https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102952
H.J. Lu <hjl.tools at gmail dot com> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|ASSIGNED |WAITING --- Comment #5 from H.J. Lu <hjl.tools at gmail dot com> --- (In reply to Andrew Cooper from comment #0) > Hello > > [FYI, this is being cross-requested of Clang too] > > Linux and other kernel level software makes use of > -mindirect-branch=thunk-extern to be able to alter the handling of indirect > branches at boot. It turns out to be advantageous to inline the thunks when > retpoline is not in use. > https://lore.kernel.org/lkml/20211026120132.613201...@infradead.org/ is some > infrastructure to make this work. > > In some cases, we want to be able to inline an `lfence; jmp *%reg` thunk. > This is fine for the low 8 registers, but not fine for %r{8..15} where the > REX prefix pushes the replacement size to being 6 bytes. > > It would be very useful to have a code-gen option to write out `call > %cs:__x86_indirect_thunk_r{8..15}` where the redundant %cs prefix will > increase the instruction length to 6, allowing the non-retpoline form to be > inlined. > -mindirect-branch-cs-prefix > Relatedly, x86 straight line speculation has been discussed before, but > without any action taken. It would be helpful to have a code gen option > which would emit `int3` following any `ret` instruction, and any indirect > jump, as neither of these two cases have following architectural execution. > > The reason these two are related is that if both options are in use, we want > an extra byte of replacement space to be able to inline `lfence; jmp *%reg; > int3`. > -mharden-sls=[none|all|return|indirect-branch] Let me know if they work. I also need testcases. > Third (and possibly only for future optimisations), Clang has been observed > to spot conditional tail calls as `Jcc __x86_indirect_thunk_*`. This is a 6 > byte source size, but needs up to 9 bytes of space for inlining including an > `int3` for straight line speculation reasons (See > https://lore.kernel.org/lkml/20211026120310.359986...@infradead.org/ for > full details). It might be enough to simply prohibit an optimisation like > this when trying to pad retpolines for inlineability. I don't think GCC does that at all.