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).
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;
====
