c8a70f4c6e
In this PR, static-data-splitter pass finds out the local-linkage global
variables in {`.rodata`, `.data.rel.ro`, `bss`, `.data`} sections by
analyzing machine instruction operands, and aggregates their accesses
from code across functions.
A follow-up item is to analyze global variable initializers and count
for access from data.
* This limitation is demonstrated by `bss2` and `data3` in
`llvm/test/CodeGen/X86/global-variable-partition.ll`.
Some stats of static-data-splitter with this patch:
**section**|**bss**|**rodata**|**data**
:-----:|:-----:|:-----:|:-----:
hot-prefixed section coverage|99.75%|97.71%|91.30%
unlikely-prefixed section size percentage|67.94%|39.37%|63.10%
1. The coverage is defined as `#perf-sample-in-hot-prefixed <data>
section / #perf-sample in <data.*> section` for each <data> section.
* The perf command samples
`MEM_INST_RETIRED.ALL_LOADS:u:pinned:precise=2` events at a high
frequency (`perf -c 2251`) for 30 seconds. The profiled binary is built
as non-PIE so `data.rel.ro` coverage data is not available.
2. The unlikely-prefixed `<data>` section size percentage is defined as
`unlikely <data> section size / the sum size of <data>.* sections` for
each `<data>` section
110 lines
4.0 KiB
C++
110 lines
4.0 KiB
C++
//===- StaticDataAnnotator - Annotate static data's section prefix --------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// To reason about module-wide data hotness in a module granularity, this file
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// implements a module pass StaticDataAnnotator to work coordinately with the
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// StaticDataSplitter pass.
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//
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// The StaticDataSplitter pass is a machine function pass. It analyzes data
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// hotness based on code and adds counters in StaticDataProfileInfo via its
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// wrapper pass StaticDataProfileInfoWrapper.
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// The StaticDataProfileInfoWrapper sits in the middle between the
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// StaticDataSplitter and StaticDataAnnotator passes.
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// The StaticDataAnnotator pass is a module pass. It iterates global variables
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// in the module, looks up counters from StaticDataProfileInfo and sets the
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// section prefix based on profiles.
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//
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// The three-pass structure is implemented for practical reasons, to work around
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// the limitation that a module pass based on legacy pass manager cannot make
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// use of MachineBlockFrequencyInfo analysis. In the future, we can consider
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// porting the StaticDataSplitter pass to a module-pass using the new pass
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// manager framework. That way, analysis are lazily computed as opposed to
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// eagerly scheduled, and a module pass can use MachineBlockFrequencyInfo.
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/ProfileSummaryInfo.h"
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#include "llvm/Analysis/StaticDataProfileInfo.h"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/IR/Analysis.h"
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#include "llvm/IR/Module.h"
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#include "llvm/IR/PassManager.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/raw_ostream.h"
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#define DEBUG_TYPE "static-data-annotator"
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using namespace llvm;
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/// A module pass which iterates global variables in the module and annotates
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/// their section prefixes based on profile-driven analysis.
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class StaticDataAnnotator : public ModulePass {
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public:
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static char ID;
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StaticDataProfileInfo *SDPI = nullptr;
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const ProfileSummaryInfo *PSI = nullptr;
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StaticDataAnnotator() : ModulePass(ID) {
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initializeStaticDataAnnotatorPass(*PassRegistry::getPassRegistry());
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}
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<StaticDataProfileInfoWrapperPass>();
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AU.addRequired<ProfileSummaryInfoWrapperPass>();
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AU.setPreservesAll();
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ModulePass::getAnalysisUsage(AU);
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}
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StringRef getPassName() const override { return "Static Data Annotator"; }
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bool runOnModule(Module &M) override;
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};
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bool StaticDataAnnotator::runOnModule(Module &M) {
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SDPI = &getAnalysis<StaticDataProfileInfoWrapperPass>()
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.getStaticDataProfileInfo();
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PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
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if (!PSI->hasProfileSummary())
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return false;
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bool Changed = false;
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for (auto &GV : M.globals()) {
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if (GV.isDeclarationForLinker())
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continue;
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// The implementation below assumes prior passes don't set section prefixes,
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// and specifically do 'assign' rather than 'update'. So report error if a
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// section prefix is already set.
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if (auto maybeSectionPrefix = GV.getSectionPrefix();
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maybeSectionPrefix && !maybeSectionPrefix->empty())
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llvm::report_fatal_error("Global variable " + GV.getName() +
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" already has a section prefix " +
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*maybeSectionPrefix);
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StringRef SectionPrefix = SDPI->getConstantSectionPrefix(&GV, PSI);
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if (SectionPrefix.empty())
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continue;
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GV.setSectionPrefix(SectionPrefix);
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Changed = true;
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}
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return Changed;
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}
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char StaticDataAnnotator::ID = 0;
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INITIALIZE_PASS(StaticDataAnnotator, DEBUG_TYPE, "Static Data Annotator", false,
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false)
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ModulePass *llvm::createStaticDataAnnotatorPass() {
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return new StaticDataAnnotator();
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}
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