C/C++ projects can benefit from using precompiled headers to improvecompile time. GCC addedsupport for precompiled headersin 2003 (version 3.4), andthe current documentation can be found at https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html.Even with the emergence of C++ modules, precompiled headers remainrelevant for several reasons:Precompiled headers share implementation aspects with modules (e.g.,AST serialization in Clang).Many C++ projects rely on the traditional compilation model and arenot converted to C++ modules.Modules may possibly use some preamble-like technology to accel ...继续阅读 (111)

C/C++ projects can benefit from using precompiled headers to improvecompile time. GCC addedsupport for precompiled headersin 2003 (version 3.4), andthe current documentation can be found at https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html.This article focuses on Clang precompiled headers (PCH). Let's beginwith an example.1234567891011121314151617181920cat > a1.cc <<'eof'#include "b.hh"intfa1() {returnfb(); }eofcat > a.cc <<'eof'#include "b.hh"int fa1();intmain() {returnfa1() + fb(); }eofcat > b.hh <<'eof'#ifndef B_HH#define B_HHinlin ...继续阅读 (113)

This article describesSHF_ALLOC|SHF_COMPRESSEDsectionsin ELF and a proposed linker option--compress-sectionstocompress arbitrary sections.Metadata sectionsIn recent years, metadata sections have gained more uses. Some usersmay find compression attractive if they weigh file sizes overdecompression overhead.Some metadata sections are non-SHF_ALLOC, like DWARFdebug sections. They may be unused by a runtime library but provideadditional information for an analysis tool to inspect the content.Other metadata sections have theSHF_ALLOCflag andtherefore are part of aPT_LOADsegment. When the programis ...继续阅读 (90)

UNDER CONSTRUCTIONThis article describesSHF_ALLOC|SHF_COMPRESSEDsectionsin ELF and a proposed linker option--compress-sectionstocompress arbitrary sections.Metadata sectionsIn recent years, metadata sections have gained more uses. Some usersmay find compression attractive if they weigh file sizes overdecompression overhead.Some metadata sections are non-SHF_ALLOC, like DWARFdebug sections. They may be unused by a runtime library but provideadditional information for an analysis tool to inspect the content.Other metadata sections have theSHF_ALLOCflag andtherefore are part of aPT_LOADsegment. W ...继续阅读 (96)

For a user who only uses one C++ standard library, such as libc++,there are typically three compatibility goals, each with increasingcompatibility requirements:Can the program, built with a specific version of libc++, work withan upgraded libc++ shared object (DSO)?Can an executable and its DSOs be compiled with different versionsof libc++ headers?Can two relocatable object files, compiled with different versionsof libc++ headers, be linked into the same executable or DSO?If we replace "different libc++ versions" with a mixture of libc++and libstdc++, we encounter additional goals:Can the prog ...继续阅读 (101)

For a user who only uses one C++ standard library, such as libc++,there are typically three compatibility goals, each with increasingcompatibility requirements:Can the program, built with a specific version of libc++, work withan upgraded libc++ shared object (DSO)?Can an executable and its DSOs be compiled with different versionsof libc++ headers?Can two relocatable object files, compiled with different versionsof libc++ headers, be linked into the same executable or DSO?If we replace "different libc++ versions" with a mixture of libc++and libstdc++, we encounter additional goals:Can the prog ...继续阅读 (103)

I do not use Apple products, but I sometimes like investigatingMach-O as an object file format and my llvm-project changes sometimesneed to work around the quirks.LLVM has a function call tracing system called XRay. It supports manyarchitectures on Linux and some BSDs but does not support Apple systems.If the target triple isx86_64-apple-darwin*, you maynotice that Clang will allow you to perform compilation, but linkingwill fail. For other architectures, Clang will reject it.123456% clang --target=x86_64-apple-darwin -fxray-instrument -fxray-instruction-threshold=1 -c a.c% clang --t ...继续阅读 (98)

UNDER CONSTRUCTIONThis article describes target-specific details about AArch32 in ELFlinkers. I described AArch64 in a previousarticle.AArch32 is the 32-bit execution state for the Arm architecture andruns the A32 and T32 instruction sets. A32 refers to the old ISA with a32-bit fixed width, while T32 refers to the mixed 16-bit and 32-bitThumb2 instructions."AArch32", "A32", and "T32" are new names. Many projects use "ARM","Arm", or "arm" as their port name.ABI documentsELFfor the Arm® ArchitectureProcedureCall Standard for the Arm® ArchitectureC++ABI for the Arm® ArchitectureExceptionHandling ...继续阅读 (86)

For a GCC or Clang command, there is typically one primary outputfile, specified by-oor the default (a.outora.exe). There can also be temporary files and auxiliaryfiles.Primary output fileWe can specify the primary output file with the-ooption. When unspecified, a default output file name is inferred fromthe input files and the final phase. When the final phase is linking,the default output file name isa.outora.exe.1234gcc -S d/a.c# a.sgcc -c d/a.c# a.ogcc d/a.c# a.outgcc d/a.c -o e/a# e/aFor-Sand-c, specifying-oin the presence of more than one input files leads to an error.123% gcc -c d/ ...继续阅读 (85)

When linking an oversized executable, it is possible to encountererrors such asrelocation truncated to fit: R_X86_64_PC32 against `.text'(GNU ld) orrelocation R_X86_64_PC32 out of range(ld.lld).These diagnostics are a result of the relocation overflow check, afeature in the linker.This article aims to explain why such issues can occur and providesinsights on how to mitigate them.Certain groups prefer static linking or mostly static linking for thesake of deployment convenience and performance. In scenarios where thedistributed program contains a significant amount of code (related:software blo ...继续阅读 (89)

UNDER CONSTRUCTIONThis archive provides a description of popular assemblers and theirarchitecture-specific differences.AssemblersGCC generates assembly code and invokes GNU Assembler (also known as"gas"), which is part of GNU Binutils, to convert the assembly code intomachine code. The GCC driver is also capable of accepting assembly inputfiles. Due to GCC's widespread usage, GNU Assembler is arguably the mostpopular assembler.Within the LLVM project, the LLVM integrated assembler is a librarythat is linked by Clang, llvm-mc, and lld (for LTO purposes) to generatemachine code. It supports a wi ...继续阅读 (97)

For a GCC or Clang command, there is typically one primary outputfile, specified by-oor the default (a.outora.exe). There can also be temporary files and auxiliaryfiles.Primary output fileWe can specify the primary output file with the-ooption. When unspecified, a default output file name is inferred fromthe input files and the final phase. When the final phase is linking,the default output file name isa.outora.exe.1234gcc -S d/a.c# a.sgcc -c d/a.c# a.ogcc d/a.c# a.outgcc d/a.c -o e/a# e/aTemporary filesFor compilation and linking in one command, the relocatable objectfiles are temporary files ...继续阅读 (89)

UNDER CONSTRUCTIONThis article describes target-specific details about AArch32 in ELFlinkers. I described AArch64 in a previousarticle.AArch32 is the 32-bit execution state for the Arm architecture andruns the A32 and T32 instruction sets. A32 refers to the old ISA with a32-bit fixed width, while T32 refers to the mixed 16-bit and 32-bitThumb2 instructions."AArch32", "A32", and "T32" are new names. Many projects use "ARM","Arm", or "arm" as their port name.ABI documentsELFfor the Arm® ArchitectureProcedureCall Standard for the Arm® ArchitectureC++ABI for the Arm® ArchitectureExceptionHandling ...继续阅读 (98)

This article describes an interesting overflow bug in the ELF hashfunction.The System V Application Binary Interface (generic ABI) specifies theELF object file format. When producing an executable or shared objectfile needing a dynamic symbol table (.dynsym), a linkergenerates a.hashsection with typeSHT_HASHto hold a symbolhash table. ADT_HASHtag is produced to hold theaddress of.hash.The hash table is used by a dynamic loader to perform symbol lookup(for dynamic relocations anddlsymfamily functions). Adetailed description of the format can be found in ELF: symbol lookup viaDT_HASH.Other use c ...继续阅读 (93)

This article describes an interesting overflow bug in the ELF hashfunction.The System V Application Binary Interface (generic ABI) specifies theELF object file format. When producing an output executable or sharedobject needing a dynamic symbol table (.dynsym), a linkergenerates a.hashsection with typeSHT_HASHto hold a symbolhash table. ADT_HASHtag is produced to hold theaddress of.hash.DT_HASHis used by a dynamic loader to perform symbollookup (for dynamic relocations anddlsymfamilyfunctions). A detailed description of the format can be found in ELF: symbol lookup viaDT_HASH.Other use casesIn ...继续阅读 (89)

A control-flow graph (CFG) is a graph representation of all pathsthat might be traversed through a program during its execution.Control-flow integrity (CFI) refers to security policy dictating thatprogram execution must follow a control-flow graph. This articledescribes some features that compilers and hardware can use to enforceCFI, with a focus on llvm-project implementations.CFI schemes are typically divided into forward-edge (e.g. indirectcalls) and backward-edge (mainly function returns). It should be notedthat exception handling and symbol interposition are not included inthese categorie ...继续阅读 (102)

UndefinedBehaviorSanitizer(UBSan) is an undefined behavior detector for C/C++. It consists of codeinstrumentation and a runtime. Both components have multiple independentimplementations.Clang implementedthe first few checks in 2009-12, initially named-fcatch-undefined-behavior. In 2012-fsanitize=undefinedwas added and-fcatch-undefined-behaviorwas removed.GCC 4.9 implemented-fsanitize=undefinedin 2013-08.The runtime used by Clang lives inllvm-project/compiler-rt/lib/ubsan. GCC from time to timesyncs its downstream fork of the sanitizers part of compiler-rt(libsanitizer). The end of the article ...继续阅读 (103)

llvm-project 16 was just released. I added some lld/ELF notes to https://github.com/llvm/llvm-project/blob/release/16.x/lld/docs/ReleaseNotes.rst.Here I will elaborate on some changes.Link speed improved greatly compared with lld 15.0. Notably inputsection initialization and relocation scanning are now parallel. (D130810) (D133003)ELFCOMPRESS_ZSTDcompressed input sections are nowsupported. (D129406)--compress-debug-sections=zstdis now available tocompress debug sections with zstd (ELFCOMPRESS_ZSTD). (D133548)--no-warnings/-wis now available tosuppress warnings. (D136569)DT_RISCV_VARIANT_CCis n ...继续阅读 (95)

llvm-project 16 was just released. I added some lld/ELF notes to https://github.com/llvm/llvm-project/blob/release/16.x/lld/docs/ReleaseNotes.rst.Here I will elaborate on some changes.Link speed improved greatly compared with lld 15.0. Notably inputsection initialization and relocation scanning are now parallel. (D130810) (D133003)ELFCOMPRESS_ZSTDcompressed input sections are nowsupported. (D129406)--compress-debug-sections=zstdis now available tocompress debug sections with zstd (ELFCOMPRESS_ZSTD). (D133548)--no-warnings/-wis now available tosuppress warnings. (D136569)DT_RISCV_VARIANT_CCis n ...继续阅读 (88)

This article describes target-specific details about x86 in ELFlinkers. I will use "x86" to refer to both x86-32 and x86-64.Global Offset TableThe Global Offset Table consists of two sections:.got.pltholds code addresses for PLT..gotholds other addresses and offsets.The symbol_GLOBAL_OFFSET_TABLE_is defined at thebeginning of the.got.pltsection..got.plthas 3 reserved entries..got.plt[0]holds the link-time address of_DYNAMICfor a legacy reason. Versions of glibc prior to2.35 have the_DYNAMICrequirement. See Allabout Global Offset Table..got.plt[1]and.got.plt[2]are for lazybinding PLT. Linkers c ...继续阅读 (92)

GCC supports some function attributes for function multi-versioning:a way for a function to have multiple implementations, each using adifferent set of ISA extensions. A function attribute specifiesdifferent requirements of ISA extensions. The generated program decodesthe CPU model and features at run-time, and picks the most restrictiveimplementation that is satisfied by the CPU, assuming that the mostrestrictive implementation has the best performance.__attribute__((target(...)))__attribute__((target(...)))has been available for along time, even before attributes for function multi-versionin ...继续阅读 (84)

This article describes target-specific details about Power ISA in ELFlinkers. Initially there was IBM POWER. The 1991 Apple–IBM–Motorolaalliance created PowerPC. In 2006, the architecture was rebranded asPower ISA. According to the ISA manual, "In 2006, Freescale and IBMcollaborated on the creation of the Power ISA Version 2.03, whichrepresented the reunification of the architecture by combining Book Econtent with the more general purpose PowerPC Version 2.02."The terms "PowerPC" and "powerpc" remain popular in numerous places,including thepowerpc-*-*-*andpowerpc64-*-*-*in official target trip ...继续阅读 (82)

This article describes target-specific details about AArch64 in ELFlinkers. AArch64 is the 64-bit execution environment for the Armarchitecture.ABI documentsELFfor the Arm® 64-bit Architecture (AArch64)SystemV ABI for the Arm® 64-bit Architecture (AArch64)Global Offset TableThe Global Offset Table consists of two sections:.got.pltholds code addresses for PLT..gotholds other addresses and offsets.The symbol_GLOBAL_OFFSET_TABLE_is defined at thebeginning of the.gotsection. GNU ld reserves a singleentry for.gotand.got[0]holds the link-timeaddress of_DYNAMICfor a legacy reason Versions of glibcpri ...继续阅读 (84)

UNDER CONSTRUCTIONThis article describes target-specific things about Power ISA in ELFlinkers. The architecture was originally named "PowerPC". In 2016 thearchitecture was rebranded as "Power ISA". The ISA manual says: "In2006, Freescale and IBM collaborated on the creation of the Power ISAVersion 2.03, which represented the reunification of the architecture bycombining Book E content with the more general purpose PowerPC Version2.02."The terms "PowerPC" and "powerpc" remain popular in numerous places,including thepowerpc-*-*-*andpowerpc64-*-*-*in official target triple names. Theabbreviation ...继续阅读 (88)

This article describes target-specific things about x86 in ELFlinkers. I will use "x86" to refer to both x86-32 and x86-64.Global Offset Table_GLOBAL_OFFSET_TABLE_is defined at the start of thesection.got.plt..got.plthas 3 reservedentries._GLOBAL_OFFSET_TABLE_[0]stores the link-time address of_DYNAMICfor a legacy reason. It's unused now. See Allabout Global Offset Table._GLOBAL_OFFSET_TABLE_[1]and_GLOBAL_OFFSET_TABLE_[2]are for lazy binding PLT.GOT optimizationSee Allabout Global Offset Table#GOT optimization.Procedure Linkage TableRetpoline and IndirectBranch Trackingld.lld supports-z retpoli ...继续阅读 (84)

Clang andGCC 4.9implemented LeakSanitizer in 2013. LeakSanitizer(LSan) is a memory leak detector. It intercepts memory allocationfunctions and by default detects memory leaks atatexittime. The implementation is purely in the runtime(compiler-rt/lib/lsan) and no instrumentation isneeded.LSan has very little architecture-specific code and supports many64-bit targets. Some 32-bit targets (e.g. Linux arm/x86-32) aresupported as well, but there may be high false negatives becausepointers with fewer bits are more easily confused with integers/floatingpoints/other data of a similar pattern. Every sup ...继续阅读 (83)

Clang andGCC 4.9implemented LeakSanitizer in 2013. LeakSanitizer(LSan) is a memory leak detector. It intercepts memory allocationfunctions and by default detects memory leaks atatexittime. The implementation is purely in the runtime(compiler-rt/lib/lsan) and no instrumentation isneeded.LSan has very little architecture-specific code and supports many64-bit targets. Some 32-bit targets (e.g. Linux arm/x86-32) aresupported as well, but there may be high false negatives because of thesmall pointer size. Every supported operating system needs to providesome way to "stop the world".UsageLSan can be ...继续阅读 (80)

Many sanitizers want to know every function in the program. Userfunctions are instrumented and therefore known by the sanitizer runtime.For library functions, some (e.g. mmap, munmap, memoryallocation/deallocation functions, longjmp, vfork) need specialtreatment. Sanitizers leverage symbol interposition to redirect suchfunction calls to its own implementation: interceptors. Other libraryfunctions can be treated as normal user code. Either instrumenting thefunction or providing an interceptor is fine.In some cases instrumenting is infeasible:Assembly source files usually do not (or are inconven ...继续阅读 (88)

GCC supports some function attributes for function multi-versioning:a way for a function to have multiple implementations, each using adifferent set of ISA extensions. A function attribute specifiesdifferent requirements of ISA extensions. The generated program decodesthe CPU model and features at run-time, and picks the most restrictiveimplementation which is satisfied by the CPU, assuming that the mostrestrictive implementation has the best performance.__attribute__((target(...)))__attribute__((target(...)))has been available for along time, even before attributes for function multi-versioni ...继续阅读 (87)

UndefinedBehaviorSanitizer(UBSan) is a undefined behavior detector for C/C++. It consists of acode instrumentation and a runtime (inllvm-project/compiler-rt).Clang and GCC have independent implementations. Clang implementedthe instrumentation first in 2009-12, initially named-fcatch-undefined-behavior. GCC 4.9 implemented-fsanitize=undefinedin 2013-08.Available checksThere are many undefined behavior checkswhich can be detected. One may specify-fsanitize=xxxwherexxx is the name of the check individually, or more commonly, specify-fsanitize=undefinedto enable all of them.Some checks are not und ...继续阅读 (79)