LLVM 17.0.0git
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1//===-- BasicBlockSections.cpp ---=========--------------------------------===//
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
9// BasicBlockSections implementation.
11// The purpose of this pass is to assign sections to basic blocks when
12// -fbasic-block-sections= option is used. Further, with profile information
13// only the subset of basic blocks with profiles are placed in separate sections
14// and the rest are grouped in a cold section. The exception handling blocks are
15// treated specially to ensure they are all in one seciton.
17// Basic Block Sections
18// ====================
20// With option, -fbasic-block-sections=list, every function may be split into
21// clusters of basic blocks. Every cluster will be emitted into a separate
22// section with its basic blocks sequenced in the given order. To get the
23// optimized performance, the clusters must form an optimal BB layout for the
24// function. We insert a symbol at the beginning of every cluster's section to
25// allow the linker to reorder the sections in any arbitrary sequence. A global
26// order of these sections would encapsulate the function layout.
27// For example, consider the following clusters for a function foo (consisting
28// of 6 basic blocks 0, 1, ..., 5).
30// 0 2
31// 1 3 5
33// * Basic blocks 0 and 2 are placed in one section with symbol `foo`
34// referencing the beginning of this section.
35// * Basic blocks 1, 3, 5 are placed in a separate section. A new symbol
36// `foo.__part.1` will reference the beginning of this section.
37// * Basic block 4 (note that it is not referenced in the list) is placed in
38// one section, and a new symbol `foo.cold` will point to it.
40// There are a couple of challenges to be addressed:
42// 1. The last basic block of every cluster should not have any implicit
43// fallthrough to its next basic block, as it can be reordered by the linker.
44// The compiler should make these fallthroughs explicit by adding
45// unconditional jumps..
47// 2. All inter-cluster branch targets would now need to be resolved by the
48// linker as they cannot be calculated during compile time. This is done
49// using static relocations. Further, the compiler tries to use short branch
50// instructions on some ISAs for small branch offsets. This is not possible
51// for inter-cluster branches as the offset is not determined at compile
52// time, and therefore, long branch instructions have to be used for those.
54// 3. Debug Information (DebugInfo) and Call Frame Information (CFI) emission
55// needs special handling with basic block sections. DebugInfo needs to be
56// emitted with more relocations as basic block sections can break a
57// function into potentially several disjoint pieces, and CFI needs to be
58// emitted per cluster. This also bloats the object file and binary sizes.
60// Basic Block Labels
61// ==================
63// With -fbasic-block-sections=labels, we encode the offsets of BB addresses of
64// every function into the .llvm_bb_addr_map section. Along with the function
65// symbols, this allows for mapping of virtual addresses in PMU profiles back to
66// the corresponding basic blocks. This logic is implemented in AsmPrinter. This
67// pass only assigns the BBSectionType of every function to ``labels``.
72#include "llvm/ADT/StringRef.h"
77#include "llvm/CodeGen/Passes.h"
81#include <optional>
83using namespace llvm;
85// Placing the cold clusters in a separate section mitigates against poor
86// profiles and allows optimizations such as hugepage mapping to be applied at a
87// section granularity. Defaults to ".text.split." which is recognized by lld
88// via the `-z keep-text-section-prefix` flag.
90 "bbsections-cold-text-prefix",
91 cl::desc("The text prefix to use for cold basic block clusters"),
92 cl::init(".text.split."), cl::Hidden);
95 "bbsections-detect-source-drift",
96 cl::desc("This checks if there is a fdo instr. profile hash "
97 "mismatch for this function"),
98 cl::init(true), cl::Hidden);
100namespace {
102class BasicBlockSections : public MachineFunctionPass {
104 static char ID;
106 BasicBlockSectionsProfileReader *BBSectionsProfileReader = nullptr;
108 BasicBlockSections() : MachineFunctionPass(ID) {
110 }
112 StringRef getPassName() const override {
113 return "Basic Block Sections Analysis";
114 }
116 void getAnalysisUsage(AnalysisUsage &AU) const override;
118 /// Identify basic blocks that need separate sections and prepare to emit them
119 /// accordingly.
120 bool runOnMachineFunction(MachineFunction &MF) override;
123} // end anonymous namespace
125char BasicBlockSections::ID = 0;
126INITIALIZE_PASS(BasicBlockSections, "bbsections-prepare",
127 "Prepares for basic block sections, by splitting functions "
128 "into clusters of basic blocks.",
129 false, false)
131// This function updates and optimizes the branching instructions of every basic
132// block in a given function to account for changes in the layout.
133static void
134updateBranches(MachineFunction &MF,
136 const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
138 for (auto &MBB : MF) {
139 auto NextMBBI = std::next(MBB.getIterator());
140 auto *FTMBB = PreLayoutFallThroughs[MBB.getNumber()];
141 // If this block had a fallthrough before we need an explicit unconditional
142 // branch to that block if either
143 // 1- the block ends a section, which means its next block may be
144 // reorderd by the linker, or
145 // 2- the fallthrough block is not adjacent to the block in the new
146 // order.
147 if (FTMBB && (MBB.isEndSection() || &*NextMBBI != FTMBB))
148 TII->insertUnconditionalBranch(MBB, FTMBB, MBB.findBranchDebugLoc());
150 // We do not optimize branches for machine basic blocks ending sections, as
151 // their adjacent block might be reordered by the linker.
152 if (MBB.isEndSection())
153 continue;
155 // It might be possible to optimize branches by flipping the branch
156 // condition.
157 Cond.clear();
158 MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
159 if (TII->analyzeBranch(MBB, TBB, FBB, Cond))
160 continue;
161 MBB.updateTerminator(FTMBB);
162 }
165// This function provides the BBCluster information associated with a function.
166// Returns true if a valid association exists and false otherwise.
168 const MachineFunction &MF,
169 BasicBlockSectionsProfileReader *BBSectionsProfileReader,
172 // Find the assoicated cluster information.
173 std::pair<bool, SmallVector<BBClusterInfo, 4>> P =
174 BBSectionsProfileReader->getBBClusterInfoForFunction(MF.getName());
175 if (!P.first)
176 return false;
178 if (P.second.empty()) {
179 // This indicates that sections are desired for all basic blocks of this
180 // function. We clear the BBClusterInfo vector to denote this.
181 V.clear();
182 return true;
183 }
185 for (const BBClusterInfo &BBCI : P.second)
187 return true;
190// This function sorts basic blocks according to the cluster's information.
191// All explicitly specified clusters of basic blocks will be ordered
192// accordingly. All non-specified BBs go into a separate "Cold" section.
193// Additionally, if exception handling landing pads end up in more than one
194// clusters, they are moved into a single "Exception" section. Eventually,
195// clusters are ordered in increasing order of their IDs, with the "Exception"
196// and "Cold" succeeding all other clusters.
197// FuncBBClusterInfo represent the cluster information for basic blocks. It
198// maps from BBID of basic blocks to their cluster information. If this is
199// empty, it means unique sections for all basic blocks in the function.
200static void
202 const DenseMap<unsigned, BBClusterInfo> &FuncBBClusterInfo) {
203 assert(MF.hasBBSections() && "BB Sections is not set for function.");
204 // This variable stores the section ID of the cluster containing eh_pads (if
205 // all eh_pads are one cluster). If more than one cluster contain eh_pads, we
206 // set it equal to ExceptionSectionID.
207 std::optional<MBBSectionID> EHPadsSectionID;
209 for (auto &MBB : MF) {
210 // With the 'all' option, every basic block is placed in a unique section.
211 // With the 'list' option, every basic block is placed in a section
212 // associated with its cluster, unless we want individual unique sections
213 // for every basic block in this function (if FuncBBClusterInfo is empty).
215 FuncBBClusterInfo.empty()) {
216 // If unique sections are desired for all basic blocks of the function, we
217 // set every basic block's section ID equal to its original position in
218 // the layout (which is equal to its number). This ensures that basic
219 // blocks are ordered canonically.
221 } else {
222 // TODO: Replace `getBBIDOrNumber` with `getBBID` once version 1 is
223 // deprecated.
224 auto I = FuncBBClusterInfo.find(MBB.getBBIDOrNumber());
225 if (I != FuncBBClusterInfo.end()) {
226 MBB.setSectionID(I->second.ClusterID);
227 } else {
228 // BB goes into the special cold section if it is not specified in the
229 // cluster info map.
231 }
232 }
234 if (MBB.isEHPad() && EHPadsSectionID != MBB.getSectionID() &&
235 EHPadsSectionID != MBBSectionID::ExceptionSectionID) {
236 // If we already have one cluster containing eh_pads, this must be updated
237 // to ExceptionSectionID. Otherwise, we set it equal to the current
238 // section ID.
239 EHPadsSectionID = EHPadsSectionID ? MBBSectionID::ExceptionSectionID
240 : MBB.getSectionID();
241 }
242 }
244 // If EHPads are in more than one section, this places all of them in the
245 // special exception section.
246 if (EHPadsSectionID == MBBSectionID::ExceptionSectionID)
247 for (auto &MBB : MF)
248 if (MBB.isEHPad())
249 MBB.setSectionID(*EHPadsSectionID);
254 [[maybe_unused]] const MachineBasicBlock *EntryBlock = &MF.front();
256 for (auto &MBB : MF)
259 MF.sort(MBBCmp);
260 assert(&MF.front() == EntryBlock &&
261 "Entry block should not be displaced by basic block sections");
263 // Set IsBeginSection and IsEndSection according to the assigned section IDs.
266 // After reordering basic blocks, we must update basic block branches to
267 // insert explicit fallthrough branches when required and optimize branches
268 // when possible.
269 updateBranches(MF, PreLayoutFallThroughs);
272// If the exception section begins with a landing pad, that landing pad will
273// assume a zero offset (relative to @LPStart) in the LSDA. However, a value of
274// zero implies "no landing pad." This function inserts a NOP just before the EH
275// pad label to ensure a nonzero offset.
277 for (auto &MBB : MF) {
278 if (MBB.isBeginSection() && MBB.isEHPad()) {
280 while (!MI->isEHLabel())
281 ++MI;
282 MCInst Nop = MF.getSubtarget().getInstrInfo()->getNop();
284 MF.getSubtarget().getInstrInfo()->get(Nop.getOpcode()));
285 }
286 }
289// This checks if the source of this function has drifted since this binary was
290// profiled previously. For now, we are piggy backing on what PGO does to
291// detect this with instrumented profiles. PGO emits an hash of the IR and
292// checks if the hash has changed. Advanced basic block layout is usually done
293// on top of PGO optimized binaries and hence this check works well in practice.
296 return false;
298 const char MetadataName[] = "instr_prof_hash_mismatch";
299 auto *Existing = MF.getFunction().getMetadata(LLVMContext::MD_annotation);
300 if (Existing) {
301 MDTuple *Tuple = cast<MDTuple>(Existing);
302 for (const auto &N : Tuple->operands())
303 if (N.equalsStr(MetadataName))
304 return true;
305 }
307 return false;
310bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
311 auto BBSectionsType = MF.getTarget().getBBSectionsType();
312 assert(BBSectionsType != BasicBlockSection::None &&
313 "BB Sections not enabled!");
315 // Check for source drift. If the source has changed since the profiles
316 // were obtained, optimizing basic blocks might be sub-optimal.
317 // This only applies to BasicBlockSection::List as it creates
318 // clusters of basic blocks using basic block ids. Source drift can
319 // invalidate these groupings leading to sub-optimal code generation with
320 // regards to performance.
321 if (BBSectionsType == BasicBlockSection::List &&
323 return true;
324 // Renumber blocks before sorting them. This is useful during sorting,
325 // basic blocks in the same section will retain the default order.
326 // This renumbering should also be done for basic block labels to match the
327 // profiles with the correct blocks.
328 // For LLVM_BB_ADDR_MAP versions 2 and higher, this renumbering serves
329 // the different purpose of accessing the original layout positions and
330 // finding the original fallthroughs.
331 // TODO: Change the above comment accordingly when version 1 is deprecated.
332 MF.RenumberBlocks();
334 if (BBSectionsType == BasicBlockSection::Labels) {
335 MF.setBBSectionsType(BBSectionsType);
336 return true;
337 }
339 BBSectionsProfileReader = &getAnalysis<BasicBlockSectionsProfileReader>();
341 // Map from BBID of blocks to their cluster information.
342 DenseMap<unsigned, BBClusterInfo> FuncBBClusterInfo;
343 if (BBSectionsType == BasicBlockSection::List &&
344 !getBBClusterInfoForFunction(MF, BBSectionsProfileReader,
345 FuncBBClusterInfo))
346 return true;
347 MF.setBBSectionsType(BBSectionsType);
348 assignSections(MF, FuncBBClusterInfo);
350 // We make sure that the cluster including the entry basic block precedes all
351 // other clusters.
352 auto EntryBBSectionID = MF.front().getSectionID();
354 // Helper function for ordering BB sections as follows:
355 // * Entry section (section including the entry block).
356 // * Regular sections (in increasing order of their Number).
357 // ...
358 // * Exception section
359 // * Cold section
360 auto MBBSectionOrder = [EntryBBSectionID](const MBBSectionID &LHS,
361 const MBBSectionID &RHS) {
362 // We make sure that the section containing the entry block precedes all the
363 // other sections.
364 if (LHS == EntryBBSectionID || RHS == EntryBBSectionID)
365 return LHS == EntryBBSectionID;
366 return LHS.Type == RHS.Type ? LHS.Number < RHS.Number : LHS.Type < RHS.Type;
367 };
369 // We sort all basic blocks to make sure the basic blocks of every cluster are
370 // contiguous and ordered accordingly. Furthermore, clusters are ordered in
371 // increasing order of their section IDs, with the exception and the
372 // cold section placed at the end of the function.
373 auto Comparator = [&](const MachineBasicBlock &X,
374 const MachineBasicBlock &Y) {
375 auto XSectionID = X.getSectionID();
376 auto YSectionID = Y.getSectionID();
377 if (XSectionID != YSectionID)
378 return MBBSectionOrder(XSectionID, YSectionID);
379 // If the two basic block are in the same section, the order is decided by
380 // their position within the section.
381 if (XSectionID.Type == MBBSectionID::SectionType::Default)
382 return FuncBBClusterInfo.lookup(X.getBBIDOrNumber()).PositionInCluster <
383 FuncBBClusterInfo.lookup(Y.getBBIDOrNumber()).PositionInCluster;
384 return X.getNumber() < Y.getNumber();
385 };
387 sortBasicBlocksAndUpdateBranches(MF, Comparator);
389 return true;
392void BasicBlockSections::getAnalysisUsage(AnalysisUsage &AU) const {
393 AU.setPreservesAll();
399 return new BasicBlockSections();
aarch64 promote const
MachineBasicBlock & MBB
SmallVector< MachineOperand, 4 > Cond
static cl::opt< bool > BBSectionsDetectSourceDrift("bbsections-detect-source-drift", cl::desc("This checks if there is a fdo instr. profile hash " "mismatch for this function"), cl::init(true), cl::Hidden)
const SmallVector< MachineBasicBlock * > & PreLayoutFallThroughs
static bool hasInstrProfHashMismatch(MachineFunction &MF)
bool getBBClusterInfoForFunction(const MachineFunction &MF, BasicBlockSectionsProfileReader *BBSectionsProfileReader, DenseMap< unsigned, BBClusterInfo > &V)
static void assignSections(MachineFunction &MF, const DenseMap< unsigned, BBClusterInfo > &FuncBBClusterInfo)
static GCMetadataPrinterRegistry::Add< ErlangGCPrinter > X("erlang", "erlang-compatible garbage collector")
const HexagonInstrInfo * TII
IRTranslator LLVM IR MI
#define I(x, y, z)
Definition: MD5.cpp:58
static GCMetadataPrinterRegistry::Add< OcamlGCMetadataPrinter > Y("ocaml", "ocaml 3.10-compatible collector")
#define P(N)
#define INITIALIZE_PASS(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:38
const SmallVectorImpl< MachineOperand > MachineBasicBlock * TBB
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
This file defines the SmallVector class.
Value * RHS
Value * LHS
Represent the analysis usage information of a pass.
AnalysisUsage & addRequired()
void setPreservesAll()
Set by analyses that do not transform their input at all.
std::pair< bool, SmallVector< BBClusterInfo > > getBBClusterInfoForFunction(StringRef FuncName) const
A debug info location.
Definition: DebugLoc.h:33
ValueT lookup(const_arg_type_t< KeyT > Val) const
lookup - Return the entry for the specified key, or a default constructed value if no such entry exis...
Definition: DenseMap.h:202
iterator find(const_arg_type_t< KeyT > Val)
Definition: DenseMap.h:155
bool empty() const
Definition: DenseMap.h:98
iterator end()
Definition: DenseMap.h:84
MDNode * getMetadata(unsigned KindID) const
Get the current metadata attachments for the given kind, if any.
Definition: Metadata.cpp:1354
bool analyzeBranch(MachineBasicBlock &MBB, MachineBasicBlock *&TBB, MachineBasicBlock *&FBB, SmallVectorImpl< MachineOperand > &Cond, bool AllowModify) const override
Analyze the branching code at the end of MBB, returning true if it cannot be understood (e....
Instances of this class represent a single low-level machine instruction.
Definition: MCInst.h:184
unsigned getOpcode() const
Definition: MCInst.h:198
const MCInstrDesc & get(unsigned Opcode) const
Return the machine instruction descriptor that corresponds to the specified instruction opcode.
Definition: MCInstrInfo.h:63
ArrayRef< MDOperand > operands() const
Definition: Metadata.h:1301
Tuple of metadata.
Definition: Metadata.h:1345
bool isEHPad() const
Returns true if the block is a landing pad.
MachineBasicBlock * getFallThrough(bool JumpToFallThrough=true)
Return the fallthrough block if the block can implicitly transfer control to the block after it by fa...
int getNumber() const
MachineBasicBlocks are uniquely numbered at the function level, unless they're not in a MachineFuncti...
void updateTerminator(MachineBasicBlock *PreviousLayoutSuccessor)
Update the terminator instructions in block to account for changes to block layout which may have bee...
MBBSectionID getSectionID() const
Returns the section ID of this basic block.
unsigned getBBIDOrNumber() const
Returns the BBID of the block when BBAddrMapVersion >= 2, otherwise returns MachineBasicBlock::Number...
void setSectionID(MBBSectionID V)
Sets the section ID for this basic block.
bool isBeginSection() const
Returns true if this block begins any section.
DebugLoc findBranchDebugLoc()
Find and return the merged DebugLoc of the branch instructions of the block.
bool isEndSection() const
Returns true if this block ends any section.
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
void getAnalysisUsage(AnalysisUsage &AU) const override
getAnalysisUsage - Subclasses that override getAnalysisUsage must call this.
virtual bool runOnMachineFunction(MachineFunction &MF)=0
runOnMachineFunction - This method must be overloaded to perform the desired machine code transformat...
void setBBSectionsType(BasicBlockSection V)
const TargetSubtargetInfo & getSubtarget() const
getSubtarget - Return the subtarget for which this machine code is being compiled.
StringRef getName() const
getName - Return the name of the corresponding LLVM function.
bool hasBBSections() const
Returns true if this function has basic block sections enabled.
Function & getFunction()
Return the LLVM function that this machine code represents.
const LLVMTargetMachine & getTarget() const
getTarget - Return the target machine this machine code is compiled with
unsigned getNumBlockIDs() const
getNumBlockIDs - Return the number of MBB ID's allocated.
void sort(Comp comp)
void RenumberBlocks(MachineBasicBlock *MBBFrom=nullptr)
RenumberBlocks - This discards all of the MachineBasicBlock numbers and recomputes them.
const MachineBasicBlock & front() const
void assignBeginEndSections()
Assign IsBeginSection IsEndSection fields for basic blocks in this function.
static PassRegistry * getPassRegistry()
getPassRegistry - Access the global registry object, which is automatically initialized at applicatio...
virtual StringRef getPassName() const
getPassName - Return a nice clean name for a pass.
Definition: Pass.cpp:81
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:50
TargetInstrInfo - Interface to description of machine instruction set.
virtual MCInst getNop() const
Return the noop instruction to use for a noop.
llvm::BasicBlockSection getBBSectionsType() const
If basic blocks should be emitted into their own section, corresponding to -fbasic-block-sections.
virtual const TargetInstrInfo * getInstrInfo() const
An efficient, type-erasing, non-owning reference to a callable.
self_iterator getIterator()
Definition: ilist_node.h:82
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
initializer< Ty > init(const Ty &Val)
Definition: CommandLine.h:445
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
MachineInstrBuilder BuildMI(MachineFunction &MF, const MIMetadata &MIMD, const MCInstrDesc &MCID)
Builder interface. Specify how to create the initial instruction itself.
MachineFunctionPass * createBasicBlockSectionsPass()
createBasicBlockSections Pass - This pass assigns sections to machine basic blocks and is enabled wit...
void initializeBasicBlockSectionsPass(PassRegistry &)
void avoidZeroOffsetLandingPad(MachineFunction &MF)
cl::opt< std::string > BBSectionsColdTextPrefix
void sortBasicBlocksAndUpdateBranches(MachineFunction &MF, MachineBasicBlockComparator MBBCmp)
#define N
static const MBBSectionID ExceptionSectionID
static const MBBSectionID ColdSectionID