Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()

On Thu, Jun 30, 2022 at 04:07:47PM +0530, Naveen N. Rao wrote:

Objtool classifies 'ud2' as INSN_BUG, and 'int3' as INSN_TRAP. In x86 
BUG(), there is no need for an annotation since objtool assumes that 
'ud2' terminates control flow. But, for __WARN_FLAGS(), since 'ud2' is 
used, an explicit annotate_reachable() is needed. That's _reachable_, to 
indicate that the control flow can continue with the next instruction.

On powerpc, we should (eventually) classify all trap variants as 
INSN_TRAP. Even in the absence of that classification today, objtool 
assumes that control flow continues with the next instruction. With your 
work to utilize asm goto for __WARN_FLAGS(), with no extra instructions 
being generated, I think it is appropriate to just use 
__builtin_unreachable() and to not use the annotation.

In any case, we are only hitting this since gcc is generating a 'bl' due 
to that annotation. We are not yet enabling full objtool validation on 
powerpc, so I think we can revisit this at that point.

See also <https://gcc.gnu.org/PR99299> that asks for a __builtin_trap()
variant that does not terminate control flow ("that is recoverable").


Segher