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/Passes.h"
32 #include "llvm/CodeGen/ScheduleHazardRecognizer.h"
33 #include "llvm/CodeGen/TargetInstrInfo.h"
34 #include "llvm/CodeGen/TargetSubtargetInfo.h"
35 #include "llvm/InitializePasses.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/ErrorHandling.h"
38 #include "llvm/Support/raw_ostream.h"
39 using namespace llvm;
40 
41 #define DEBUG_TYPE "post-RA-hazard-rec"
42 
43 STATISTIC(NumNoops, "Number of noops inserted");
44 
45 namespace {
46   class PostRAHazardRecognizer : public MachineFunctionPass {
47 
48   public:
49     static char ID;
50     PostRAHazardRecognizer() : MachineFunctionPass(ID) {}
51 
52     void getAnalysisUsage(AnalysisUsage &AU) const override {
53       AU.setPreservesCFG();
54       MachineFunctionPass::getAnalysisUsage(AU);
55     }
56 
57     bool runOnMachineFunction(MachineFunction &Fn) override;
58 
59   };
60   char PostRAHazardRecognizer::ID = 0;
61 
62 }
63 
64 char &llvm::PostRAHazardRecognizerID = PostRAHazardRecognizer::ID;
65 
66 INITIALIZE_PASS(PostRAHazardRecognizer, DEBUG_TYPE,
67                 "Post RA hazard recognizer", false, false)
68 
69 bool PostRAHazardRecognizer::runOnMachineFunction(MachineFunction &Fn) {
70   const TargetInstrInfo *TII = Fn.getSubtarget().getInstrInfo();
71   std::unique_ptr<ScheduleHazardRecognizer> HazardRec(
72       TII->CreateTargetPostRAHazardRecognizer(Fn));
73 
74   // Return if the target has not implemented a hazard recognizer.
75   if (!HazardRec.get())
76     return false;
77 
78   // Loop over all of the basic blocks
79   for (auto &MBB : Fn) {
80     // We do not call HazardRec->reset() here to make sure we are handling noop
81     // hazards at the start of basic blocks.
82     for (MachineInstr &MI : MBB) {
83       // If we need to emit noops prior to this instruction, then do so.
84       unsigned NumPreNoops = HazardRec->PreEmitNoops(&MI);
85       for (unsigned i = 0; i != NumPreNoops; ++i) {
86         HazardRec->EmitNoop();
87         TII->insertNoop(MBB, MachineBasicBlock::iterator(MI));
88         ++NumNoops;
89       }
90 
91       HazardRec->EmitInstruction(&MI);
92       if (HazardRec->atIssueLimit()) {
93         HazardRec->AdvanceCycle();
94       }
95     }
96   }
97   return true;
98 }
99