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//===- AMDGPUPerfHintAnalysis.cpp - analysis of functions memory traffic --===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
/// \file
/// \brief Analyzes if a function potentially memory bound and if a kernel
/// kernel may benefit from limiting number of waves to reduce cache thrashing.
///
//===----------------------------------------------------------------------===//
#include "AMDGPU.h"
#include "AMDGPUPerfHintAnalysis.h"
#include "Utils/AMDGPUBaseInfo.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/CodeGen/TargetLowering.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
#define DEBUG_TYPE "amdgpu-perf-hint"
static cl::opt<unsigned>
MemBoundThresh("amdgpu-membound-threshold", cl::init(50), cl::Hidden,
cl::desc("Function mem bound threshold in %"));
static cl::opt<unsigned>
LimitWaveThresh("amdgpu-limit-wave-threshold", cl::init(50), cl::Hidden,
cl::desc("Kernel limit wave threshold in %"));
static cl::opt<unsigned>
IAWeight("amdgpu-indirect-access-weight", cl::init(1000), cl::Hidden,
cl::desc("Indirect access memory instruction weight"));
static cl::opt<unsigned>
LSWeight("amdgpu-large-stride-weight", cl::init(1000), cl::Hidden,
cl::desc("Large stride memory access weight"));
static cl::opt<unsigned>
LargeStrideThresh("amdgpu-large-stride-threshold", cl::init(64), cl::Hidden,
cl::desc("Large stride memory access threshold"));
STATISTIC(NumMemBound, "Number of functions marked as memory bound");
STATISTIC(NumLimitWave, "Number of functions marked as needing limit wave");
char llvm::AMDGPUPerfHintAnalysis::ID = 0;
char &llvm::AMDGPUPerfHintAnalysisID = AMDGPUPerfHintAnalysis::ID;
INITIALIZE_PASS(AMDGPUPerfHintAnalysis, DEBUG_TYPE,
"Analysis if a function is memory bound", true, true)
namespace {
struct AMDGPUPerfHint {
friend AMDGPUPerfHintAnalysis;
public:
AMDGPUPerfHint(AMDGPUPerfHintAnalysis::FuncInfoMap &FIM_,
const TargetLowering *TLI_)
: FIM(FIM_), DL(nullptr), TLI(TLI_) {}
bool runOnFunction(Function &F);
private:
struct MemAccessInfo {
const Value *V = nullptr;
const Value *Base = nullptr;
int64_t Offset = 0;
MemAccessInfo() = default;
bool isLargeStride(MemAccessInfo &Reference) const;
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
Printable print() const {
return Printable([this](raw_ostream &OS) {
OS << "Value: " << *V << '\n'
<< "Base: " << *Base << " Offset: " << Offset << '\n';
});
}
#endif
};
MemAccessInfo makeMemAccessInfo(Instruction *) const;
MemAccessInfo LastAccess; // Last memory access info
AMDGPUPerfHintAnalysis::FuncInfoMap &FIM;
const DataLayout *DL;
const TargetLowering *TLI;
AMDGPUPerfHintAnalysis::FuncInfo *visit(const Function &F);
static bool isMemBound(const AMDGPUPerfHintAnalysis::FuncInfo &F);
static bool needLimitWave(const AMDGPUPerfHintAnalysis::FuncInfo &F);
bool isIndirectAccess(const Instruction *Inst) const;
/// Check if the instruction is large stride.
/// The purpose is to identify memory access pattern like:
/// x = a[i];
/// y = a[i+1000];
/// z = a[i+2000];
/// In the above example, the second and third memory access will be marked
/// large stride memory access.
bool isLargeStride(const Instruction *Inst);
bool isGlobalAddr(const Value *V) const;
bool isLocalAddr(const Value *V) const;
bool isGlobalLoadUsedInBB(const Instruction &) const;
};
static std::pair<const Value *, const Type *> getMemoryInstrPtrAndType(
const Instruction *Inst) {
if (auto LI = dyn_cast<LoadInst>(Inst))
return {LI->getPointerOperand(), LI->getType()};
if (auto SI = dyn_cast<StoreInst>(Inst))
return {SI->getPointerOperand(), SI->getValueOperand()->getType()};
if (auto AI = dyn_cast<AtomicCmpXchgInst>(Inst))
return {AI->getPointerOperand(), AI->getCompareOperand()->getType()};
if (auto AI = dyn_cast<AtomicRMWInst>(Inst))
return {AI->getPointerOperand(), AI->getValOperand()->getType()};
if (auto MI = dyn_cast<AnyMemIntrinsic>(Inst))
return {MI->getRawDest(), Type::getInt8Ty(MI->getContext())};
return {nullptr, nullptr};
}
bool AMDGPUPerfHint::isIndirectAccess(const Instruction *Inst) const {
LLVM_DEBUG(dbgs() << "[isIndirectAccess] " << *Inst << '\n');
SmallSet<const Value *, 32> WorkSet;
SmallSet<const Value *, 32> Visited;
if (const Value *MO = getMemoryInstrPtrAndType(Inst).first) {
if (isGlobalAddr(MO))
WorkSet.insert(MO);
}
while (!WorkSet.empty()) {
const Value *V = *WorkSet.begin();
WorkSet.erase(*WorkSet.begin());
if (!Visited.insert(V).second)
continue;
LLVM_DEBUG(dbgs() << " check: " << *V << '\n');
if (auto LD = dyn_cast<LoadInst>(V)) {
auto M = LD->getPointerOperand();
if (isGlobalAddr(M)) {
LLVM_DEBUG(dbgs() << " is IA\n");
return true;
}
continue;
}
if (auto GEP = dyn_cast<GetElementPtrInst>(V)) {
auto P = GEP->getPointerOperand();
WorkSet.insert(P);
for (unsigned I = 1, E = GEP->getNumIndices() + 1; I != E; ++I)
WorkSet.insert(GEP->getOperand(I));
continue;
}
if (auto U = dyn_cast<UnaryInstruction>(V)) {
WorkSet.insert(U->getOperand(0));
continue;
}
if (auto BO = dyn_cast<BinaryOperator>(V)) {
WorkSet.insert(BO->getOperand(0));
WorkSet.insert(BO->getOperand(1));
continue;
}
if (auto S = dyn_cast<SelectInst>(V)) {
WorkSet.insert(S->getFalseValue());
WorkSet.insert(S->getTrueValue());
continue;
}
if (auto E = dyn_cast<ExtractElementInst>(V)) {
WorkSet.insert(E->getVectorOperand());
continue;
}
LLVM_DEBUG(dbgs() << " dropped\n");
}
LLVM_DEBUG(dbgs() << " is not IA\n");
return false;
}
// Returns true if the global load `I` is used in its own basic block.
bool AMDGPUPerfHint::isGlobalLoadUsedInBB(const Instruction &I) const {
const auto *Ld = dyn_cast<LoadInst>(&I);
if (!Ld)
return false;
if (!isGlobalAddr(Ld->getPointerOperand()))
return false;
for (const User *Usr : Ld->users()) {
if (const Instruction *UsrInst = dyn_cast<Instruction>(Usr)) {
if (UsrInst->getParent() == I.getParent())
return true;
}
}
return false;
}
AMDGPUPerfHintAnalysis::FuncInfo *AMDGPUPerfHint::visit(const Function &F) {
AMDGPUPerfHintAnalysis::FuncInfo &FI = FIM[&F];
LLVM_DEBUG(dbgs() << "[AMDGPUPerfHint] process " << F.getName() << '\n');
for (auto &B : F) {
LastAccess = MemAccessInfo();
unsigned UsedGlobalLoadsInBB = 0;
for (auto &I : B) {
if (const Type *Ty = getMemoryInstrPtrAndType(&I).second) {
unsigned Size = divideCeil(Ty->getPrimitiveSizeInBits(), 32);
// TODO: Check if the global load and its user are close to each other
// instead (Or do this analysis in GCNSchedStrategy?).
if (isGlobalLoadUsedInBB(I))
UsedGlobalLoadsInBB += Size;
if (isIndirectAccess(&I))
FI.IAMInstCost += Size;
if (isLargeStride(&I))
FI.LSMInstCost += Size;
FI.MemInstCost += Size;
FI.InstCost += Size;
continue;
}
if (auto *CB = dyn_cast<CallBase>(&I)) {
Function *Callee = CB->getCalledFunction();
if (!Callee || Callee->isDeclaration()) {
++FI.InstCost;
continue;
}
if (&F == Callee) // Handle immediate recursion
continue;
auto Loc = FIM.find(Callee);
if (Loc == FIM.end())
continue;
FI.MemInstCost += Loc->second.MemInstCost;
FI.InstCost += Loc->second.InstCost;
FI.IAMInstCost += Loc->second.IAMInstCost;
FI.LSMInstCost += Loc->second.LSMInstCost;
} else if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) {
TargetLoweringBase::AddrMode AM;
auto *Ptr = GetPointerBaseWithConstantOffset(GEP, AM.BaseOffs, *DL);
AM.BaseGV = dyn_cast_or_null<GlobalValue>(const_cast<Value *>(Ptr));
AM.HasBaseReg = !AM.BaseGV;
if (TLI->isLegalAddressingMode(*DL, AM, GEP->getResultElementType(),
GEP->getPointerAddressSpace()))
// Offset will likely be folded into load or store
continue;
++FI.InstCost;
} else {
++FI.InstCost;
}
}
if (!FI.HasDenseGlobalMemAcc) {
unsigned GlobalMemAccPercentage = UsedGlobalLoadsInBB * 100 / B.size();
if (GlobalMemAccPercentage > 50) {
LLVM_DEBUG(dbgs() << "[HasDenseGlobalMemAcc] Set to true since "
<< B.getName() << " has " << GlobalMemAccPercentage
<< "% global memory access\n");
FI.HasDenseGlobalMemAcc = true;
}
}
}
return &FI;
}
bool AMDGPUPerfHint::runOnFunction(Function &F) {
const Module &M = *F.getParent();
DL = &M.getDataLayout();
if (F.hasFnAttribute("amdgpu-wave-limiter") &&
F.hasFnAttribute("amdgpu-memory-bound"))
return false;
const AMDGPUPerfHintAnalysis::FuncInfo *Info = visit(F);
LLVM_DEBUG(dbgs() << F.getName() << " MemInst cost: " << Info->MemInstCost
<< '\n'
<< " IAMInst cost: " << Info->IAMInstCost << '\n'
<< " LSMInst cost: " << Info->LSMInstCost << '\n'
<< " TotalInst cost: " << Info->InstCost << '\n');
bool Changed = false;
if (isMemBound(*Info)) {
LLVM_DEBUG(dbgs() << F.getName() << " is memory bound\n");
NumMemBound++;
F.addFnAttr("amdgpu-memory-bound", "true");
Changed = true;
}
if (AMDGPU::isEntryFunctionCC(F.getCallingConv()) && needLimitWave(*Info)) {
LLVM_DEBUG(dbgs() << F.getName() << " needs limit wave\n");
NumLimitWave++;
F.addFnAttr("amdgpu-wave-limiter", "true");
Changed = true;
}
return Changed;
}
bool AMDGPUPerfHint::isMemBound(const AMDGPUPerfHintAnalysis::FuncInfo &FI) {
// Reverting optimal scheduling in favour of occupancy with basic block(s)
// having dense global memory access can potentially hurt performance.
if (FI.HasDenseGlobalMemAcc)
return true;
return FI.MemInstCost * 100 / FI.InstCost > MemBoundThresh;
}
bool AMDGPUPerfHint::needLimitWave(const AMDGPUPerfHintAnalysis::FuncInfo &FI) {
return ((FI.MemInstCost + FI.IAMInstCost * IAWeight +
FI.LSMInstCost * LSWeight) * 100 / FI.InstCost) > LimitWaveThresh;
}
bool AMDGPUPerfHint::isGlobalAddr(const Value *V) const {
if (auto PT = dyn_cast<PointerType>(V->getType())) {
unsigned As = PT->getAddressSpace();
// Flat likely points to global too.
return As == AMDGPUAS::GLOBAL_ADDRESS || As == AMDGPUAS::FLAT_ADDRESS;
}
return false;
}
bool AMDGPUPerfHint::isLocalAddr(const Value *V) const {
if (auto PT = dyn_cast<PointerType>(V->getType()))
return PT->getAddressSpace() == AMDGPUAS::LOCAL_ADDRESS;
return false;
}
bool AMDGPUPerfHint::isLargeStride(const Instruction *Inst) {
LLVM_DEBUG(dbgs() << "[isLargeStride] " << *Inst << '\n');
MemAccessInfo MAI = makeMemAccessInfo(const_cast<Instruction *>(Inst));
bool IsLargeStride = MAI.isLargeStride(LastAccess);
if (MAI.Base)
LastAccess = std::move(MAI);
return IsLargeStride;
}
AMDGPUPerfHint::MemAccessInfo
AMDGPUPerfHint::makeMemAccessInfo(Instruction *Inst) const {
MemAccessInfo MAI;
const Value *MO = getMemoryInstrPtrAndType(Inst).first;
LLVM_DEBUG(dbgs() << "[isLargeStride] MO: " << *MO << '\n');
// Do not treat local-addr memory access as large stride.
if (isLocalAddr(MO))
return MAI;
MAI.V = MO;
MAI.Base = GetPointerBaseWithConstantOffset(MO, MAI.Offset, *DL);
return MAI;
}
bool AMDGPUPerfHint::MemAccessInfo::isLargeStride(
MemAccessInfo &Reference) const {
if (!Base || !Reference.Base || Base != Reference.Base)
return false;
uint64_t Diff = Offset > Reference.Offset ? Offset - Reference.Offset
: Reference.Offset - Offset;
bool Result = Diff > LargeStrideThresh;
LLVM_DEBUG(dbgs() << "[isLargeStride compare]\n"
<< print() << "<=>\n"
<< Reference.print() << "Result:" << Result << '\n');
return Result;
}
} // namespace
bool AMDGPUPerfHintAnalysis::runOnSCC(CallGraphSCC &SCC) {
auto *TPC = getAnalysisIfAvailable<TargetPassConfig>();
if (!TPC)
return false;
const TargetMachine &TM = TPC->getTM<TargetMachine>();
bool Changed = false;
for (CallGraphNode *I : SCC) {
Function *F = I->getFunction();
if (!F || F->isDeclaration())
continue;
const TargetSubtargetInfo *ST = TM.getSubtargetImpl(*F);
AMDGPUPerfHint Analyzer(FIM, ST->getTargetLowering());
if (Analyzer.runOnFunction(*F))
Changed = true;
}
return Changed;
}
bool AMDGPUPerfHintAnalysis::isMemoryBound(const Function *F) const {
auto FI = FIM.find(F);
if (FI == FIM.end())
return false;
return AMDGPUPerfHint::isMemBound(FI->second);
}
bool AMDGPUPerfHintAnalysis::needsWaveLimiter(const Function *F) const {
auto FI = FIM.find(F);
if (FI == FIM.end())
return false;
return AMDGPUPerfHint::needLimitWave(FI->second);
}