Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

On Mon, Sep 12, 2022 at 06:31:14AM -0500, Segher Boessenkool wrote:

Hi!

On Fri, Sep 09, 2022 at 11:07:04AM -0700, Josh Poimboeuf wrote:

quoted

2) Noreturn functions:
   
   There's no reliable way to determine which functions are designated
   by the compiler to be noreturn (either explictly via function
   attribute, or implicitly via a static function which is a wrapper
   around a noreturn function.)

Or just a function that does not return for any other reason.

The compiler makes no difference between functions that have the
attribute and functions that do not.  There are good reasons to not
have the attribute on functions that do in fact not return.  The
not-returningness of the function may be just an implementation
accident, something you do not want part of the API, so it *should* not
have that attribute; or you may want the callers to a function to not be
optimised according to this knowledge (you cannot *prevent* that, the
compiler can figure it out it other ways, but still) for any other
reason.

Yes, many static functions that are wrappers around noreturn functions
have this "implicit noreturn" property.  I agree we would need to know
about those functions (or, as Michael suggested, their call sites) as
well.

quoted

   This information is needed because the
   code after the call to such a function is optimized out as
   unreachable and objtool has no way of knowing that.

Since June we (GCC) have -funreachable-traps.  This creates a trap insn
wherever control flow would otherwise go into limbo.

Ah, that's interesting, though I'm not sure if we'd be able to
distinguish between "call doesn't return" traps and other traps or
reasons for UD2.

-- 
Josh