[Bug target/84908] retpoline weirdness: 7.3.0-1 and 4.8.5-16: with -fPIC: R_X86_64_PC32 against undefined symbol `__x86_indirect_thunk_rax'

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84908

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Jason Vas Dias from comment #2)
> Thanks H.J. -
> 
> RE:
> > vDSO isn't compiled with -mindirect-branch=thunk-extern in kernel
> > 4.16-rc5.  Why isn't it the case for you? 
> 
> All I know is ,  when submitting a patched vclock_gettime.c 
> in which the switch in vdso_clock_gettime() had 6 clauses
> instead of 5, I received in response a mail from 

4.16-rc5 has

ifdef CONFIG_RETPOLINE
ifneq ($(RETPOLINE_CFLAGS),)
  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
endif
endif

vDSO isn't compiled with $(KBUILD_CFLAGS).  Why does your kernel do it?

> 
> But I don't want my patch to have to depend on this being removed -
> the removal of these flags effectively removes retpoline support, no ?
> It is a shame to have to do that after it was carefully added.
> 
> My gripe is really that nothing in GCC should work with a switch
> with < 6 clauses, and fail with a switch with >= 6 clauses.
> 

This is if statement vs jump table for switch implementation.