[Bug inline-asm/93981] No EH information generated for asm statements

[jwjagersma at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93981

--- Comment #5 from jwjagersma at gmail dot com ---
(In reply to Segher Boessenkool from comment #4)
> Pretending any asm can throw would be a pretty serious code degradation.
> 
> Any asm that is not volatile cannot throw (and be correct code).  But
> most volatile asm in the wild can never throw, either.

The intention is to only produce EH info for volatile asms, and only if
-fnon-call-exceptions is given. Asms that take volatile memory operands
should be covered too. There are multiple instances in the gcc code
that suggest this should be possible, but it seems it was never
implemented fully. For example this comment in stmt_could_throw_p:

```
    /* The only statements that can throw an exception are assignments,
         conditionals, calls, resx, and asms.  */
```

And the fact that tree_could_trap_p, stmt_could_throw_p, may_trap_p_1
have cases that return true for asm.

Do note that the proposed patch is still incomplete, I ran into an ICE
while building libgcc with -fnon-call-exceptions:

```
during GIMPLE pass: ehcleanup
../../../gnu/gcc-9.2.0/libgcc/config/i386/sfp-exceptions.c: In function
'__sfp_handle_exceptions':
../../../gnu/gcc-9.2.0/libgcc/config/i386/sfp-exceptions.c:107:1: internal
compiler error: in mark_reachable_handlers, at tree-eh.c:3929
  107 | }
      | ^
libbacktrace could not find executable to open
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://gcc.gnu.org/bugs/> for instructions.
```

However I am not knowledgable enough about gcc's inner workings to know
what exactly is missing. I'm hoping someone can help me out with this.