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glaucus Linux

mold

  • Depends on tbb
  • Uses 12 GB when building and basically freezes the build process (mold should speed up linking times and reduce the overall bootstrap time but building mold takes up all the saved time and more…)
  • Segfaults at random
  • Used to add libgcc to all linked binaries
  • Used to not be able to link the kernel
  • Used to not support LTO
  • Don’t forget to include mold in your compiler’s custom search directory

Resources

Notes

  • You can pass linker flags like this by using a single -Wl, check output.txt:
Terminal window
gcc -v main.c -o test -Wl,-s,--as-needed,--build-id,sha256,--compress-debug-sections,zstd,--gc-sections,--hash-style=gnu,-z,now,-z,noexecstack,-z,relro 2> output.txt
  • MOLD flags:
RAD_FLAGS_LINKER* = "-fuse-ld=mold -Wl,-s,--as-needed,--compress-debug-sections=zstd,--gc-sections,--hash-style=gnu,-z,now,-z,noexecstack,-z,relro"
  • BFD flags:
RAD_FLAGS_LINKER* = "-Wl,-O1,-s,-z,noexecstack,-z,now,-z,relro,-z,x86-64-v3,--as-needed,--gc-sections,--sort-common,--hash-style=gnu,--compress-debug-sections=zstd"

Custom GCC Search Directory

  • Custom built GCCs have custom search directories. Even though I have version 12.2.0, adding -fuse-ld=mold was not sufficient for my custom gcc to find the host’s mold (provided by distribution’s package manager) or my custom mold (as I was getting the following error collect2: fatal error: cannot find 'ld' mold, which is why I had to add -B/path/to/directory/where/custom/mold/is to my CFLAGS along with -fuse-ld=mold to get it working.
  • Most packages linked just fine, until I stumbled upon packages which used autoreconf and libtool to create .la files. Apparently libtool likes to strip flags when linking. This was troublesome because I am passing -B/path/to/directory/where/custom/mold/is in my CFLAGS and it was being removed, so my custom gcc could no longer find mold, giving the same error as above.
  • You can workaround this by modifying GCC_EXEC_PREFIX which is not desirable for many reasons.
  • The solution I found was to provide a symbolic link to mold in the search directories of the custom built gcc (which can be listed by running gcc -print-search-dirs).
  • The symbolic link was installed in /custom/toolchain/tuple/bin where binutils already keeps its hardlinks to ../../bin including ld.bfd, which is why I issued the command ln -fs ../../bin/mold /custom/toolchain/tuple/bin/ld.mold.