On Mon, Apr 8, 2024 at 4:04 PM Iain Sandoe <idsan...@googlemail.com> wrote:
>
> Hi
>
> PR 109627 is about functions that have had their bodies completely elided,
> but still have the wrappers for EH frames (either .cfi_xxx or LFSxx/LFExx).
I was thinking about how to fix this once and for all. The easiest
method I could think of was if __builtin_unreachable is the only thing
in the CFG expand it as __builtin_trap.
And then it should just work.
It should not to hard to add that check in expand_gimple_basic_block
and handle it that way.
What do you think of that? I can code this up for GCC 15 if you want.
Thanks,
Andrew Pinski
>
> These are causing issues for some linkers because such functions result in
> FDEs with a 0 code extent.
>
> The simplest representation of this is (from PR109527)
>
> void foo () { __builtin_unreachable (); }
>
> The solution (so far) is to detect this case during final lowering and
> replace the unreachable (which is expanded to nothing, at least for the
> targets I’ve dealt with) by a trap; this results in two positive improvements
> (1) the FDE is now finite-sized so the linker consumes it and (2) actually
> the trap is considerably more user-friendly UB than falling through to some
> other arbitrary place.
>
> I was looking into using -funreachable-traps to do this for aarch64 Darwin -
> because the ad-hoc solutions that were applied to X86 and PPC are not easily
> usable for aarch64.
>
> -funreachabe-traps was added for similar reasons (helping make missing
> returns less unexpected) in r13-1204-gd68d3664253696 by Jason (and then there
> have been further improvements resulting in the use of __builtin_unreachable
> trap () from Jakub)
>
> As I read the commit message for r13-1204, I would expect -funreachable-traps
> to work for the simple case above, but it does not. I think that is because
> the incremental patch below is needed. however, I am not sure if there was
> some reason this was not done at the time?
>
> PR 109627 is currently a show-stopper for the aarch64-darwin branch since
> libgomp and libgm2 fail to bootstrap - and other workarounds (e.g.
> -D__builtin_unreachable=__builtin_trap) do not work got m2 (since it does not
> use the C preprocessor by default).
>
> Setting -funreachable-traps either per affected file, or globally for a
> target resolves the issue in a neater manner.
>
> Any guidance / comments would be most welcome - if the direction seems sane,
> I can repost this patch formally.
>
> (I have tested quite widely on Darwin and on a small number of Linux cases
> too)
>
> thanks
> Iain
>
> * I will note that applying this does result in some regressions in several
> contracts test cases - but they also regress for -fsanitize=undefined
> -fsanitise-traps (not yet clear if that’s expected or we’ve uncovered a bug
> in the contracts impl.).
>
> ----------
>
>
> diff --git a/gcc/builtins.cc b/gcc/builtins.cc
> index f8d94c4b435..e2d26e45744 100644
> --- a/gcc/builtins.cc
> +++ b/gcc/builtins.cc
> @@ -5931,7 +5931,8 @@ expand_builtin_unreachable (void)
> {
> /* Use gimple_build_builtin_unreachable or builtin_decl_unreachable
> to avoid this. */
> - gcc_checking_assert (!sanitize_flags_p (SANITIZE_UNREACHABLE));
> + gcc_checking_assert (!sanitize_flags_p (SANITIZE_UNREACHABLE)
> + && !flag_unreachable_traps);
> emit_barrier ();
> }
>
> @@ -10442,7 +10443,7 @@ fold_builtin_0 (location_t loc, tree fndecl)
>
> case BUILT_IN_UNREACHABLE:
> /* Rewrite any explicit calls to __builtin_unreachable. */
> - if (sanitize_flags_p (SANITIZE_UNREACHABLE))
> + if (sanitize_flags_p (SANITIZE_UNREACHABLE) || flag_unreachable_traps)
> return build_builtin_unreachable (loc);
> break;
>
> ====
