------------===//
     27 
     28 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
     29 #include "Utils/WebAssemblyUtilities.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   StringRef getPassName() const override { return "WebAssembly Argument Move"; }
     49 
     50   void getAnalysisUsage(AnalysisUsage &AU) const override {
     51     AU.setPreservesCFG();
     52     AU.addPreserved<MachineBlockFrequencyInfo>();
     53     AU.addPreservedID(MachineDominatorsID);
     54     MachineFunctionPass::getAnalysisUsage(AU);
     55   }
     56 
     57   bool runOnMachineFunction(MachineFunction &MF) override;
     58 };
     59 } // end anonymous namespace
     60 
     61 char WebAssemblyArgumentMove::ID = 0;
     62 INITIALIZE_PASS(WebAssemblyArgumentMove, DEBUG_TYPE,
     63                 "Move ARGUMENT instructions for WebAssembly", false, false)
     64 
     65 FunctionPass *llvm::createWebAssemblyArgumentMove() {
     66   return new WebAssemblyArgumentMove();
     67 }
     68 
     69 bool WebAssemblyArgumentMove::runOnMachineFunction(MachineFunction &MF) {
     70   LLVM_DEBUG({
     71     dbgs() << "********** Argument Move **********\n"
     72            << "********** Function: " << MF.getName() << '\n';
     73   });
     74 
     75   bool Changed = false;
     76   MachineBasicBlock &EntryMBB = MF.front();
     77   MachineBasicBlock::iterator InsertPt = EntryMBB.end();
     78 
     79   // Look for the first NonArg instruction.
     80   for (MachineInstr &MI : EntryMBB) {
     81     if (!WebAssembly::isArgument(MI.getOpcode())) {
     82       InsertPt = MI;
     83       break;
     84     }
     85   }
     86 
     87   // Now move any argument instructions later in the block
     88   // to before our first NonArg instruction.
     89   for (MachineInstr &MI : llvm::make_range(InsertPt, EntryMBB.end())) {
     90     if (WebAssembly::isArgument(MI.getOpcode())) {
     91       EntryMBB.insert(InsertPt, MI.removeFromParent());
     92       Changed = true;
     93     }
     94   }
     95 
     96   return Changed;
     97 }
     98