How to use LLVM linker (ld.lld) instead of GNU linker (ld)

[Pete Carter]

My sys info:

Operating System: Fedora Linux 38
KDE Plasma Version: 5.27.7
KDE Frameworks Version: 5.109.0
Qt Version: 5.15.10
Kernel Version: 6.4.13-200.fc38.x86_64 (64-bit)
Graphics Platform: Wayland

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GCC and ld are the default settings for building a C++ project. I managed to use clang and configure it alongside GCC, How can i add the LLVM linker (ld.lld) instead of GCC linker (ld) to be used by my project ?

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This is the commands being run in the compile output:

clang++ -c -pipe -g -std=gnu++2a -Wall -Wextra -DQT_QML_DEBUG -I../../cpp_for_dummies -I. -I../../../../Qt/6.5.2/gcc_64/mkspecs/linux-clang -o main.o ../main.cpp

clang++ -ccc-gcc-name g++ -o cpp_for_dummies add.o main.o

In the second command -ccc-gcc-name g++ is being used, which probably means to use the GNU linker ld. How can i change that to LLVM linker ld.lld ?

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These are my GCC and Clang kits configs:

space

I see no option to configure the linker ?!

[SimonSchroeder]

Similar to your use case I have the following line in my qmake project file:

QMAKE_LFLAGS += -fuse-ld=gold

This seems to translate to -fuse-ld=lld for CMAKE_EXE_LINKER_FLAGS.

[Pete Carter]

@SimonSchroeder

So in my case it should be QMAKE_LFLAGS += -fuse-ld=lld ?

Isn't there an option to specify this in the settings so it's used in all the projects, like hardcode it in the kit settings ?

[cristian-adam]

In the kit you have the Qt mkspec: field which could point to a mkspec that includes this feature.

I have the mkspecs here: c:\Qt\6.4.3\mingw_64\mkspecs, and a "grep" after lld pointed to:

C:\Qt\6.4.3\mingw_64\mkspecs
$ findstr /spin /c:"lld" /l *
common\gcc-base-unix.conf:25:QMAKE_LFLAGS_USE_LLD    = -fuse-ld=lld
features\default_post.prf:85:    use_lld_linker: QMAKE_LFLAGS += $$QMAKE_LFLAGS_USE_LLD
features\qt_configure.prf:399:    use_lld_linker: \
features\qt_configure.prf:400:        LFLAGS = -fuse-ld=lld
features\qt_configure.prf:1216:    use_lld_linker: \
features\qt_configure.prf:1217:        qmake_configs += "use_lld_linker"
features\qt_docs.prf:68:    QDOC += -installdir $$shell_quote($$[QT_INSTALL_DOCS])
features\qt_helper_lib.prf:28:DLLDESTDIR = $$MODULE_BASE_OUTDIR/bin
features\qt_module.prf:94:DLLDESTDIR       = $$MODULE_BASE_OUTDIR/bin
features\toolchain.prf:333:            dir = $$(VSINSTALLDIR)
features\toolchain.prf:349:            dir = $$(VCINSTALLDIR)
macx-xcode\default.xcscheme:26:      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
macx-xcode\default.xcscheme:27:      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
macx-xcode\default.xcscheme:60:      selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
macx-xcode\default.xcscheme:61:      selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
modules\qt_lib_core_private.pri:11:QT.core_private.disabled_features = clock-gettime system-doubleconversion dladdr futimens futimes getauxval getentropy glib glibc icu inotify journald system-libb2 linkat system-pcre2 poll_ppoll poll_pollts poll_poll poll_select qqnx_pps renameat2 slog2 statx syslog backtrace cpp-winrt lttng etw forkfd_pidfd use_bfd_linker use_gold_linker use_lld_linker use_mold_linker android-style-assets gc_binaries developer-build no-prefix private_tests debug no_direct_extern_access mips_dsp mips_dspr2 neon arm_crc32 arm_crypto posix_fallocate alloca_h stack-protector-strong system-zlib stdlib-libcpp dbus-linked libudev dlopen intelcet
qmodule.pri:3:QT.global_private.disabled_features = use_bfd_linker use_gold_linker use_lld_linker use_mold_linker android-style-assets gc_binaries developer-build no-prefix private_tests debug no_direct_extern_access mips_dsp mips_dspr2 neon arm_crc32 arm_crypto posix_fallocate alloca_h stack-protector-strong system-zlib stdlib-libcpp dbus-linked libudev dlopen intelcet
win32-clang-msvc\qmake.conf:40:QMAKE_LINK              = lld-link
win32-clang-msvc\qmake.conf:52:# Leave QMAKE_LFLAGS_LTCG empty because lld-link doesn't need any additional parameters

Maybe it works pointing to a custom mkspec not being part of the system ones.

[ChrisW67]

@Pete-Carter Looks to me like:

CONFIG += use_lld_linker

in your PRO file would switch linker without affecting the GCC compiler..

I am not certain of the CMakeLists.txt equivalent, perhaps:

set(QT_FEATURE_use_lld_linker ON)

[Pete Carter]

@ChrisW67

Is there a way to check that it worked, to check if the executable was linked with ld or lld ?

[cristian-adam]

For CMake you can pass --verbose for the build line, with QMake and make you can pass VERBOSE=1 to the make line in project settings.

[Pete Carter]

@cristian-adam I remember that there was a command used to check the executable and with what it was linked!