1 //===----- PostRAHazardRecognizer.cpp - hazard recognizer -----------------===//
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 /// \file
10 /// This runs the hazard recognizer and emits noops when necessary. This
11 /// gives targets a way to run the hazard recognizer without running one of
12 /// the schedulers. Example use cases for this pass would be:
13 ///
14 /// - Targets that need the hazard recognizer to be run at -O0.
15 /// - Targets that want to guarantee that hazards at the beginning of
16 /// scheduling regions are handled correctly. The post-RA scheduler is
17 /// a top-down scheduler, but when there are multiple scheduling regions
18 /// in a basic block, it visits the regions in bottom-up order. This
19 /// makes it impossible for the scheduler to gauranttee it can correctly
20 /// handle hazards at the beginning of scheduling regions.
21 ///
22 /// This pass traverses all the instructions in a program in top-down order.
23 /// In contrast to the instruction scheduling passes, this pass never resets
24 /// the hazard recognizer to ensure it can correctly handles noop hazards at
25 /// the beginning of blocks.
26 //
27 //===----------------------------------------------------------------------===//
28
29 #include "llvm/ADT/Statistic.h"
30 #include "llvm/CodeGen/MachineFunctionPass.h"
31 #include "llvm/CodeGen/ScheduleHazardRecognizer.h"
32 #include "llvm/CodeGen/TargetInstrInfo.h"
33 #include "llvm/CodeGen/TargetSubtargetInfo.h"
34 #include "llvm/InitializePasses.h"
35 #include "llvm/Pass.h"
36 using namespace llvm;
37
38 #define DEBUG_TYPE "post-RA-hazard-rec"
39
40 STATISTIC(NumNoops, "Number of noops inserted");
41
42 namespace {
43 class PostRAHazardRecognizer : public MachineFunctionPass {
44
45 public:
46 static char ID;
PostRAHazardRecognizer()47 PostRAHazardRecognizer() : MachineFunctionPass(ID) {}
48
getAnalysisUsage(AnalysisUsage & AU) const49 void getAnalysisUsage(AnalysisUsage &AU) const override {
50 AU.setPreservesCFG();
51 MachineFunctionPass::getAnalysisUsage(AU);
52 }
53
54 bool runOnMachineFunction(MachineFunction &Fn) override;
55
56 };
57 char PostRAHazardRecognizer::ID = 0;
58
59 }
60
61 char &llvm::PostRAHazardRecognizerID = PostRAHazardRecognizer::ID;
62
63 INITIALIZE_PASS(PostRAHazardRecognizer, DEBUG_TYPE,
64 "Post RA hazard recognizer", false, false)
65
runOnMachineFunction(MachineFunction & Fn)66 bool PostRAHazardRecognizer::runOnMachineFunction(MachineFunction &Fn) {
67 const TargetInstrInfo *TII = Fn.getSubtarget().getInstrInfo();
68 std::unique_ptr<ScheduleHazardRecognizer> HazardRec(
69 TII->CreateTargetPostRAHazardRecognizer(Fn));
70
71 // Return if the target has not implemented a hazard recognizer.
72 if (!HazardRec)
73 return false;
74
75 // Loop over all of the basic blocks
76 bool Changed = false;
77 for (auto &MBB : Fn) {
78 // We do not call HazardRec->reset() here to make sure we are handling noop
79 // hazards at the start of basic blocks.
80 for (MachineInstr &MI : MBB) {
81 // If we need to emit noops prior to this instruction, then do so.
82 unsigned NumPreNoops = HazardRec->PreEmitNoops(&MI);
83 HazardRec->EmitNoops(NumPreNoops);
84 TII->insertNoops(MBB, MachineBasicBlock::iterator(MI), NumPreNoops);
85 NumNoops += NumPreNoops;
86 if (NumPreNoops)
87 Changed = true;
88
89 HazardRec->EmitInstruction(&MI);
90 if (HazardRec->atIssueLimit()) {
91 HazardRec->AdvanceCycle();
92 }
93 }
94 }
95 return Changed;
96 }
97