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//=- SyntheticCountsPropagation.cpp - Propagate function counts --*- C++ -*-=// |
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// |
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// The LLVM Compiler Infrastructure |
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// |
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// This file is distributed under the University of Illinois Open Source |
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// License. See LICENSE.TXT for details. |
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// |
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//===----------------------------------------------------------------------===// |
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// |
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// This file implements a transformation that synthesizes entry counts for |
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// functions and attaches !prof metadata to functions with the synthesized |
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// counts. The presence of !prof metadata with counter name set to |
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// 'synthesized_function_entry_count' indicate that the value of the counter is |
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// an estimation of the likely execution count of the function. This transform |
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// is applied only in non PGO mode as functions get 'real' profile-based |
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// function entry counts in the PGO mode. |
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// |
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// The transformation works by first assigning some initial values to the entry |
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// counts of all functions and then doing a top-down traversal of the |
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// callgraph-scc to propagate the counts. For each function the set of callsites |
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// and their relative block frequency is gathered. The relative block frequency |
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// multiplied by the entry count of the caller and added to the callee's entry |
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// count. For non-trivial SCCs, the new counts are computed from the previous |
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// counts and updated in one shot. |
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// |
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//===----------------------------------------------------------------------===// |
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#include "llvm/Transforms/IPO/SyntheticCountsPropagation.h" |
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#include "llvm/ADT/DenseSet.h" |
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#include "llvm/ADT/STLExtras.h" |
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#include "llvm/Analysis/BlockFrequencyInfo.h" |
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#include "llvm/Analysis/CallGraph.h" |
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#include "llvm/Analysis/SyntheticCountsUtils.h" |
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#include "llvm/IR/CallSite.h" |
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#include "llvm/IR/Function.h" |
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#include "llvm/IR/Instructions.h" |
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#include "llvm/IR/Module.h" |
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#include "llvm/Support/CommandLine.h" |
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#include "llvm/Support/Debug.h" |
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#include "llvm/Support/raw_ostream.h" |
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using namespace llvm; |
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using Scaled64 = ScaledNumber<uint64_t>; |
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#define DEBUG_TYPE "synthetic-counts-propagation" |
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/// Initial synthetic count assigned to functions. |
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static cl::opt<int> |
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InitialSyntheticCount("initial-synthetic-count", cl::Hidden, cl::init(10), |
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cl::ZeroOrMore, |
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cl::desc("Initial value of synthetic entry count.")); |
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/// Initial synthetic count assigned to inline functions. |
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static cl::opt<int> InlineSyntheticCount( |
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"inline-synthetic-count", cl::Hidden, cl::init(15), cl::ZeroOrMore, |
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cl::desc("Initial synthetic entry count for inline functions.")); |
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/// Initial synthetic count assigned to cold functions. |
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static cl::opt<int> ColdSyntheticCount( |
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"cold-synthetic-count", cl::Hidden, cl::init(5), cl::ZeroOrMore, |
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cl::desc("Initial synthetic entry count for cold functions.")); |
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// Assign initial synthetic entry counts to functions. |
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static void |
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initializeCounts(Module &M, function_ref<void(Function *, uint64_t)> SetCount) { |
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auto MayHaveIndirectCalls = [](Function &F) { |
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for (auto *U : F.users()) { |
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if (!isa<CallInst>(U) && !isa<InvokeInst>(U)) |
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return true; |
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} |
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return false; |
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}; |
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for (Function &F : M) { |
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uint64_t InitialCount = InitialSyntheticCount; |
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if (F.isDeclaration()) |
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continue; |
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if (F.hasFnAttribute(Attribute::AlwaysInline) || |
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F.hasFnAttribute(Attribute::InlineHint)) { |
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// Use a higher value for inline functions to account for the fact that |
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// these are usually beneficial to inline. |
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InitialCount = InlineSyntheticCount; |
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} else if (F.hasLocalLinkage() && !MayHaveIndirectCalls(F)) { |
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// Local functions without inline hints get counts only through |
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// propagation. |
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InitialCount = 0; |
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} else if (F.hasFnAttribute(Attribute::Cold) || |
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F.hasFnAttribute(Attribute::NoInline)) { |
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// Use a lower value for noinline and cold functions. |
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InitialCount = ColdSyntheticCount; |
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} |
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SetCount(&F, InitialCount); |
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} |
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} |
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PreservedAnalyses SyntheticCountsPropagation::run(Module &M, |
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ModuleAnalysisManager &MAM) { |
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FunctionAnalysisManager &FAM = |
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MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager(); |
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DenseMap<Function *, uint64_t> Counts; |
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// Set initial entry counts. |
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initializeCounts(M, [&](Function *F, uint64_t Count) { Counts[F] = Count; }); |
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// Compute the relative block frequency for a callsite. Use scaled numbers |
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// and not integers since the relative block frequency could be less than 1. |
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auto GetCallSiteRelFreq = [&](CallSite CS) { |
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Function *Caller = CS.getCaller(); |
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auto &BFI = FAM.getResult<BlockFrequencyAnalysis>(*Caller); |
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BasicBlock *CSBB = CS.getInstruction()->getParent(); |
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Scaled64 EntryFreq(BFI.getEntryFreq(), 0); |
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Scaled64 BBFreq(BFI.getBlockFreq(CSBB).getFrequency(), 0); |
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BBFreq /= EntryFreq; |
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return BBFreq; |
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}; |
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CallGraph CG(M); |
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// Propgate the entry counts on the callgraph. |
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propagateSyntheticCounts( |
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CG, GetCallSiteRelFreq, [&](Function *F) { return Counts[F]; }, |
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[&](Function *F, uint64_t New) { Counts[F] += New; }); |
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// Set the counts as metadata. |
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for (auto Entry : Counts) |
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Entry.first->setEntryCount(Entry.second, true); |
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return PreservedAnalyses::all(); |
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} |