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

[Segher Boessenkool]

On Wed, Sep 14, 2022 at 04:55:27PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 14, 2022 at 02:28:26PM +0000, Michael Matz wrote:
> > Don't mix DWARF debug info with DWARF-based unwinding info, the latter 
> > doesn't imply the former.  Out of interest: how does ORC get around the 
> > need for CFI annotations (or equivalents to restore registers) and what 
> 
> Objtool 'interprets' the stackops. So it follows the call-graph and is
> an interpreter for all instructions that modify the stack. Doing that it
> konws what the stackframe is at 'most' places.

To get correct backtraces on e.g. PowerPC you need to emulate many of
the integer insns.  That is why GCC enables -fasynchronous-unwind-tables
by default for us.

The same is true for s390, aarch64, and x86 (unless 32-bit w/ frame
pointer).

The problem is that you do not know how to access anything on the stack,
whether in the current frame or in a previous frame, from a random point
in the program.  GDB has many heuristics for this, and it still does not
get it right in all cases.

> > makes it fast?  I want faster unwinding for DWARF as well, when there's 
> > feature parity :-)  Maybe something can be learned for integration into 
> > dwarf-unwind.
> 
> I think we have some details here:
> 
>  https://www.kernel.org/doc/html/latest/x86/orc-unwinder.html

It is faster because it does a whole lot less.  Is that still enough?
It's not clear (to me) what exact information it wants to provide :-(


Segher