Re: [PATCH] lto: pass through -funwind-tables and -fasynchronous-unwind-tables

[Michael Matz <matz@suse.de>]

Hello,

On Wed, 18 Jan 2023, Jakub Jelinek wrote:

> > > > > Partly OT, what is riscv not defaulting that on as well?  Does it have
> > > > > usable unwind info even without that option, something else?
> > > > 
> > > > The RISC-V ABI does not address this, AFAICS.
> > > 
> > > And neither do many other ABIs, still we default there to
> > > -fasynchronous-unwind-tables because we've decided it is a good idea.
> > 
> > That might or might not be, but in the context of this thread that's 
> > immaterial.  Doing the same as the other archs will then simply hide the 
> > problem on risc-v as well, instead of fixing it.
> 
> Yeah, that is why I've mentioned "Partly OT".  We want this bug to be fixed
> (but the fix is not what has been posted but rather decide what we want to
> ask there; if it is at the end of compilation, whether it is at least one
> function with that flag has been compiled, or all functions have been with
> that flag, something else),

The answer to this should be guided by normal use cases.  The normal use 
case is that a whole input file is compiled with or without 
-funwind-tables, and not that individual functions change this.  So any 
solution in which that usecase doesn't work is not a solution.

The purest solution is to emit unwind tables for all functions that 
request it into .eh_frame and for those that don't request it put 
into .debug_frame (if also -g is on).  If that requires enabling 
unwind-tables globally first (i.e. if even just one input function 
requests it) then that is what needs to be done.  (this seems to be the 
problem currently, that the unwind-table activation on a per-function 
basis comes too late).

The easier solution might be to make funwind-tables also be a global 
special-cased option for LTO (so that the usual use-case above works), 
that would trade one or another bug, but I'd say the current bug is more 
serious than the other bug that would be introduced.

> and IMHO riscv should switch to
> -fasynchronous-unwind-tables by default.

I don't disagree, as long as that doesn't lead to the bug being ignored :)


Ciao,
Michael.