1 //- X86Insertwait.cpp - Strict-Fp:Insert wait instruction X87 instructions --//
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 // This file defines the pass which insert x86 wait instructions after each
10 // X87 instructions when strict float is enabled.
11 //
12 // The logic to insert a wait instruction after an X87 instruction is as below:
13 // 1. If the X87 instruction don't raise float exception nor is a load/store
14 // instruction, or is a x87 control instruction, don't insert wait.
15 // 2. If the X87 instruction is an instruction which the following instruction
16 // is an X87 exception synchronizing X87 instruction, don't insert wait.
17 // 3. For other situations, insert wait instruction.
18 //
19 //===----------------------------------------------------------------------===//
20
21 #include "X86.h"
22 #include "X86InstrInfo.h"
23 #include "X86Subtarget.h"
24 #include "llvm/CodeGen/MachineBasicBlock.h"
25 #include "llvm/CodeGen/MachineFunction.h"
26 #include "llvm/CodeGen/MachineFunctionPass.h"
27 #include "llvm/CodeGen/MachineInstr.h"
28 #include "llvm/CodeGen/MachineInstrBuilder.h"
29 #include "llvm/CodeGen/MachineOperand.h"
30 #include "llvm/IR/DebugLoc.h"
31 #include "llvm/Support/Debug.h"
32
33 using namespace llvm;
34
35 #define DEBUG_TYPE "x86-insert-wait"
36
37 namespace {
38
39 class WaitInsert : public MachineFunctionPass {
40 public:
41 static char ID;
42
WaitInsert()43 WaitInsert() : MachineFunctionPass(ID) {}
44
45 bool runOnMachineFunction(MachineFunction &MF) override;
46
getPassName() const47 StringRef getPassName() const override {
48 return "X86 insert wait instruction";
49 }
50 };
51
52 } // namespace
53
54 char WaitInsert::ID = 0;
55
createX86InsertX87waitPass()56 FunctionPass *llvm::createX86InsertX87waitPass() { return new WaitInsert(); }
57
isX87ControlInstruction(MachineInstr & MI)58 static bool isX87ControlInstruction(MachineInstr &MI) {
59 switch (MI.getOpcode()) {
60 case X86::FNINIT:
61 case X86::FLDCW16m:
62 case X86::FNSTCW16m:
63 case X86::FNSTSW16r:
64 case X86::FNSTSWm:
65 case X86::FNCLEX:
66 case X86::FLDENVm:
67 case X86::FSTENVm:
68 case X86::FRSTORm:
69 case X86::FSAVEm:
70 case X86::FINCSTP:
71 case X86::FDECSTP:
72 case X86::FFREE:
73 case X86::FFREEP:
74 case X86::FNOP:
75 case X86::WAIT:
76 return true;
77 default:
78 return false;
79 }
80 }
81
isX87NonWaitingControlInstruction(MachineInstr & MI)82 static bool isX87NonWaitingControlInstruction(MachineInstr &MI) {
83 // a few special control instructions don't perform a wait operation
84 switch (MI.getOpcode()) {
85 case X86::FNINIT:
86 case X86::FNSTSW16r:
87 case X86::FNSTSWm:
88 case X86::FNSTCW16m:
89 case X86::FNCLEX:
90 return true;
91 default:
92 return false;
93 }
94 }
95
runOnMachineFunction(MachineFunction & MF)96 bool WaitInsert::runOnMachineFunction(MachineFunction &MF) {
97 if (!MF.getFunction().hasFnAttribute(Attribute::StrictFP))
98 return false;
99
100 const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
101 const X86InstrInfo *TII = ST.getInstrInfo();
102 bool Changed = false;
103
104 for (MachineBasicBlock &MBB : MF) {
105 for (MachineBasicBlock::iterator MI = MBB.begin(); MI != MBB.end(); ++MI) {
106 // Jump non X87 instruction.
107 if (!X86::isX87Instruction(*MI))
108 continue;
109 // If the instruction instruction neither has float exception nor is
110 // a load/store instruction, or the instruction is x87 control
111 // instruction, do not insert wait.
112 if (!(MI->mayRaiseFPException() || MI->mayLoadOrStore()) ||
113 isX87ControlInstruction(*MI))
114 continue;
115 // If the following instruction is an X87 instruction and isn't an X87
116 // non-waiting control instruction, we can omit insert wait instruction.
117 MachineBasicBlock::iterator AfterMI = std::next(MI);
118 if (AfterMI != MBB.end() && X86::isX87Instruction(*AfterMI) &&
119 !isX87NonWaitingControlInstruction(*AfterMI))
120 continue;
121
122 BuildMI(MBB, AfterMI, MI->getDebugLoc(), TII->get(X86::WAIT));
123 LLVM_DEBUG(dbgs() << "\nInsert wait after:\t" << *MI);
124 // Jump the newly inserting wait
125 ++MI;
126 Changed = true;
127 }
128 }
129 return Changed;
130 }
131