1 //===-- PHIEliminationUtils.cpp - Helper functions for PHI elimination ----===// 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 #include "PHIEliminationUtils.h" 10 #include "llvm/ADT/SmallPtrSet.h" 11 #include "llvm/CodeGen/MachineBasicBlock.h" 12 #include "llvm/CodeGen/MachineFunction.h" 13 #include "llvm/CodeGen/MachineRegisterInfo.h" 14 using namespace llvm; 15 16 // findCopyInsertPoint - Find a safe place in MBB to insert a copy from SrcReg 17 // when following the CFG edge to SuccMBB. This needs to be after any def of 18 // SrcReg, but before any subsequent point where control flow might jump out of 19 // the basic block. 20 MachineBasicBlock::iterator 21 llvm::findPHICopyInsertPoint(MachineBasicBlock* MBB, MachineBasicBlock* SuccMBB, 22 unsigned SrcReg) { 23 // Handle the trivial case trivially. 24 if (MBB->empty()) 25 return MBB->begin(); 26 27 // Usually, we just want to insert the copy before the first terminator 28 // instruction. However, for the edge going to a landing pad, we must insert 29 // the copy before the call/invoke instruction. 30 if (!SuccMBB->isEHPad()) 31 return MBB->getFirstTerminator(); 32 33 // Discover any defs/uses in this basic block. 34 SmallPtrSet<MachineInstr*, 8> DefUsesInMBB; 35 MachineRegisterInfo& MRI = MBB->getParent()->getRegInfo(); 36 for (MachineInstr &RI : MRI.reg_instructions(SrcReg)) { 37 if (RI.getParent() == MBB) 38 DefUsesInMBB.insert(&RI); 39 } 40 41 MachineBasicBlock::iterator InsertPoint; 42 if (DefUsesInMBB.empty()) { 43 // No defs. Insert the copy at the start of the basic block. 44 InsertPoint = MBB->begin(); 45 } else if (DefUsesInMBB.size() == 1) { 46 // Insert the copy immediately after the def/use. 47 InsertPoint = *DefUsesInMBB.begin(); 48 ++InsertPoint; 49 } else { 50 // Insert the copy immediately after the last def/use. 51 InsertPoint = MBB->end(); 52 while (!DefUsesInMBB.count(&*--InsertPoint)) {} 53 ++InsertPoint; 54 } 55 56 // Make sure the copy goes after any phi nodes but before 57 // any debug nodes. 58 return MBB->SkipPHIsAndLabels(InsertPoint); 59 } 60