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//===-- BasicBlockSections.cpp ---=========--------------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// BasicBlockSections implementation.
// The purpose of this pass is to assign sections to basic blocks when
// -fbasic-block-sections= option is used. Further, with profile information
// only the subset of basic blocks with profiles are placed in separate sections
// and the rest are grouped in a cold section. The exception handling blocks are
// treated specially to ensure they are all in one seciton.
// Basic Block Sections
// ====================
// With option, -fbasic-block-sections=list, every function may be split into
// clusters of basic blocks. Every cluster will be emitted into a separate
// section with its basic blocks sequenced in the given order. To get the
// optimized performance, the clusters must form an optimal BB layout for the
// function. We insert a symbol at the beginning of every cluster's section to
// allow the linker to reorder the sections in any arbitrary sequence. A global
// order of these sections would encapsulate the function layout.
// For example, consider the following clusters for a function foo (consisting
// of 6 basic blocks 0, 1, ..., 5).
// 0 2
// 1 3 5
// * Basic blocks 0 and 2 are placed in one section with symbol `foo`
// referencing the beginning of this section.
// * Basic blocks 1, 3, 5 are placed in a separate section. A new symbol
// `foo.__part.1` will reference the beginning of this section.
// * Basic block 4 (note that it is not referenced in the list) is placed in
// one section, and a new symbol `foo.cold` will point to it.
// There are a couple of challenges to be addressed:
// 1. The last basic block of every cluster should not have any implicit
// fallthrough to its next basic block, as it can be reordered by the linker.
// The compiler should make these fallthroughs explicit by adding
// unconditional jumps..
// 2. All inter-cluster branch targets would now need to be resolved by the
// linker as they cannot be calculated during compile time. This is done
// using static relocations. Further, the compiler tries to use short branch
// instructions on some ISAs for small branch offsets. This is not possible
// for inter-cluster branches as the offset is not determined at compile
// time, and therefore, long branch instructions have to be used for those.
// 3. Debug Information (DebugInfo) and Call Frame Information (CFI) emission
// needs special handling with basic block sections. DebugInfo needs to be
// emitted with more relocations as basic block sections can break a
// function into potentially several disjoint pieces, and CFI needs to be
// emitted per cluster. This also bloats the object file and binary sizes.
// Basic Block Labels
// ==================
// With -fbasic-block-sections=labels, we encode the offsets of BB addresses of
// every function into the .llvm_bb_addr_map section. Along with the function
// symbols, this allows for mapping of virtual addresses in PMU profiles back to
// the corresponding basic blocks. This logic is implemented in AsmPrinter. This
// pass only assigns the BBSectionType of every function to ``labels``.
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CodeGen/BasicBlockSectionUtils.h"
#include "llvm/CodeGen/BasicBlockSectionsProfileReader.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Target/TargetMachine.h"
#include <optional>
using namespace llvm;
// Placing the cold clusters in a separate section mitigates against poor
// profiles and allows optimizations such as hugepage mapping to be applied at a
// section granularity. Defaults to ".text.split." which is recognized by lld
// via the `-z keep-text-section-prefix` flag.
cl::opt<std::string> llvm::BBSectionsColdTextPrefix(
cl::desc("The text prefix to use for cold basic block clusters"),
cl::init(".text.split."), cl::Hidden);
static cl::opt<bool> BBSectionsDetectSourceDrift(
cl::desc("This checks if there is a fdo instr. profile hash "
"mismatch for this function"),
cl::init(true), cl::Hidden);
namespace {
class BasicBlockSections : public MachineFunctionPass {
static char ID;
BasicBlockSectionsProfileReader *BBSectionsProfileReader = nullptr;
BasicBlockSections() : MachineFunctionPass(ID) {
StringRef getPassName() const override {
return "Basic Block Sections Analysis";
void getAnalysisUsage(AnalysisUsage &AU) const override;
/// Identify basic blocks that need separate sections and prepare to emit them
/// accordingly.
bool runOnMachineFunction(MachineFunction &MF) override;
} // end anonymous namespace
char BasicBlockSections::ID = 0;
INITIALIZE_PASS(BasicBlockSections, "bbsections-prepare",
"Prepares for basic block sections, by splitting functions "
"into clusters of basic blocks.",
false, false)
// This function updates and optimizes the branching instructions of every basic
// block in a given function to account for changes in the layout.
static void
updateBranches(MachineFunction &MF,
const SmallVector<MachineBasicBlock *> &PreLayoutFallThroughs) {
const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
SmallVector<MachineOperand, 4> Cond;
for (auto &MBB : MF) {
auto NextMBBI = std::next(MBB.getIterator());
auto *FTMBB = PreLayoutFallThroughs[MBB.getNumber()];
// If this block had a fallthrough before we need an explicit unconditional
// branch to that block if either
// 1- the block ends a section, which means its next block may be
// reorderd by the linker, or
// 2- the fallthrough block is not adjacent to the block in the new
// order.
if (FTMBB && (MBB.isEndSection() || &*NextMBBI != FTMBB))
TII->insertUnconditionalBranch(MBB, FTMBB, MBB.findBranchDebugLoc());
// We do not optimize branches for machine basic blocks ending sections, as
// their adjacent block might be reordered by the linker.
if (MBB.isEndSection())
// It might be possible to optimize branches by flipping the branch
// condition.
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
if (TII->analyzeBranch(MBB, TBB, FBB, Cond))
// This function provides the BBCluster information associated with a function.
// Returns true if a valid association exists and false otherwise.
bool getBBClusterInfoForFunction(
const MachineFunction &MF,
BasicBlockSectionsProfileReader *BBSectionsProfileReader,
DenseMap<unsigned, BBClusterInfo> &V) {
// Find the assoicated cluster information.
std::pair<bool, SmallVector<BBClusterInfo, 4>> P =
if (!P.first)
return false;
if (P.second.empty()) {
// This indicates that sections are desired for all basic blocks of this
// function. We clear the BBClusterInfo vector to denote this.
return true;
for (const BBClusterInfo &BBCI : P.second)
return true;
// This function sorts basic blocks according to the cluster's information.
// All explicitly specified clusters of basic blocks will be ordered
// accordingly. All non-specified BBs go into a separate "Cold" section.
// Additionally, if exception handling landing pads end up in more than one
// clusters, they are moved into a single "Exception" section. Eventually,
// clusters are ordered in increasing order of their IDs, with the "Exception"
// and "Cold" succeeding all other clusters.
// FuncBBClusterInfo represent the cluster information for basic blocks. It
// maps from BBID of basic blocks to their cluster information. If this is
// empty, it means unique sections for all basic blocks in the function.
static void
assignSections(MachineFunction &MF,
const DenseMap<unsigned, BBClusterInfo> &FuncBBClusterInfo) {
assert(MF.hasBBSections() && "BB Sections is not set for function.");
// This variable stores the section ID of the cluster containing eh_pads (if
// all eh_pads are one cluster). If more than one cluster contain eh_pads, we
// set it equal to ExceptionSectionID.
std::optional<MBBSectionID> EHPadsSectionID;
for (auto &MBB : MF) {
// With the 'all' option, every basic block is placed in a unique section.
// With the 'list' option, every basic block is placed in a section
// associated with its cluster, unless we want individual unique sections
// for every basic block in this function (if FuncBBClusterInfo is empty).
if (MF.getTarget().getBBSectionsType() == llvm::BasicBlockSection::All ||
FuncBBClusterInfo.empty()) {
// If unique sections are desired for all basic blocks of the function, we
// set every basic block's section ID equal to its original position in
// the layout (which is equal to its number). This ensures that basic
// blocks are ordered canonically.
} else {
// TODO: Replace `getBBIDOrNumber` with `getBBID` once version 1 is
// deprecated.
auto I = FuncBBClusterInfo.find(MBB.getBBIDOrNumber());
if (I != FuncBBClusterInfo.end()) {
} else {
// BB goes into the special cold section if it is not specified in the
// cluster info map.
if (MBB.isEHPad() && EHPadsSectionID != MBB.getSectionID() &&
EHPadsSectionID != MBBSectionID::ExceptionSectionID) {
// If we already have one cluster containing eh_pads, this must be updated
// to ExceptionSectionID. Otherwise, we set it equal to the current
// section ID.
EHPadsSectionID = EHPadsSectionID ? MBBSectionID::ExceptionSectionID
: MBB.getSectionID();
// If EHPads are in more than one section, this places all of them in the
// special exception section.
if (EHPadsSectionID == MBBSectionID::ExceptionSectionID)
for (auto &MBB : MF)
if (MBB.isEHPad())
void llvm::sortBasicBlocksAndUpdateBranches(
MachineFunction &MF, MachineBasicBlockComparator MBBCmp) {
[[maybe_unused]] const MachineBasicBlock *EntryBlock = &MF.front();
SmallVector<MachineBasicBlock *> PreLayoutFallThroughs(MF.getNumBlockIDs());
for (auto &MBB : MF)
PreLayoutFallThroughs[MBB.getNumber()] = MBB.getFallThrough();
assert(&MF.front() == EntryBlock &&
"Entry block should not be displaced by basic block sections");
// Set IsBeginSection and IsEndSection according to the assigned section IDs.
// After reordering basic blocks, we must update basic block branches to
// insert explicit fallthrough branches when required and optimize branches
// when possible.
updateBranches(MF, PreLayoutFallThroughs);
// If the exception section begins with a landing pad, that landing pad will
// assume a zero offset (relative to @LPStart) in the LSDA. However, a value of
// zero implies "no landing pad." This function inserts a NOP just before the EH
// pad label to ensure a nonzero offset.
void llvm::avoidZeroOffsetLandingPad(MachineFunction &MF) {
for (auto &MBB : MF) {
if (MBB.isBeginSection() && MBB.isEHPad()) {
MachineBasicBlock::iterator MI = MBB.begin();
while (!MI->isEHLabel())
MCInst Nop = MF.getSubtarget().getInstrInfo()->getNop();
BuildMI(MBB, MI, DebugLoc(),
// This checks if the source of this function has drifted since this binary was
// profiled previously. For now, we are piggy backing on what PGO does to
// detect this with instrumented profiles. PGO emits an hash of the IR and
// checks if the hash has changed. Advanced basic block layout is usually done
// on top of PGO optimized binaries and hence this check works well in practice.
static bool hasInstrProfHashMismatch(MachineFunction &MF) {
if (!BBSectionsDetectSourceDrift)
return false;
const char MetadataName[] = "instr_prof_hash_mismatch";
auto *Existing = MF.getFunction().getMetadata(LLVMContext::MD_annotation);
if (Existing) {
MDTuple *Tuple = cast<MDTuple>(Existing);
for (const auto &N : Tuple->operands())
if (N.equalsStr(MetadataName))
return true;
return false;
bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
auto BBSectionsType = MF.getTarget().getBBSectionsType();
assert(BBSectionsType != BasicBlockSection::None &&
"BB Sections not enabled!");
// Check for source drift. If the source has changed since the profiles
// were obtained, optimizing basic blocks might be sub-optimal.
// This only applies to BasicBlockSection::List as it creates
// clusters of basic blocks using basic block ids. Source drift can
// invalidate these groupings leading to sub-optimal code generation with
// regards to performance.
if (BBSectionsType == BasicBlockSection::List &&
return true;
// Renumber blocks before sorting them. This is useful during sorting,
// basic blocks in the same section will retain the default order.
// This renumbering should also be done for basic block labels to match the
// profiles with the correct blocks.
// For LLVM_BB_ADDR_MAP versions 2 and higher, this renumbering serves
// the different purpose of accessing the original layout positions and
// finding the original fallthroughs.
// TODO: Change the above comment accordingly when version 1 is deprecated.
if (BBSectionsType == BasicBlockSection::Labels) {
return true;
BBSectionsProfileReader = &getAnalysis<BasicBlockSectionsProfileReader>();
// Map from BBID of blocks to their cluster information.
DenseMap<unsigned, BBClusterInfo> FuncBBClusterInfo;
if (BBSectionsType == BasicBlockSection::List &&
!getBBClusterInfoForFunction(MF, BBSectionsProfileReader,
return true;
assignSections(MF, FuncBBClusterInfo);
// We make sure that the cluster including the entry basic block precedes all
// other clusters.
auto EntryBBSectionID = MF.front().getSectionID();
// Helper function for ordering BB sections as follows:
// * Entry section (section including the entry block).
// * Regular sections (in increasing order of their Number).
// ...
// * Exception section
// * Cold section
auto MBBSectionOrder = [EntryBBSectionID](const MBBSectionID &LHS,
const MBBSectionID &RHS) {
// We make sure that the section containing the entry block precedes all the
// other sections.
if (LHS == EntryBBSectionID || RHS == EntryBBSectionID)
return LHS == EntryBBSectionID;
return LHS.Type == RHS.Type ? LHS.Number < RHS.Number : LHS.Type < RHS.Type;
// We sort all basic blocks to make sure the basic blocks of every cluster are
// contiguous and ordered accordingly. Furthermore, clusters are ordered in
// increasing order of their section IDs, with the exception and the
// cold section placed at the end of the function.
auto Comparator = [&](const MachineBasicBlock &X,
const MachineBasicBlock &Y) {
auto XSectionID = X.getSectionID();
auto YSectionID = Y.getSectionID();
if (XSectionID != YSectionID)
return MBBSectionOrder(XSectionID, YSectionID);
// If the two basic block are in the same section, the order is decided by
// their position within the section.
if (XSectionID.Type == MBBSectionID::SectionType::Default)
return FuncBBClusterInfo.lookup(X.getBBIDOrNumber()).PositionInCluster <
return X.getNumber() < Y.getNumber();
sortBasicBlocksAndUpdateBranches(MF, Comparator);
return true;
void BasicBlockSections::getAnalysisUsage(AnalysisUsage &AU) const {
MachineFunctionPass *llvm::createBasicBlockSectionsPass() {
return new BasicBlockSections();