[PATCH/RFC] On the use of -funreachable-traps to deal with PR 109627

[Iain Sandoe <idsandoe@googlemail.com>]

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;
 
====