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/ADT/MapVector.h"
16 #include "llvm/CodeGen/MachineBasicBlock.h"
17 #include "llvm/CodeGen/MachineFunction.h"
18 #include "llvm/CodeGen/MachineFunctionPass.h"
19 #include "llvm/CodeGen/MachineModuleInfo.h"
20 using namespace llvm;
21 
22 #define DEBUG_TYPE "funclet-layout"
23 
24 namespace {
25 class FuncletLayout : public MachineFunctionPass {
26 public:
27   static char ID; // Pass identification, replacement for typeid
28   FuncletLayout() : MachineFunctionPass(ID) {
29     initializeFuncletLayoutPass(*PassRegistry::getPassRegistry());
30   }
31 
32   bool runOnMachineFunction(MachineFunction &F) override;
33 };
34 }
35 
36 static void
37 collectFuncletMembers(MapVector<MachineBasicBlock *, int> &FuncletMembership,
38                       int Funclet, MachineBasicBlock *MBB) {
39   // Don't revisit blocks.
40   if (FuncletMembership.count(MBB) > 0)
41     return;
42 
43   // Add this MBB to our funclet.
44   FuncletMembership[MBB] = Funclet;
45 
46   bool IsReturn = false;
47   int NumTerminators = 0;
48   for (MachineInstr &MI : MBB->terminators()) {
49     IsReturn |= MI.isReturn();
50     ++NumTerminators;
51   }
52   assert((!IsReturn || NumTerminators == 1) &&
53          "Expected only one terminator when a return is present!");
54 
55   // Returns are boundaries where funclet transfer can occur, don't follow
56   // successors.
57   if (IsReturn)
58     return;
59 
60   for (MachineBasicBlock *SMBB : MBB->successors())
61     if (!SMBB->isEHPad())
62       collectFuncletMembers(FuncletMembership, Funclet, SMBB);
63 }
64 
65 char FuncletLayout::ID = 0;
66 char &llvm::FuncletLayoutID = FuncletLayout::ID;
67 INITIALIZE_PASS(FuncletLayout, "funclet-layout",
68                 "Contiguously Lay Out Funclets", false, false)
69 
70 bool FuncletLayout::runOnMachineFunction(MachineFunction &F) {
71   // We don't have anything to do if there aren't any EH pads.
72   if (!F.getMMI().hasEHFunclets())
73     return false;
74 
75   SmallVector<MachineBasicBlock *, 16> FuncletBlocks;
76   for (MachineBasicBlock &MBB : F)
77     if (MBB.isEHFuncletEntry())
78       FuncletBlocks.push_back(&MBB);
79 
80   // We don't have anything to do if there aren't any EH pads.
81   if (FuncletBlocks.empty())
82     return false;
83 
84   MapVector<MachineBasicBlock *, int> FuncletMembership;
85   for (MachineBasicBlock *MBB : FuncletBlocks)
86     collectFuncletMembers(FuncletMembership, MBB->getNumber(), MBB);
87 
88   for (std::pair<llvm::MachineBasicBlock *, int> &FuncletMember :
89        FuncletMembership) {
90     // Move this block to the end of the function.
91     MachineBasicBlock *MBB = FuncletMember.first;
92     MBB->moveAfter(--F.end());
93   }
94 
95   // Conservatively assume we changed something.
96   return true;
97 }
98