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[lld][MachO] Refactor handling of subsections 

This diff is a preparation for fixing FunStabs (incorrect size calculation).
std::map<uint32_t, InputSection*> (SubsectionMap) is replaced with
a sorted vector + binary search. If .subsections_via_symbols is set
this vector will contain the list of subsections, otherwise,
the offsets will be used for calculating the symbols sizes.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D98837
This commit is contained in:
Alexander Shaposhnikov 2021-03-31 15:23:19 -07:00
parent 4af4828a6e
commit f1e4e2fb20
4 changed files with 78 additions and 70 deletions
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9
lld/MachO/Driver.cpp
View File

@ -1119,12 +1119,9 @@ bool macho::link(ArrayRef<const char *> argsArr, bool canExitEarly,
TimeTraceScope timeScope("Gathering input sections");
// Gather all InputSections into one vector.
for (const InputFile *file : inputFiles) {
for (const SubsectionMap &map : file->subsections) {
for (const auto &p : map) {
InputSection *isec = p.second;
inputSections.push_back(isec);
}
}
for (const SubsectionMapping &map : file->subsections)
for (const SubsectionEntry &subsectionEntry : map)
inputSections.push_back(subsectionEntry.isec);
}
}

123
lld/MachO/InputFiles.cpp
View File

@ -194,11 +194,14 @@ void ObjFile::parseSections(ArrayRef<section_64> sections) {
// any subsection splitting has occurred). It will be updated to represent the
// same location as an offset relative to the start of the containing
// subsection.
static InputSection *findContainingSubsection(SubsectionMap &map,
uint32_t *offset) {
auto it = std::prev(map.upper_bound(*offset));
*offset -= it->first;
return it->second;
static InputSection *findContainingSubsection(SubsectionMapping &map,
uint64_t *offset) {
auto it = std::prev(llvm::upper_bound(
map, *offset, [](uint64_t value, SubsectionEntry subsectionEntry) {
return value < subsectionEntry.offset;
}));
*offset -= it->offset;
return it->isec;
}
static bool validateRelocationInfo(InputFile *file, const section_64 &sec,
@ -233,7 +236,7 @@ static bool validateRelocationInfo(InputFile *file, const section_64 &sec,
}
void ObjFile::parseRelocations(const section_64 &sec,
SubsectionMap &subsecMap) {
SubsectionMapping &subsecMap) {
auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
ArrayRef<relocation_info> relInfos(
reinterpret_cast<const relocation_info *>(buf + sec.reloff), sec.nreloc);
@ -288,9 +291,10 @@ void ObjFile::parseRelocations(const section_64 &sec,
r.referent = symbols[relInfo.r_symbolnum];
r.addend = totalAddend;
} else {
SubsectionMap &referentSubsecMap = subsections[relInfo.r_symbolnum - 1];
SubsectionMapping &referentSubsecMap =
subsections[relInfo.r_symbolnum - 1];
const section_64 &referentSec = sectionHeaders[relInfo.r_symbolnum - 1];
uint32_t referentOffset;
uint64_t referentOffset;
if (relInfo.r_pcrel) {
// The implicit addend for pcrel section relocations is the pcrel offset
// in terms of the addresses in the input file. Here we adjust it so
@ -328,7 +332,7 @@ void ObjFile::parseRelocations(const section_64 &sec,
static macho::Symbol *createDefined(const structs::nlist_64 &sym,
StringRef name, InputSection *isec,
uint32_t value) {
uint64_t value) {
// Symbol scope is determined by sym.n_type & (N_EXT | N_PEXT):
// N_EXT: Global symbols
// N_EXT | N_PEXT: Linkage unit (think: dylib) scoped
@ -410,11 +414,51 @@ macho::Symbol *ObjFile::parseNonSectionSymbol(const structs::nlist_64 &sym,
void ObjFile::parseSymbols(ArrayRef<structs::nlist_64> nList,
const char *strtab, bool subsectionsViaSymbols) {
// resize(), not reserve(), because we are going to create N_ALT_ENTRY symbols
// out-of-sequence.
symbols.resize(nList.size());
std::vector<size_t> altEntrySymIdxs;
// Precompute the boundaries of symbols within a section.
// If subsectionsViaSymbols is True then the corresponding subsections will be
// created, otherwise these boundaries are used for the calculation of symbols
// sizes only.
for (const structs::nlist_64 &sym : nList) {
if ((sym.n_type & N_TYPE) == N_SECT && !(sym.n_desc & N_ALT_ENTRY) &&
!subsections[sym.n_sect - 1].empty()) {
SubsectionMapping &subsectionMapping = subsections[sym.n_sect - 1];
subsectionMapping.push_back(
{sym.n_value - sectionHeaders[sym.n_sect - 1].addr,
subsectionMapping.front().isec});
}
}
for (SubsectionMapping &subsectionMap : subsections) {
if (subsectionMap.empty())
continue;
llvm::sort(subsectionMap,
[](const SubsectionEntry &lhs, const SubsectionEntry &rhs) {
return lhs.offset < rhs.offset;
});
subsectionMap.erase(
std::unique(subsectionMap.begin(), subsectionMap.end(),
[](const SubsectionEntry &lhs, const SubsectionEntry &rhs) {
return lhs.offset == rhs.offset;
}),
subsectionMap.end());
if (!subsectionsViaSymbols)
continue;
for (size_t i = 0; i < subsectionMap.size(); ++i) {
uint32_t offset = subsectionMap[i].offset;
InputSection *&isec = subsectionMap[i].isec;
uint32_t end = i + 1 < subsectionMap.size() ? subsectionMap[i + 1].offset
: isec->data.size();
isec = make<InputSection>(*isec);
isec->data = isec->data.slice(offset, end - offset);
// TODO: ld64 appears to preserve the original alignment as well as each
// subsection's offset from the last aligned address. We should consider
// emulating that behavior.
isec->align = MinAlign(isec->align, offset);
}
}
symbols.resize(nList.size());
for (size_t i = 0, n = nList.size(); i < n; ++i) {
const structs::nlist_64 &sym = nList[i];
StringRef name = strtab + sym.n_strx;
@ -425,7 +469,7 @@ void ObjFile::parseSymbols(ArrayRef<structs::nlist_64> nList,
}
const section_64 &sec = sectionHeaders[sym.n_sect - 1];
SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
SubsectionMapping &subsecMap = subsections[sym.n_sect - 1];
// parseSections() may have chosen not to parse this section.
if (subsecMap.empty())
@ -437,55 +481,18 @@ void ObjFile::parseSymbols(ArrayRef<structs::nlist_64> nList,
// use the same subsection. Otherwise, we must split the sections along
// symbol boundaries.
if (!subsectionsViaSymbols) {
symbols[i] = createDefined(sym, name, subsecMap[0], offset);
symbols[i] = createDefined(sym, name, subsecMap.front().isec, offset);
continue;
}
// nList entries aren't necessarily arranged in address order. Therefore,
// we can't create alt-entry symbols at this point because a later symbol
// may split its section, which may affect which subsection the alt-entry
// symbol is assigned to. So we need to handle them in a second pass below.
if (sym.n_desc & N_ALT_ENTRY) {
altEntrySymIdxs.push_back(i);
continue;
}
// Find the subsection corresponding to the greatest section offset that is
// <= that of the current symbol. The subsection that we find either needs
// to be used directly or split in two.
uint32_t firstSize = offset;
InputSection *firstIsec = findContainingSubsection(subsecMap, &firstSize);
if (firstSize == 0) {
// Alias of an existing symbol, or the first symbol in the section. These
// are handled by reusing the existing section.
symbols[i] = createDefined(sym, name, firstIsec, 0);
continue;
}
// We saw a symbol definition at a new offset. Split the section into two
// subsections. The new symbol uses the second subsection.
auto *secondIsec = make<InputSection>(*firstIsec);
secondIsec->data = firstIsec->data.slice(firstSize);
firstIsec->data = firstIsec->data.slice(0, firstSize);
// TODO: ld64 appears to preserve the original alignment as well as each
// subsection's offset from the last aligned address. We should consider
// emulating that behavior.
secondIsec->align = MinAlign(firstIsec->align, offset);
subsecMap[offset] = secondIsec;
// By construction, the symbol will be at offset zero in the new section.
symbols[i] = createDefined(sym, name, secondIsec, 0);
InputSection *subsec = findContainingSubsection(subsecMap, &offset);
symbols[i] = createDefined(sym, name, subsec, offset);
}
for (size_t idx : altEntrySymIdxs) {
const structs::nlist_64 &sym = nList[idx];
StringRef name = strtab + sym.n_strx;
SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
uint32_t off = sym.n_value - sectionHeaders[sym.n_sect - 1].addr;
InputSection *subsec = findContainingSubsection(subsecMap, &off);
symbols[idx] = createDefined(sym, name, subsec, off);
}
if (!subsectionsViaSymbols)
for (SubsectionMapping &subsectionMap : subsections)
if (!subsectionMap.empty())
subsectionMap = {subsectionMap.front()};
}
OpaqueFile::OpaqueFile(MemoryBufferRef mb, StringRef segName,

14
lld/MachO/InputFiles.h
View File

@ -47,9 +47,13 @@ enum class RefState : uint8_t;
extern std::unique_ptr<llvm::TarWriter> tar;
// If .subsections_via_symbols is set, each InputSection will be split along
// symbol boundaries. The keys of a SubsectionMap represent the offsets of
// each subsection from the start of the original pre-split InputSection.
using SubsectionMap = std::map<uint32_t, InputSection *>;
// symbol boundaries. The field offset represents the offset of the subsection
// from the start of the original pre-split InputSection.
struct SubsectionEntry {
uint64_t offset;
InputSection *isec;
};
using SubsectionMapping = std::vector<SubsectionEntry>;
class InputFile {
public:
@ -68,7 +72,7 @@ public:
MemoryBufferRef mb;
std::vector<Symbol *> symbols;
std::vector<SubsectionMap> subsections;
std::vector<SubsectionMapping> subsections;
// Provides an easy way to sort InputFiles deterministically.
const int id;
@ -105,7 +109,7 @@ private:
void parseSymbols(ArrayRef<lld::structs::nlist_64> nList, const char *strtab,
bool subsectionsViaSymbols);
Symbol *parseNonSectionSymbol(const structs::nlist_64 &sym, StringRef name);
void parseRelocations(const llvm::MachO::section_64 &, SubsectionMap &);
void parseRelocations(const llvm::MachO::section_64 &, SubsectionMapping &);
void parseDebugInfo();
};

2
lld/MachO/Relocations.h
View File

@ -56,7 +56,7 @@ struct Reloc {
uint8_t length = 0;
// The offset from the start of the subsection that this relocation belongs
// to.
uint32_t offset = 0;
uint64_t offset = 0;
// Adding this offset to the address of the referent symbol or subsection
// gives the destination that this relocation refers to.
int64_t addend = 0;