1//===-- AMDGPUMarkLastScratchLoad.cpp -------------------------------------===//
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// Mark scratch load/spill instructions which are guaranteed to be the last time
10// this scratch slot is used so it can be evicted from caches.
11//
12// TODO: Handle general stack accesses not just spilling.
13//
14//===----------------------------------------------------------------------===//
15
16#include "AMDGPU.h"
17#include "GCNSubtarget.h"
18#include "llvm/CodeGen/LiveIntervals.h"
19#include "llvm/CodeGen/LiveStacks.h"
20#include "llvm/CodeGen/MachineOperand.h"
21
22using namespace llvm;
23
24#define DEBUG_TYPE "amdgpu-mark-last-scratch-load"
25
26namespace {
27
28class AMDGPUMarkLastScratchLoad : public MachineFunctionPass {
29private:
30 LiveStacks *LS = nullptr;
31 LiveIntervals *LIS = nullptr;
32 SlotIndexes *SI = nullptr;
33 const SIInstrInfo *SII = nullptr;
34
35public:
36 static char ID;
37
38 AMDGPUMarkLastScratchLoad() : MachineFunctionPass(ID) {
39 initializeAMDGPUMarkLastScratchLoadPass(*PassRegistry::getPassRegistry());
40 }
41
42 bool runOnMachineFunction(MachineFunction &MF) override;
43
44 void getAnalysisUsage(AnalysisUsage &AU) const override {
45 AU.addRequired<SlotIndexes>();
46 AU.addRequired<LiveIntervals>();
47 AU.addRequired<LiveStacks>();
48 AU.setPreservesAll();
49 MachineFunctionPass::getAnalysisUsage(AU);
50 }
51
52 StringRef getPassName() const override {
53 return "AMDGPU Mark Last Scratch Load";
54 }
55};
56
57} // end anonymous namespace
58
59bool AMDGPUMarkLastScratchLoad::runOnMachineFunction(MachineFunction &MF) {
60 if (skipFunction(F: MF.getFunction()))
61 return false;
62
63 const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
64 if (ST.getGeneration() < AMDGPUSubtarget::GFX12)
65 return false;
66
67 LS = &getAnalysis<LiveStacks>();
68 LIS = &getAnalysis<LiveIntervals>();
69 SI = &getAnalysis<SlotIndexes>();
70 SII = ST.getInstrInfo();
71 SlotIndexes &Slots = *LIS->getSlotIndexes();
72
73 const unsigned NumSlots = LS->getNumIntervals();
74 if (NumSlots == 0) {
75 LLVM_DEBUG(dbgs() << "No live slots, skipping\n");
76 return false;
77 }
78
79 LLVM_DEBUG(dbgs() << LS->getNumIntervals() << " intervals\n");
80
81 bool Changed = false;
82
83 for (auto &[SS, LI] : *LS) {
84 for (const LiveRange::Segment &Segment : LI.segments) {
85
86 // Ignore segments that run to the end of basic block because in this case
87 // slot is still live at the end of it.
88 if (Segment.end.isBlock())
89 continue;
90
91 const int FrameIndex = Register::stackSlot2Index(Reg: LI.reg());
92 MachineInstr *LastLoad = nullptr;
93
94 MachineInstr *MISegmentEnd = SI->getInstructionFromIndex(index: Segment.end);
95
96 // If there is no instruction at this slot because it was deleted take the
97 // instruction from the next slot.
98 if (!MISegmentEnd) {
99 SlotIndex NextSlot = Slots.getNextNonNullIndex(Index: Segment.end);
100 MISegmentEnd = SI->getInstructionFromIndex(index: NextSlot);
101 }
102
103 MachineInstr *MISegmentStart = SI->getInstructionFromIndex(index: Segment.start);
104 MachineBasicBlock *BB = MISegmentEnd->getParent();
105
106 // Start iteration backwards from segment end until the start of basic
107 // block or start of segment if it is in the same basic block.
108 auto End = BB->rend();
109 if (MISegmentStart && MISegmentStart->getParent() == BB)
110 End = MISegmentStart->getReverseIterator();
111
112 for (auto MI = MISegmentEnd->getReverseIterator(); MI != End; ++MI) {
113 int LoadFI = 0;
114
115 if (SII->isLoadFromStackSlot(MI: *MI, FrameIndex&: LoadFI) && LoadFI == FrameIndex) {
116 LastLoad = &*MI;
117 break;
118 }
119 }
120
121 if (LastLoad && !LastLoad->memoperands_empty()) {
122 MachineMemOperand *MMO = *LastLoad->memoperands_begin();
123 MMO->setFlags(MOLastUse);
124 Changed = true;
125 LLVM_DEBUG(dbgs() << " Found last load: " << *LastLoad);
126 }
127 }
128 }
129
130 return Changed;
131}
132
133char AMDGPUMarkLastScratchLoad::ID = 0;
134
135char &llvm::AMDGPUMarkLastScratchLoadID = AMDGPUMarkLastScratchLoad::ID;
136
137INITIALIZE_PASS_BEGIN(AMDGPUMarkLastScratchLoad, DEBUG_TYPE,
138 "AMDGPU Mark last scratch load", false, false)
139INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
140INITIALIZE_PASS_DEPENDENCY(LiveStacks)
141INITIALIZE_PASS_END(AMDGPUMarkLastScratchLoad, DEBUG_TYPE,
142 "AMDGPU Mark last scratch load", false, false)
143

source code of llvm/lib/Target/AMDGPU/AMDGPUMarkLastScratchLoad.cpp