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llvm-project/lld/ELF/ICF.cpp

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//===- ICF.cpp ------------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// ICF is short for Identical Code Folding. This is a size optimization to
// identify and merge two or more read-only sections (typically functions)
// that happened to have the same contents. It usually reduces output size
// by a few percent.
//
// In ICF, two sections are considered identical if they have the same
// section flags, section data, and relocations. Relocations are tricky,
// because two relocations are considered the same if they have the same
// relocation types, values, and if they point to the same sections *in
// terms of ICF*.
//
// Here is an example. If foo and bar defined below are compiled to the
// same machine instructions, ICF can and should merge the two, although
// their relocations point to each other.
//
// void foo() { bar(); }
// void bar() { foo(); }
//
// If you merge the two, their relocations point to the same section and
// thus you know they are mergeable, but how do you know they are
// mergeable in the first place? This is not an easy problem to solve.
//
// What we are doing in LLD is to partition sections into equivalence
// classes. Sections in the same equivalence class when the algorithm
// terminates are considered identical. Here are details:
//
// 1. First, we partition sections using their hash values as keys. Hash
// values contain section types, section contents and numbers of
// relocations. During this step, relocation targets are not taken into
// account. We just put sections that apparently differ into different
// equivalence classes.
//
// 2. Next, for each equivalence class, we visit sections to compare
// relocation targets. Relocation targets are considered equivalent if
// their targets are in the same equivalence class. Sections with
// different relocation targets are put into different equivalence
// classes.
//
// 3. If we split an equivalence class in step 2, two relocations
// previously target the same equivalence class may now target
// different equivalence classes. Therefore, we repeat step 2 until a
// convergence is obtained.
//
// 4. For each equivalence class C, pick an arbitrary section in C, and
// merge all the other sections in C with it.
//
// For small programs, this algorithm needs 3-5 iterations. For large
// programs such as Chromium, it takes more than 20 iterations.
//
// This algorithm was mentioned as an "optimistic algorithm" in [1],
// though gold implements a different algorithm than this.
//
// We parallelize each step so that multiple threads can work on different
// equivalence classes concurrently. That gave us a large performance
// boost when applying ICF on large programs. For example, MSVC link.exe
// or GNU gold takes 10-20 seconds to apply ICF on Chromium, whose output
// size is about 1.5 GB, but LLD can finish it in less than 2 seconds on a
// 2.8 GHz 40 core machine. Even without threading, LLD's ICF is still
// faster than MSVC or gold though.
//
// [1] Safe ICF: Pointer Safe and Unwinding aware Identical Code Folding
// in the Gold Linker
// http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/36912.pdf
//
//===----------------------------------------------------------------------===//
#include "ICF.h"
#include "Config.h"
#include "LinkerScript.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
#include "SyntheticSections.h"
#include "Writer.h"
#include "lld/Common/Threads.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/Object/ELF.h"
#include "llvm/Support/TimeProfiler.h"
#include "llvm/Support/xxhash.h"
#include <algorithm>
#include <atomic>
using namespace llvm;
using namespace llvm::ELF;
using namespace llvm::object;
namespace lld {
namespace elf {
namespace {
template <class ELFT> class ICF {
public:
void run();
private:
void segregate(size_t begin, size_t end, bool constant);
template <class RelTy>
bool constantEq(const InputSection *a, ArrayRef<RelTy> relsA,
const InputSection *b, ArrayRef<RelTy> relsB);
template <class RelTy>
bool variableEq(const InputSection *a, ArrayRef<RelTy> relsA,
const InputSection *b, ArrayRef<RelTy> relsB);
bool equalsConstant(const InputSection *a, const InputSection *b);
bool equalsVariable(const InputSection *a, const InputSection *b);
size_t findBoundary(size_t begin, size_t end);
void forEachClassRange(size_t begin, size_t end,
llvm::function_ref<void(size_t, size_t)> fn);
void forEachClass(llvm::function_ref<void(size_t, size_t)> fn);
std::vector<InputSection *> sections;
// We repeat the main loop while `Repeat` is true.
std::atomic<bool> repeat;
// The main loop counter.
int cnt = 0;
// We have two locations for equivalence classes. On the first iteration
// of the main loop, Class[0] has a valid value, and Class[1] contains
// garbage. We read equivalence classes from slot 0 and write to slot 1.
// So, Class[0] represents the current class, and Class[1] represents
// the next class. On each iteration, we switch their roles and use them
// alternately.
//
// Why are we doing this? Recall that other threads may be working on
// other equivalence classes in parallel. They may read sections that we
// are updating. We cannot update equivalence classes in place because
// it breaks the invariance that all possibly-identical sections must be
// in the same equivalence class at any moment. In other words, the for
// loop to update equivalence classes is not atomic, and that is
// observable from other threads. By writing new classes to other
// places, we can keep the invariance.
//
// Below, `Current` has the index of the current class, and `Next` has
// the index of the next class. If threading is enabled, they are either
// (0, 1) or (1, 0).
//
// Note on single-thread: if that's the case, they are always (0, 0)
// because we can safely read the next class without worrying about race
// conditions. Using the same location makes this algorithm converge
// faster because it uses results of the same iteration earlier.
int current = 0;
int next = 0;
};
}
// Returns true if section S is subject of ICF.
static bool isEligible(InputSection *s) {
if (!s->isLive() || s->keepUnique || !(s->flags & SHF_ALLOC))
return false;
// Don't merge writable sections. .data.rel.ro sections are marked as writable
// but are semantically read-only.
if ((s->flags & SHF_WRITE) && s->name != ".data.rel.ro" &&
!s->name.startswith(".data.rel.ro."))
return false;
// SHF_LINK_ORDER sections are ICF'd as a unit with their dependent sections,
// so we don't consider them for ICF individually.
if (s->flags & SHF_LINK_ORDER)
return false;
// Don't merge synthetic sections as their Data member is not valid and empty.
// The Data member needs to be valid for ICF as it is used by ICF to determine
// the equality of section contents.
if (isa<SyntheticSection>(s))
return false;
// .init and .fini contains instructions that must be executed to initialize
// and finalize the process. They cannot and should not be merged.
if (s->name == ".init" || s->name == ".fini")
return false;
// A user program may enumerate sections named with a C identifier using
// __start_* and __stop_* symbols. We cannot ICF any such sections because
// that could change program semantics.
if (isValidCIdentifier(s->name))
return false;
return true;
}
// Split an equivalence class into smaller classes.
template <class ELFT>
void ICF<ELFT>::segregate(size_t begin, size_t end, bool constant) {
// This loop rearranges sections in [Begin, End) so that all sections
// that are equal in terms of equals{Constant,Variable} are contiguous
// in [Begin, End).
//
// The algorithm is quadratic in the worst case, but that is not an
// issue in practice because the number of the distinct sections in
// each range is usually very small.
while (begin < end) {
// Divide [Begin, End) into two. Let Mid be the start index of the
// second group.
auto bound =
std::stable_partition(sections.begin() + begin + 1,
sections.begin() + end, [&](InputSection *s) {
if (constant)
return equalsConstant(sections[begin], s);
return equalsVariable(sections[begin], s);
});
size_t mid = bound - sections.begin();
// Now we split [Begin, End) into [Begin, Mid) and [Mid, End) by
// updating the sections in [Begin, Mid). We use Mid as an equivalence
// class ID because every group ends with a unique index.
for (size_t i = begin; i < mid; ++i)
sections[i]->eqClass[next] = mid;
// If we created a group, we need to iterate the main loop again.
if (mid != end)
repeat = true;
begin = mid;
}
}
// Compare two lists of relocations.
template <class ELFT>
template <class RelTy>
bool ICF<ELFT>::constantEq(const InputSection *secA, ArrayRef<RelTy> ra,
const InputSection *secB, ArrayRef<RelTy> rb) {
for (size_t i = 0; i < ra.size(); ++i) {
if (ra[i].r_offset != rb[i].r_offset ||
ra[i].getType(config->isMips64EL) != rb[i].getType(config->isMips64EL))
return false;
uint64_t addA = getAddend<ELFT>(ra[i]);
uint64_t addB = getAddend<ELFT>(rb[i]);
Symbol &sa = secA->template getFile<ELFT>()->getRelocTargetSym(ra[i]);
Symbol &sb = secB->template getFile<ELFT>()->getRelocTargetSym(rb[i]);
if (&sa == &sb) {
if (addA == addB)
continue;
return false;
}
auto *da = dyn_cast<Defined>(&sa);
auto *db = dyn_cast<Defined>(&sb);
// Placeholder symbols generated by linker scripts look the same now but
// may have different values later.
if (!da || !db || da->scriptDefined || db->scriptDefined)
return false;
// When comparing a pair of relocations, if they refer to different symbols,
// and either symbol is preemptible, the containing sections should be
// considered different. This is because even if the sections are identical
// in this DSO, they may not be after preemption.
if (da->isPreemptible || db->isPreemptible)
return false;
// Relocations referring to absolute symbols are constant-equal if their
// values are equal.
if (!da->section && !db->section && da->value + addA == db->value + addB)
continue;
if (!da->section || !db->section)
return false;
if (da->section->kind() != db->section->kind())
return false;
// Relocations referring to InputSections are constant-equal if their
// section offsets are equal.
if (isa<InputSection>(da->section)) {
if (da->value + addA == db->value + addB)
continue;
return false;
}
// Relocations referring to MergeInputSections are constant-equal if their
// offsets in the output section are equal.
auto *x = dyn_cast<MergeInputSection>(da->section);
if (!x)
return false;
auto *y = cast<MergeInputSection>(db->section);
if (x->getParent() != y->getParent())
return false;
uint64_t offsetA =
sa.isSection() ? x->getOffset(addA) : x->getOffset(da->value) + addA;
uint64_t offsetB =
sb.isSection() ? y->getOffset(addB) : y->getOffset(db->value) + addB;
if (offsetA != offsetB)
return false;
}
return true;
}
// Compare "non-moving" part of two InputSections, namely everything
// except relocation targets.
template <class ELFT>
bool ICF<ELFT>::equalsConstant(const InputSection *a, const InputSection *b) {
if (a->numRelocations != b->numRelocations || a->flags != b->flags ||
a->getSize() != b->getSize() || a->data() != b->data())
return false;
// If two sections have different output sections, we cannot merge them.
assert(a->getParent() && b->getParent());
if (a->getParent() != b->getParent())
return false;
if (a->areRelocsRela)
return constantEq(a, a->template relas<ELFT>(), b,
b->template relas<ELFT>());
return constantEq(a, a->template rels<ELFT>(), b, b->template rels<ELFT>());
}
// Compare two lists of relocations. Returns true if all pairs of
// relocations point to the same section in terms of ICF.
template <class ELFT>
template <class RelTy>
bool ICF<ELFT>::variableEq(const InputSection *secA, ArrayRef<RelTy> ra,
const InputSection *secB, ArrayRef<RelTy> rb) {
assert(ra.size() == rb.size());
for (size_t i = 0; i < ra.size(); ++i) {
// The two sections must be identical.
Symbol &sa = secA->template getFile<ELFT>()->getRelocTargetSym(ra[i]);
Symbol &sb = secB->template getFile<ELFT>()->getRelocTargetSym(rb[i]);
if (&sa == &sb)
continue;
auto *da = cast<Defined>(&sa);
auto *db = cast<Defined>(&sb);
// We already dealt with absolute and non-InputSection symbols in
// constantEq, and for InputSections we have already checked everything
// except the equivalence class.
if (!da->section)
continue;
auto *x = dyn_cast<InputSection>(da->section);
if (!x)
continue;
auto *y = cast<InputSection>(db->section);
// Ineligible sections are in the special equivalence class 0.
// They can never be the same in terms of the equivalence class.
if (x->eqClass[current] == 0)
return false;
if (x->eqClass[current] != y->eqClass[current])
return false;
};
return true;
}
// Compare "moving" part of two InputSections, namely relocation targets.
template <class ELFT>
bool ICF<ELFT>::equalsVariable(const InputSection *a, const InputSection *b) {
if (a->areRelocsRela)
return variableEq(a, a->template relas<ELFT>(), b,
b->template relas<ELFT>());
return variableEq(a, a->template rels<ELFT>(), b, b->template rels<ELFT>());
}
template <class ELFT> size_t ICF<ELFT>::findBoundary(size_t begin, size_t end) {
uint32_t eqClass = sections[begin]->eqClass[current];
for (size_t i = begin + 1; i < end; ++i)
if (eqClass != sections[i]->eqClass[current])
return i;
return end;
}
// Sections in the same equivalence class are contiguous in Sections
// vector. Therefore, Sections vector can be considered as contiguous
// groups of sections, grouped by the class.
//
// This function calls Fn on every group within [Begin, End).
template <class ELFT>
void ICF<ELFT>::forEachClassRange(size_t begin, size_t end,
llvm::function_ref<void(size_t, size_t)> fn) {
while (begin < end) {
size_t mid = findBoundary(begin, end);
fn(begin, mid);
begin = mid;
}
}
// Call Fn on each equivalence class.
template <class ELFT>
void ICF<ELFT>::forEachClass(llvm::function_ref<void(size_t, size_t)> fn) {
// If threading is disabled or the number of sections are
// too small to use threading, call Fn sequentially.
if (!threadsEnabled || sections.size() < 1024) {
forEachClassRange(0, sections.size(), fn);
++cnt;
return;
}
current = cnt % 2;
next = (cnt + 1) % 2;
// Shard into non-overlapping intervals, and call Fn in parallel.
// The sharding must be completed before any calls to Fn are made
// so that Fn can modify the Chunks in its shard without causing data
// races.
const size_t numShards = 256;
size_t step = sections.size() / numShards;
size_t boundaries[numShards + 1];
boundaries[0] = 0;
boundaries[numShards] = sections.size();
parallelForEachN(1, numShards, [&](size_t i) {
boundaries[i] = findBoundary((i - 1) * step, sections.size());
});
parallelForEachN(1, numShards + 1, [&](size_t i) {
if (boundaries[i - 1] < boundaries[i])
forEachClassRange(boundaries[i - 1], boundaries[i], fn);
});
++cnt;
}
// Combine the hashes of the sections referenced by the given section into its
// hash.
template <class ELFT, class RelTy>
static void combineRelocHashes(unsigned cnt, InputSection *isec,
ArrayRef<RelTy> rels) {
uint32_t hash = isec->eqClass[cnt % 2];
for (RelTy rel : rels) {
Symbol &s = isec->template getFile<ELFT>()->getRelocTargetSym(rel);
if (auto *d = dyn_cast<Defined>(&s))
if (auto *relSec = dyn_cast_or_null<InputSection>(d->section))
hash += relSec->eqClass[cnt % 2];
}
// Set MSB to 1 to avoid collisions with non-hash IDs.
isec->eqClass[(cnt + 1) % 2] = hash | (1U << 31);
}
static void print(const Twine &s) {
if (config->printIcfSections)
message(s);
}
// The main function of ICF.
template <class ELFT> void ICF<ELFT>::run() {
// Compute isPreemptible early. We may add more symbols later, so this loop
// cannot be merged with the later computeIsPreemptible() pass which is used
// by scanRelocations().
for (Symbol *sym : symtab->symbols())
sym->isPreemptible = computeIsPreemptible(*sym);
// Collect sections to merge.
for (InputSectionBase *sec : inputSections) {
auto *s = cast<InputSection>(sec);
if (isEligible(s))
sections.push_back(s);
}
// Initially, we use hash values to partition sections.
parallelForEach(sections, [&](InputSection *s) {
s->eqClass[0] = xxHash64(s->data());
});
for (unsigned cnt = 0; cnt != 2; ++cnt) {
parallelForEach(sections, [&](InputSection *s) {
if (s->areRelocsRela)
combineRelocHashes<ELFT>(cnt, s, s->template relas<ELFT>());
else
combineRelocHashes<ELFT>(cnt, s, s->template rels<ELFT>());
});
}
// From now on, sections in Sections vector are ordered so that sections
// in the same equivalence class are consecutive in the vector.
llvm::stable_sort(sections, [](const InputSection *a, const InputSection *b) {
return a->eqClass[0] < b->eqClass[0];
});
// Compare static contents and assign unique IDs for each static content.
forEachClass([&](size_t begin, size_t end) { segregate(begin, end, true); });
// Split groups by comparing relocations until convergence is obtained.
do {
repeat = false;
forEachClass(
[&](size_t begin, size_t end) { segregate(begin, end, false); });
} while (repeat);
log("ICF needed " + Twine(cnt) + " iterations");
// Merge sections by the equivalence class.
forEachClassRange(0, sections.size(), [&](size_t begin, size_t end) {
if (end - begin == 1)
return;
print("selected section " + toString(sections[begin]));
for (size_t i = begin + 1; i < end; ++i) {
print(" removing identical section " + toString(sections[i]));
sections[begin]->replace(sections[i]);
// At this point we know sections merged are fully identical and hence
// we want to remove duplicate implicit dependencies such as link order
// and relocation sections.
for (InputSection *isec : sections[i]->dependentSections)
isec->markDead();
}
});
// InputSectionDescription::sections is populated by processSectionCommands().
// ICF may fold some input sections assigned to output sections. Remove them.
for (BaseCommand *base : script->sectionCommands)
if (auto *sec = dyn_cast<OutputSection>(base))
for (BaseCommand *sub_base : sec->sectionCommands)
if (auto *isd = dyn_cast<InputSectionDescription>(sub_base))
llvm::erase_if(isd->sections,
[](InputSection *isec) { return !isec->isLive(); });
}
// ICF entry point function.
template <class ELFT> void doIcf() {
llvm::TimeTraceScope timeScope("ICF");
ICF<ELFT>().run();
}
template void doIcf<ELF32LE>();
template void doIcf<ELF32BE>();
template void doIcf<ELF64LE>();
template void doIcf<ELF64BE>();
} // namespace elf
} // namespace lld
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