lib/CodeGen/FuncletLayout.cpp

   1 //===-- FuncletLayout.cpp - Contiguously lay out funclets -----------------===//
   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 // This file implements basic block placement transformations which result in
  11 // funclets being contiguous.
  12 //
  13 //===----------------------------------------------------------------------===//
  14 #include "llvm/CodeGen/Passes.h"
  15 #include "llvm/CodeGen/MachineBasicBlock.h"
  16 #include "llvm/CodeGen/MachineFunction.h"
  17 #include "llvm/CodeGen/MachineFunctionPass.h"
  18 #include "llvm/CodeGen/MachineModuleInfo.h"
  19 using namespace llvm;
  20
  21 #define DEBUG_TYPE "funclet-layout"
  22
  23 namespace {
  24 class FuncletLayout : public MachineFunctionPass {
  25 public:
  26   static char ID; // Pass identification, replacement for typeid
  27   FuncletLayout() : MachineFunctionPass(ID) {
  28     initializeFuncletLayoutPass(*PassRegistry::getPassRegistry());
  29   }
  30
  31   bool runOnMachineFunction(MachineFunction &F) override;
  32 };
  33 }
  34
  35 static void
  36 collectFuncletMembers(DenseMap<MachineBasicBlock *, int> &FuncletMembership,
  37                       int Funclet, MachineBasicBlock *MBB) {
  38   // Don't revisit blocks.
  39   if (FuncletMembership.count(MBB) > 0)
  40     return;
  41
  42   // Add this MBB to our funclet.
  43   FuncletMembership[MBB] = Funclet;
  44
  45   bool IsReturn = false;
  46   int NumTerminators = 0;
  47   for (MachineInstr &MI : MBB->terminators()) {
  48     IsReturn |= MI.isReturn();
  49     ++NumTerminators;
  50   }
  51   assert((!IsReturn || NumTerminators == 1) &&
  52          "Expected only one terminator when a return is present!");
  53
  54   // Returns are boundaries where funclet transfer can occur, don't follow
  55   // successors.
  56   if (IsReturn)
  57     return;
  58
  59   for (MachineBasicBlock *SMBB : MBB->successors())
  60     if (!SMBB->isEHPad())
  61       collectFuncletMembers(FuncletMembership, Funclet, SMBB);
  62 }
  63
  64 char FuncletLayout::ID = 0;
  65 char &llvm::FuncletLayoutID = FuncletLayout::ID;
  66 INITIALIZE_PASS(FuncletLayout, "funclet-layout",
  67                 "Contiguously Lay Out Funclets", false, false)
  68
  69 bool FuncletLayout::runOnMachineFunction(MachineFunction &F) {
  70   // We don't have anything to do if there aren't any EH pads.
  71   if (!F.getMMI().hasEHFunclets())
  72     return false;
  73
  74   SmallVector<MachineBasicBlock *, 16> FuncletBlocks;
  75   for (MachineBasicBlock &MBB : F)
  76     if (MBB.isEHFuncletEntry())
  77       FuncletBlocks.push_back(&MBB);
  78
  79   // We don't have anything to do if there aren't any EH pads.
  80   if (FuncletBlocks.empty())
  81     return false;
  82
  83   DenseMap<MachineBasicBlock *, int> FuncletMembership;
  84   for (MachineBasicBlock *MBB : FuncletBlocks)
  85     collectFuncletMembers(FuncletMembership, MBB->getNumber(), MBB);
  86
  87   F.sort([&](MachineBasicBlock &x, MachineBasicBlock &y) {
  88     return FuncletMembership[&x] < FuncletMembership[&y];
  89   });
  90
  91   // Conservatively assume we changed something.
  92   return true;
  93 }