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