llvm-project/lld/ELF/Symbols.h

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//===- Symbols.h ------------------------------------------------*- C++ -*-===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
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//
// This file defines various types of Symbols.
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//
//===----------------------------------------------------------------------===//
#ifndef LLD_ELF_SYMBOLS_H
#define LLD_ELF_SYMBOLS_H
#include "InputSection.h"
#include "lld/Common/LLVM.h"
#include "lld/Common/Strings.h"
#include "llvm/Object/Archive.h"
#include "llvm/Object/ELF.h"
namespace lld {
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namespace elf {
class ArchiveFile;
ELF: New symbol table design. This patch implements a new design for the symbol table that stores SymbolBodies within a memory region of the Symbol object. Symbols are mutated by constructing SymbolBodies in place over existing SymbolBodies, rather than by mutating pointers. As mentioned in the initial proposal [1], this memory layout helps reduce the cache miss rate by improving memory locality. Performance numbers: old(s) new(s) Without debug info: chrome 7.178 6.432 (-11.5%) LLVMgold.so 0.505 0.502 (-0.5%) clang 0.954 0.827 (-15.4%) llvm-as 0.052 0.045 (-15.5%) With debug info: scylla 5.695 5.613 (-1.5%) clang 14.396 14.143 (-1.8%) Performance counter results show that the fewer required indirections is indeed the cause of the improved performance. For example, when linking chrome, stalled cycles decreases from 14,556,444,002 to 12,959,238,310, and instructions per cycle increases from 0.78 to 0.83. We are also executing many fewer instructions (15,516,401,933 down to 15,002,434,310), probably because we spend less time allocating SymbolBodies. The new mechanism by which symbols are added to the symbol table is by calling add* functions on the SymbolTable. In this patch, I handle local symbols by storing them inside "unparented" SymbolBodies. This is suboptimal, but if we do want to try to avoid allocating these SymbolBodies, we can probably do that separately. I also removed a few members from the SymbolBody class that were only being used to pass information from the input file to the symbol table. This patch implements the new design for the ELF linker only. I intend to prepare a similar patch for the COFF linker. [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098832.html Differential Revision: http://reviews.llvm.org/D19752 llvm-svn: 268178
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class BitcodeFile;
class BssSection;
class InputFile;
class LazyObjFile;
template <class ELFT> class ObjFile;
class OutputSection;
template <class ELFT> class SharedFile;
// This is a StringRef-like container that doesn't run strlen().
//
// ELF string tables contain a lot of null-terminated strings. Most of them
// are not necessary for the linker because they are names of local symbols,
// and the linker doesn't use local symbol names for name resolution. So, we
// use this class to represents strings read from string tables.
struct StringRefZ {
StringRefZ(const char *S) : Data(S), Size(-1) {}
StringRefZ(StringRef S) : Data(S.data()), Size(S.size()) {}
const char *Data;
const uint32_t Size;
};
// The base class for real symbol classes.
class Symbol {
public:
enum Kind {
DefinedKind,
SharedKind,
UndefinedKind,
LazyArchiveKind,
LazyObjectKind,
};
Kind kind() const { return static_cast<Kind>(SymbolKind); }
// The file from which this symbol was created.
InputFile *File;
protected:
const char *NameData;
mutable uint32_t NameSize;
public:
uint32_t DynsymIndex = 0;
uint32_t GotIndex = -1;
uint32_t PltIndex = -1;
uint32_t GlobalDynIndex = -1;
// This field is a index to the symbol's version definition.
uint32_t VerdefIndex = -1;
// Version definition index.
uint16_t VersionId;
// Symbol binding. This is not overwritten by replaceSymbol to track
// changes during resolution. In particular:
// - An undefined weak is still weak when it resolves to a shared library.
// - An undefined weak will not fetch archive members, but we have to
// remember it is weak.
uint8_t Binding;
ELF: New symbol table design. This patch implements a new design for the symbol table that stores SymbolBodies within a memory region of the Symbol object. Symbols are mutated by constructing SymbolBodies in place over existing SymbolBodies, rather than by mutating pointers. As mentioned in the initial proposal [1], this memory layout helps reduce the cache miss rate by improving memory locality. Performance numbers: old(s) new(s) Without debug info: chrome 7.178 6.432 (-11.5%) LLVMgold.so 0.505 0.502 (-0.5%) clang 0.954 0.827 (-15.4%) llvm-as 0.052 0.045 (-15.5%) With debug info: scylla 5.695 5.613 (-1.5%) clang 14.396 14.143 (-1.8%) Performance counter results show that the fewer required indirections is indeed the cause of the improved performance. For example, when linking chrome, stalled cycles decreases from 14,556,444,002 to 12,959,238,310, and instructions per cycle increases from 0.78 to 0.83. We are also executing many fewer instructions (15,516,401,933 down to 15,002,434,310), probably because we spend less time allocating SymbolBodies. The new mechanism by which symbols are added to the symbol table is by calling add* functions on the SymbolTable. In this patch, I handle local symbols by storing them inside "unparented" SymbolBodies. This is suboptimal, but if we do want to try to avoid allocating these SymbolBodies, we can probably do that separately. I also removed a few members from the SymbolBody class that were only being used to pass information from the input file to the symbol table. This patch implements the new design for the ELF linker only. I intend to prepare a similar patch for the COFF linker. [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098832.html Differential Revision: http://reviews.llvm.org/D19752 llvm-svn: 268178
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// The following fields have the same meaning as the ELF symbol attributes.
uint8_t Type; // symbol type
uint8_t StOther; // st_other field value
const uint8_t SymbolKind;
// Symbol visibility. This is the computed minimum visibility of all
// observed non-DSO symbols.
unsigned Visibility : 2;
// True if the symbol was used for linking and thus need to be added to the
// output file's symbol table. This is true for all symbols except for
// unreferenced DSO symbols and bitcode symbols that are unreferenced except
// by other bitcode objects.
unsigned IsUsedInRegularObj : 1;
// If this flag is true and the symbol has protected or default visibility, it
// will appear in .dynsym. This flag is set by interposable DSO symbols in
// executables, by most symbols in DSOs and executables built with
// --export-dynamic, and by dynamic lists.
unsigned ExportDynamic : 1;
// False if LTO shouldn't inline whatever this symbol points to. If a symbol
// is overwritten after LTO, LTO shouldn't inline the symbol because it
// doesn't know the final contents of the symbol.
unsigned CanInline : 1;
// True if this symbol is specified by --trace-symbol option.
unsigned Traced : 1;
bool includeInDynsym() const;
uint8_t computeBinding() const;
bool isWeak() const { return Binding == llvm::ELF::STB_WEAK; }
bool isUndefined() const { return SymbolKind == UndefinedKind; }
bool isDefined() const { return SymbolKind == DefinedKind; }
bool isShared() const { return SymbolKind == SharedKind; }
bool isLocal() const { return Binding == llvm::ELF::STB_LOCAL; }
bool isLazy() const {
return SymbolKind == LazyArchiveKind || SymbolKind == LazyObjectKind;
}
// True if this is an undefined weak symbol.
bool isUndefWeak() const { return isWeak() && isUndefined(); }
StringRef getName() const {
if (NameSize == (uint32_t)-1)
NameSize = strlen(NameData);
return {NameData, NameSize};
}
void parseSymbolVersion();
bool isInGot() const { return GotIndex != -1U; }
bool isInPlt() const { return PltIndex != -1U; }
uint64_t getVA(int64_t Addend = 0) const;
uint64_t getGotOffset() const;
uint64_t getGotVA() const;
uint64_t getGotPltOffset() const;
uint64_t getGotPltVA() const;
uint64_t getPltVA() const;
uint64_t getPltOffset() const;
uint64_t getSize() const;
OutputSection *getOutputSection() const;
protected:
Symbol(Kind K, InputFile *File, StringRefZ Name, uint8_t Binding,
uint8_t StOther, uint8_t Type)
: File(File), NameData(Name.Data), NameSize(Name.Size), Binding(Binding),
Type(Type), StOther(StOther), SymbolKind(K), NeedsPltAddr(false),
[ELF][MIPS] Multi-GOT implementation Almost all entries inside MIPS GOT are referenced by signed 16-bit index. Zero entry lies approximately in the middle of the GOT. So the total number of GOT entries cannot exceed ~16384 for 32-bit architecture and ~8192 for 64-bit architecture. This limitation makes impossible to link rather large application like for example LLVM+Clang. There are two workaround for this problem. The first one is using the -mxgot compiler's flag. It enables using a 32-bit index to access GOT entries. But each access requires two assembly instructions two load GOT entry index to a register. Another workaround is multi-GOT. This patch implements it. Here is a brief description of multi-GOT for detailed one see the following link https://dmz-portal.mips.com/wiki/MIPS_Multi_GOT. If the sum of local, global and tls entries is less than 64K only single got is enough. Otherwise, multi-got is created. Series of primary and multiple secondary GOTs have the following layout: ``` - Primary GOT Header Local entries Global entries Relocation only entries TLS entries - Secondary GOT Local entries Global entries TLS entries ... ``` All GOT entries required by relocations from a single input file entirely belong to either primary or one of secondary GOTs. To reference GOT entries each GOT has its own _gp value points to the "middle" of the GOT. In the code this value loaded to the register which is used for GOT access. MIPS 32 function's prologue: ``` lui v0,0x0 0: R_MIPS_HI16 _gp_disp addiu v0,v0,0 4: R_MIPS_LO16 _gp_disp ``` MIPS 64 function's prologue: ``` lui at,0x0 14: R_MIPS_GPREL16 main ``` Dynamic linker does not know anything about secondary GOTs and cannot use a regular MIPS mechanism for GOT entries initialization. So we have to use an approach accepted by other architectures and create dynamic relocations R_MIPS_REL32 to initialize global entries (and local in case of PIC code) in secondary GOTs. But ironically MIPS dynamic linker requires GOT entries and correspondingly ordered dynamic symbol table entries to deal with dynamic relocations. To handle this problem relocation-only section in the primary GOT contains entries for all symbols referenced in global parts of secondary GOTs. Although the sum of local and normal global entries of the primary got should be less than 64K, the size of the primary got (including relocation-only entries can be greater than 64K, because parts of the primary got that overflow the 64K limit are used only by the dynamic linker at dynamic link-time and not by 16-bit gp-relative addressing at run-time. The patch affects common LLD code in the following places: - Added new hidden -mips-got-size flag. This flag required to set low maximum size of a single GOT to be able to test the implementation using small test cases. - Added InputFile argument to the getRelocTargetVA function. The same symbol referenced by GOT relocation from different input file might be allocated in different GOT. So result of relocation depends on the file. - Added new ctor to the DynamicReloc class. This constructor records settings of dynamic relocation which used to adjust address of 64kb page lies inside a specific output section. With the patch LLD is able to link all LLVM+Clang+LLD applications and libraries for MIPS 32/64 targets. Differential revision: https://reviews.llvm.org/D31528 llvm-svn: 334390
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IsInIplt(false), IsInIgot(false), IsPreemptible(false),
Used(!Config->GcSections), NeedsTocRestore(false) {}
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public:
// True the symbol should point to its PLT entry.
// For SharedSymbol only.
unsigned NeedsPltAddr : 1;
// True if this symbol is in the Iplt sub-section of the Plt.
unsigned IsInIplt : 1;
// True if this symbol is in the Igot sub-section of the .got.plt or .got.
unsigned IsInIgot : 1;
// True if this symbol is preemptible at load time.
unsigned IsPreemptible : 1;
// True if an undefined or shared symbol is used from a live section.
unsigned Used : 1;
// True if a call to this symbol needs to be followed by a restore of the
// PPC64 toc pointer.
unsigned NeedsTocRestore : 1;
// The Type field may also have this value. It means that we have not yet seen
// a non-Lazy symbol with this name, so we don't know what its type is. The
// Type field is normally set to this value for Lazy symbols unless we saw a
// weak undefined symbol first, in which case we need to remember the original
// symbol's type in order to check for TLS mismatches.
enum { UnknownType = 255 };
bool isSection() const { return Type == llvm::ELF::STT_SECTION; }
bool isTls() const { return Type == llvm::ELF::STT_TLS; }
bool isFunc() const { return Type == llvm::ELF::STT_FUNC; }
bool isGnuIFunc() const { return Type == llvm::ELF::STT_GNU_IFUNC; }
bool isObject() const { return Type == llvm::ELF::STT_OBJECT; }
bool isFile() const { return Type == llvm::ELF::STT_FILE; }
};
// Represents a symbol that is defined in the current output file.
class Defined : public Symbol {
public:
Defined(InputFile *File, StringRefZ Name, uint8_t Binding, uint8_t StOther,
uint8_t Type, uint64_t Value, uint64_t Size, SectionBase *Section)
: Symbol(DefinedKind, File, Name, Binding, StOther, Type), Value(Value),
Size(Size), Section(Section) {}
static bool classof(const Symbol *S) { return S->isDefined(); }
uint64_t Value;
uint64_t Size;
SectionBase *Section;
};
class Undefined : public Symbol {
public:
Undefined(InputFile *File, StringRefZ Name, uint8_t Binding, uint8_t StOther,
uint8_t Type)
: Symbol(UndefinedKind, File, Name, Binding, StOther, Type) {}
static bool classof(const Symbol *S) { return S->kind() == UndefinedKind; }
};
class SharedSymbol : public Symbol {
public:
static bool classof(const Symbol *S) { return S->kind() == SharedKind; }
SharedSymbol(InputFile &File, StringRef Name, uint8_t Binding,
uint8_t StOther, uint8_t Type, uint64_t Value, uint64_t Size,
uint32_t Alignment, uint32_t VerdefIndex)
: Symbol(SharedKind, &File, Name, Binding, StOther, Type),
Alignment(Alignment), Value(Value), Size(Size) {
this->VerdefIndex = VerdefIndex;
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// GNU ifunc is a mechanism to allow user-supplied functions to
// resolve PLT slot values at load-time. This is contrary to the
// regular symbol resolution scheme in which symbols are resolved just
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// by name. Using this hook, you can program how symbols are solved
// for you program. For example, you can make "memcpy" to be resolved
// to a SSE-enabled version of memcpy only when a machine running the
// program supports the SSE instruction set.
//
// Naturally, such symbols should always be called through their PLT
// slots. What GNU ifunc symbols point to are resolver functions, and
// calling them directly doesn't make sense (unless you are writing a
// loader).
//
// For DSO symbols, we always call them through PLT slots anyway.
// So there's no difference between GNU ifunc and regular function
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// symbols if they are in DSOs. So we can handle GNU_IFUNC as FUNC.
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if (this->Type == llvm::ELF::STT_GNU_IFUNC)
this->Type = llvm::ELF::STT_FUNC;
}
template <class ELFT> SharedFile<ELFT> &getFile() const {
return *cast<SharedFile<ELFT>>(File);
}
uint32_t Alignment;
uint64_t Value; // st_value
uint64_t Size; // st_size
};
// LazyArchive and LazyObject represent a symbols that is not yet in the link,
// but we know where to find it if needed. If the resolver finds both Undefined
// and Lazy for the same name, it will ask the Lazy to load a file.
//
// A special complication is the handling of weak undefined symbols. They should
// not load a file, but we have to remember we have seen both the weak undefined
// and the lazy. We represent that with a lazy symbol with a weak binding. This
// means that code looking for undefined symbols normally also has to take lazy
// symbols into consideration.
// This class represents a symbol defined in an archive file. It is
// created from an archive file header, and it knows how to load an
// object file from an archive to replace itself with a defined
// symbol.
class LazyArchive : public Symbol {
public:
LazyArchive(InputFile &File, uint8_t Type,
const llvm::object::Archive::Symbol S)
: Symbol(LazyArchiveKind, &File, S.getName(), llvm::ELF::STB_GLOBAL,
llvm::ELF::STV_DEFAULT, Type),
Sym(S) {}
static bool classof(const Symbol *S) { return S->kind() == LazyArchiveKind; }
InputFile *fetch();
private:
const llvm::object::Archive::Symbol Sym;
};
// LazyObject symbols represents symbols in object files between
// --start-lib and --end-lib options.
class LazyObject : public Symbol {
public:
LazyObject(InputFile &File, uint8_t Type, StringRef Name)
: Symbol(LazyObjectKind, &File, Name, llvm::ELF::STB_GLOBAL,
llvm::ELF::STV_DEFAULT, Type) {}
static bool classof(const Symbol *S) { return S->kind() == LazyObjectKind; }
};
// Some linker-generated symbols need to be created as
// Defined symbols.
struct ElfSym {
// __bss_start
static Defined *Bss;
// etext and _etext
static Defined *Etext1;
static Defined *Etext2;
// edata and _edata
static Defined *Edata1;
static Defined *Edata2;
// end and _end
static Defined *End1;
static Defined *End2;
// The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention to
// be at some offset from the base of the .got section, usually 0 or
// the end of the .got.
static Defined *GlobalOffsetTable;
// _gp, _gp_disp and __gnu_local_gp symbols. Only for MIPS.
static Defined *MipsGp;
static Defined *MipsGpDisp;
static Defined *MipsLocalGp;
// __rela_iplt_end or __rel_iplt_end
static Defined *RelaIpltEnd;
};
// A buffer class that is large enough to hold any Symbol-derived
// object. We allocate memory using this class and instantiate a symbol
// using the placement new.
union SymbolUnion {
alignas(Defined) char A[sizeof(Defined)];
alignas(Undefined) char C[sizeof(Undefined)];
alignas(SharedSymbol) char D[sizeof(SharedSymbol)];
alignas(LazyArchive) char E[sizeof(LazyArchive)];
alignas(LazyObject) char F[sizeof(LazyObject)];
ELF: New symbol table design. This patch implements a new design for the symbol table that stores SymbolBodies within a memory region of the Symbol object. Symbols are mutated by constructing SymbolBodies in place over existing SymbolBodies, rather than by mutating pointers. As mentioned in the initial proposal [1], this memory layout helps reduce the cache miss rate by improving memory locality. Performance numbers: old(s) new(s) Without debug info: chrome 7.178 6.432 (-11.5%) LLVMgold.so 0.505 0.502 (-0.5%) clang 0.954 0.827 (-15.4%) llvm-as 0.052 0.045 (-15.5%) With debug info: scylla 5.695 5.613 (-1.5%) clang 14.396 14.143 (-1.8%) Performance counter results show that the fewer required indirections is indeed the cause of the improved performance. For example, when linking chrome, stalled cycles decreases from 14,556,444,002 to 12,959,238,310, and instructions per cycle increases from 0.78 to 0.83. We are also executing many fewer instructions (15,516,401,933 down to 15,002,434,310), probably because we spend less time allocating SymbolBodies. The new mechanism by which symbols are added to the symbol table is by calling add* functions on the SymbolTable. In this patch, I handle local symbols by storing them inside "unparented" SymbolBodies. This is suboptimal, but if we do want to try to avoid allocating these SymbolBodies, we can probably do that separately. I also removed a few members from the SymbolBody class that were only being used to pass information from the input file to the symbol table. This patch implements the new design for the ELF linker only. I intend to prepare a similar patch for the COFF linker. [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098832.html Differential Revision: http://reviews.llvm.org/D19752 llvm-svn: 268178
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};
void printTraceSymbol(Symbol *Sym);
ELF: New symbol table design. This patch implements a new design for the symbol table that stores SymbolBodies within a memory region of the Symbol object. Symbols are mutated by constructing SymbolBodies in place over existing SymbolBodies, rather than by mutating pointers. As mentioned in the initial proposal [1], this memory layout helps reduce the cache miss rate by improving memory locality. Performance numbers: old(s) new(s) Without debug info: chrome 7.178 6.432 (-11.5%) LLVMgold.so 0.505 0.502 (-0.5%) clang 0.954 0.827 (-15.4%) llvm-as 0.052 0.045 (-15.5%) With debug info: scylla 5.695 5.613 (-1.5%) clang 14.396 14.143 (-1.8%) Performance counter results show that the fewer required indirections is indeed the cause of the improved performance. For example, when linking chrome, stalled cycles decreases from 14,556,444,002 to 12,959,238,310, and instructions per cycle increases from 0.78 to 0.83. We are also executing many fewer instructions (15,516,401,933 down to 15,002,434,310), probably because we spend less time allocating SymbolBodies. The new mechanism by which symbols are added to the symbol table is by calling add* functions on the SymbolTable. In this patch, I handle local symbols by storing them inside "unparented" SymbolBodies. This is suboptimal, but if we do want to try to avoid allocating these SymbolBodies, we can probably do that separately. I also removed a few members from the SymbolBody class that were only being used to pass information from the input file to the symbol table. This patch implements the new design for the ELF linker only. I intend to prepare a similar patch for the COFF linker. [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098832.html Differential Revision: http://reviews.llvm.org/D19752 llvm-svn: 268178
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template <typename T, typename... ArgT>
void replaceSymbol(Symbol *S, ArgT &&... Arg) {
static_assert(std::is_trivially_destructible<T>(),
"Symbol types must be trivially destructible");
static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
static_assert(alignof(T) <= alignof(SymbolUnion),
"SymbolUnion not aligned enough");
assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
"Not a Symbol");
Symbol Sym = *S;
new (S) T(std::forward<ArgT>(Arg)...);
S->VersionId = Sym.VersionId;
S->Visibility = Sym.Visibility;
S->IsUsedInRegularObj = Sym.IsUsedInRegularObj;
S->ExportDynamic = Sym.ExportDynamic;
S->CanInline = Sym.CanInline;
S->Traced = Sym.Traced;
// Print out a log message if --trace-symbol was specified.
// This is for debugging.
if (S->Traced)
printTraceSymbol(S);
ELF: New symbol table design. This patch implements a new design for the symbol table that stores SymbolBodies within a memory region of the Symbol object. Symbols are mutated by constructing SymbolBodies in place over existing SymbolBodies, rather than by mutating pointers. As mentioned in the initial proposal [1], this memory layout helps reduce the cache miss rate by improving memory locality. Performance numbers: old(s) new(s) Without debug info: chrome 7.178 6.432 (-11.5%) LLVMgold.so 0.505 0.502 (-0.5%) clang 0.954 0.827 (-15.4%) llvm-as 0.052 0.045 (-15.5%) With debug info: scylla 5.695 5.613 (-1.5%) clang 14.396 14.143 (-1.8%) Performance counter results show that the fewer required indirections is indeed the cause of the improved performance. For example, when linking chrome, stalled cycles decreases from 14,556,444,002 to 12,959,238,310, and instructions per cycle increases from 0.78 to 0.83. We are also executing many fewer instructions (15,516,401,933 down to 15,002,434,310), probably because we spend less time allocating SymbolBodies. The new mechanism by which symbols are added to the symbol table is by calling add* functions on the SymbolTable. In this patch, I handle local symbols by storing them inside "unparented" SymbolBodies. This is suboptimal, but if we do want to try to avoid allocating these SymbolBodies, we can probably do that separately. I also removed a few members from the SymbolBody class that were only being used to pass information from the input file to the symbol table. This patch implements the new design for the ELF linker only. I intend to prepare a similar patch for the COFF linker. [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098832.html Differential Revision: http://reviews.llvm.org/D19752 llvm-svn: 268178
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}
void warnUnorderableSymbol(const Symbol *Sym);
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} // namespace elf
std::string toString(const elf::Symbol &B);
} // namespace lld
#endif