1 //===- MipsMulMulBugPass.cpp - Mips VR4300 mulmul bugfix pass -------------===//
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 // Early revisions of the VR4300 have a hardware bug where two consecutive
10 // multiplications can produce an incorrect result in the second multiply.
11 //
12 // This pass scans for mul instructions in each basic block and inserts
13 // a nop whenever the following conditions are met:
14 //
15 // - The current instruction is a single or double-precision floating-point
16 //   mul instruction.
17 // - The next instruction is either a mul instruction (any kind)
18 //   or a branch instruction.
19 //===----------------------------------------------------------------------===//
20 
21 #include "Mips.h"
22 #include "MipsInstrInfo.h"
23 #include "MipsSubtarget.h"
24 #include "llvm/CodeGen/MachineBasicBlock.h"
25 #include "llvm/CodeGen/MachineFunction.h"
26 #include "llvm/CodeGen/MachineFunctionPass.h"
27 #include "llvm/Support/CommandLine.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Target/TargetMachine.h"
30 
31 #define DEBUG_TYPE "mips-vr4300-mulmul-fix"
32 
33 using namespace llvm;
34 
35 namespace {
36 
37 class MipsMulMulBugFix : public MachineFunctionPass {
38 public:
39   MipsMulMulBugFix() : MachineFunctionPass(ID) {
40     initializeMipsMulMulBugFixPass(*PassRegistry::getPassRegistry());
41   }
42 
43   StringRef getPassName() const override { return "Mips VR4300 mulmul bugfix"; }
44 
45   MachineFunctionProperties getRequiredProperties() const override {
46     return MachineFunctionProperties().set(
47         MachineFunctionProperties::Property::NoVRegs);
48   }
49 
50   bool runOnMachineFunction(MachineFunction &MF) override;
51 
52   static char ID;
53 
54 private:
55   bool fixMulMulBB(MachineBasicBlock &MBB, const MipsInstrInfo &MipsII);
56 };
57 
58 } // namespace
59 
60 INITIALIZE_PASS(MipsMulMulBugFix, "mips-vr4300-mulmul-fix",
61                 "Mips VR4300 mulmul bugfix", false, false)
62 
63 char MipsMulMulBugFix::ID = 0;
64 
65 bool MipsMulMulBugFix::runOnMachineFunction(MachineFunction &MF) {
66   const MipsInstrInfo &MipsII =
67       *static_cast<const MipsInstrInfo *>(MF.getSubtarget().getInstrInfo());
68 
69   bool Modified = false;
70 
71   for (auto &MBB : MF)
72     Modified |= fixMulMulBB(MBB, MipsII);
73 
74   return Modified;
75 }
76 
77 static bool isFirstMul(const MachineInstr &MI) {
78   switch (MI.getOpcode()) {
79   case Mips::FMUL_S:
80   case Mips::FMUL_D:
81   case Mips::FMUL_D32:
82   case Mips::FMUL_D64:
83     return true;
84   default:
85     return false;
86   }
87 }
88 
89 static bool isSecondMulOrBranch(const MachineInstr &MI) {
90   if (MI.isBranch() || MI.isIndirectBranch() || MI.isCall())
91     return true;
92 
93   switch (MI.getOpcode()) {
94   case Mips::MUL:
95   case Mips::FMUL_S:
96   case Mips::FMUL_D:
97   case Mips::FMUL_D32:
98   case Mips::FMUL_D64:
99   case Mips::MULT:
100   case Mips::MULTu:
101   case Mips::DMULT:
102   case Mips::DMULTu:
103     return true;
104   default:
105     return false;
106   }
107 }
108 
109 bool MipsMulMulBugFix::fixMulMulBB(MachineBasicBlock &MBB,
110                                    const MipsInstrInfo &MipsII) {
111   bool Modified = false;
112 
113   MachineBasicBlock::instr_iterator NextMII;
114 
115   // Iterate through the instructions in the basic block
116   for (MachineBasicBlock::instr_iterator MII = MBB.instr_begin(),
117                                          E = MBB.instr_end();
118        MII != E; MII = NextMII) {
119 
120     NextMII = next_nodbg(MII, E);
121 
122     // Trigger when the current instruction is a mul and the next instruction
123     // is either a mul or a branch in case the branch target start with a mul
124     if (NextMII != E && isFirstMul(*MII) && isSecondMulOrBranch(*NextMII)) {
125       LLVM_DEBUG(dbgs() << "Found mulmul!\n");
126 
127       const MCInstrDesc &NewMCID = MipsII.get(Mips::NOP);
128       BuildMI(MBB, NextMII, DebugLoc(), NewMCID);
129       Modified = true;
130     }
131   }
132 
133   return Modified;
134 }
135 
136 FunctionPass *llvm::createMipsMulMulBugPass() { return new MipsMulMulBugFix(); }
137