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/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 }