GCC(/Clang): Merging functions with identical instructions (COMDAT folding)

Question

Just curious, do the GCC or Clang toolsets implement the equivalent of MSVC\'s identical COMDAT folding (ICF) currently? If not, are there any plans to? I can\'t seem to fin

Answer 1

Neither GCC nor Clang is a linker, and ICF needs to be done by the linker, or at least with cooperation with the linker. Edit: They don't do ICF, so yes, distinct instantiations produce distinct code. The GNU gold linker supports ICF with the --icf option, which needs the GCC option -ffunction-sections to be used.

Distinct functions must have distinct addresses ... I can't remember if ICF is disabled for any function that has its address taken, but if not it should be possible to put a load of no-op instructions before the combined function and make each distinct instantiation start on a different instruction, so they have different addresses. Edit: gold's --icf=safe option only enables ICF for functions that can be proven not to have their address taken, so code that relies on distinct addresses will still work.

ICF is a neat optimization, but not essential. With a bit of effort you can hoist out non-dependent code to a non-template, or a template with fewer parameters, to reduce the quantity of duplicated code in the executable. There's more information on this in the slides for a Diet Templates talk I did a couple of years ago.