llvm/lib/CodeGen/PHIEliminationUtils.cpp

   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/MachineFunction.h"
  12 #include "llvm/CodeGen/MachineRegisterInfo.h"
  13
  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 }