with – • --no-allow-shlib-undefined (on the exe) • Operates recursively on ld, not on gold/lld. • -z defs (on the shlib) • --no-undefined (on the shlib) 19 bar foo exe lib1 lib2 lib3 lib4 Mach-O and PE/COFF have many problems but this may be a place where they got right.” Fangrui Song, LLD maintainer https://maskray.me/blog/2021-06-13-dependency-related-linker-options 21Position Independent x86-64 instructions is not covered by the Standard? Of course not. It just means that that’s a low-level concern (relative to the semantics of C++), and it's up to the platform to make a conforming C++

158 wr) ==131579== LLd misses: 3,336,167 ( 2,266,653 rd + 1,069,514 wr) ==131579== D1 miss rate: 7.9% ( 9.6% + 0.7% ) ==131579== LLd miss rate: 158 wr) ==131579== LLd misses: 3,336,167 ( 2,266,653 rd + 1,069,514 wr) ==131579== D1 miss rate: 7.9% ( 9.6% + 0.7% ) ==131579== LLd miss rate: ==131579== LLd misses: 3,336,167 ( 2,266,653 rd + 1,069,514 wr) 12 ==131579== D1 miss rate: 7.9% ( 9.6% + 0.7% ) 13 ==131579== LLd miss rate:

of the systemThe linker [ld] ● Program is typically called ld ○ other implementations: ld.gold, lld, mold ● On Linux, typically invoked by gcc itself via the collect2 tool g++ my_program.cpp -o the collect2 invocation with -v ● GNU Linker is typically the default, there’s others: ○ gold, lld, mold ○ They may have slightly different behaviorsThe linker [ld] - search procedure (cont’d)The dynamic

Clang/LLVMThe Big 3 – Compiler Toolchains Clang/LLVM clang / clang++ Creates object files (.o) lld Creates executables and shared libraries (.so) ar Creates static libraries (.a) llvm Attaches

use Build Tools Source Code Tools Dependency Packages Dependency Tools ld, ld.bfd, ld.gold, lld, link.exe, …Copyright © 2020 Bob Steagall K E W B C O M P U T I N G C++ Programming Ecosystem – Building

when re-compiled from source in the machine. This shouldn’t be com- mon, but some extreme cases of low-level, close to the hardware, binaries, might be difficult to have reusable binaries between different flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:tools.apple:enable_bitcode boolean value to enable/disable Bitcode Apple Clang flags flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:defines list of preprocessor definitions that will be used by CPPFLAGS. • tools.build:sysroot

when re-compiled from source in the machine. This shouldn’t be com- mon, but some extreme cases of low-level, close to the hardware, binaries, might be difficult to have reusable binaries between different flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:tools.apple:enable_bitcode boolean value to enable/disable Bitcode Apple Clang flags flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:defines list of preprocessor definitions that will be used by CPPFLAGS. • tools.build:sysroot

when re-compiled from source in the machine. This shouldn’t be com- mon, but some extreme cases of low-level, close to the hardware, binaries, might be difficult to have reusable binaries between different flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:tools.apple:enable_bitcode boolean value to enable/disable Bitcode Apple Clang flags flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:defines list of preprocessor definitions that will be used by CPPFLAGS. • tools.build:sysroot

when re-compiled from source in the machine. This shouldn’t be com- mon, but some extreme cases of low-level, close to the hardware, binaries, might be difficult to have reusable binaries between different flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:tools.apple:enable_bitcode boolean value to enable/disable Bitcode Apple Clang flags flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:defines list of preprocessor definitions that will be used by CPPFLAGS. • tools.build:sysroot

when re-compiled from source in the machine. This shouldn’t be com- mon, but some extreme cases of low-level, close to the hardware, binaries, might be difficult to have reusable binaries between different flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:tools.apple:enable_bitcode boolean value to enable/disable Bitcode Apple Clang flags flag with linkers that supports specifying linker scripts with the -T flag, such as ld, gold, and lld. • tools.build:defines list of preprocessor definitions that will be used by CPPFLAGS. • tools.build:sysroot