lib/Target/WebAssembly/WebAssemblyArgumentMove.cpp

   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 "WebAssembly.h"
  30 #include "WebAssemblyMachineFunctionInfo.h"
  31 #include "WebAssemblySubtarget.h"
  32 #include "WebAssemblyUtilities.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 }