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. Similarly for an INLINEASM_BR 30 // going to an indirect target. This is similar to SplitKit.cpp's 31 // computeLastInsertPoint, and similarly assumes that there cannot be multiple 32 // instructions that are Calls with EHPad successors or INLINEASM_BR in a 33 // block. 34 bool EHPadSuccessor = SuccMBB->isEHPad(); 35 if (!EHPadSuccessor && !SuccMBB->isInlineAsmBrIndirectTarget()) 36 return MBB->getFirstTerminator(); 37 38 // Discover any defs in this basic block. 39 SmallPtrSet<MachineInstr *, 8> DefsInMBB; 40 MachineRegisterInfo& MRI = MBB->getParent()->getRegInfo(); 41 for (MachineInstr &RI : MRI.def_instructions(SrcReg)) 42 if (RI.getParent() == MBB) 43 DefsInMBB.insert(&RI); 44 45 MachineBasicBlock::iterator InsertPoint = MBB->begin(); 46 // Insert the copy at the _latest_ point of: 47 // 1. Immediately AFTER the last def 48 // 2. Immediately BEFORE a call/inlineasm_br. 49 for (auto I = MBB->rbegin(), E = MBB->rend(); I != E; ++I) { 50 if (DefsInMBB.contains(&*I)) { 51 InsertPoint = std::next(I.getReverse()); 52 break; 53 } 54 if ((EHPadSuccessor && I->isCall()) || 55 I->getOpcode() == TargetOpcode::INLINEASM_BR) { 56 InsertPoint = I.getReverse(); 57 break; 58 } 59 } 60 61 // Make sure the copy goes after any phi nodes but before 62 // any debug nodes. 63 return MBB->SkipPHIsAndLabels(InsertPoint); 64 } 65