GCN: Set 'UI_TARGET' for 'TARGET_EXCEPT_UNWIND_INFO' [PR94282, PR113331]

GCN: Set 'UI_TARGET' for 'TARGET_EXCEPT_UNWIND_INFO' [PR94282, PR113331]

In commit 7f1989249e25af6fc0f124452efa24b3796b767a
"[gcn] Set 'UI_NONE' for 'TARGET_EXCEPT_UNWIND_INFO' [PR94282]", we've copied
the 'UI_NONE' idea from nvptx to GCN.

I understand the intention of using 'UI_NONE' like this, and it happens to work
in a lot of cases, but there are ICEs elsewhere: code paths where we run into
'internal compiler error: in get_personality_function, at expr.cc:13512':

    13494 /* Extracts the personality function of DECL and returns the corresponding
    13495    libfunc.  */
    13496
    13497 rtx
    13498 get_personality_function (tree decl)
    13499 {
    13500   tree personality = DECL_FUNCTION_PERSONALITY (decl);
    13501   enum eh_personality_kind pk;
    13502
    13503   pk = function_needs_eh_personality (DECL_STRUCT_FUNCTION (decl));
    13504   if (pk == eh_personality_none)
    13505     return NULL;
    13506
    13507   if (!personality
    13508       && pk == eh_personality_any)
    13509     personality = lang_hooks.eh_personality ();
    13510
    13511   if (pk == eh_personality_lang)
    13512     gcc_assert (personality != NULL_TREE);
    13513
    13514   return XEXP (DECL_RTL (personality), 0);
    13515 }

..., where 'lang_hooks.eh_personality ()' ends up calling
'gcc/expr.cc:build_personality_function', and we 'return NULL;' for 'UI_NONE':

    13448 /* Build a decl for a personality function given a language prefix.  */
    13449
    13450 tree
    13451 build_personality_function (const char *lang)
    13452 {
    13453   const char *unwind_and_version;
    13454   tree decl, type;
    13455   char *name;
    13456
    13457   switch (targetm_common.except_unwind_info (&global_options))
    13458     {
    13459     case UI_NONE:
    13460       return NULL;
    [...]

(Comparing to nvptx' current use of 'UI_NONE', this problem (ICEs mentioned
above) is way more prevalent for GCN.)

The GCC internals documentation indeed states, 'gcc/doc/tm.texi':

    @deftypefn {Common Target Hook} {enum unwind_info_type} TARGET_EXCEPT_UNWIND_INFO (struct gcc_options *@var{opts})
    This hook defines the mechanism that will be used for exception handling
    by the target.  If the target has ABI specified unwind tables, the hook
    should return @code{UI_TARGET}.  If the target is to use the
    @code{setjmp}/@code{longjmp}-based exception handling scheme, the hook
    should return @code{UI_SJLJ}.  If the target supports DWARF 2 frame unwind
    information, the hook should return @code{UI_DWARF2}.

    A target may, if exceptions are disabled, choose to return @code{UI_NONE}.
    This may end up simplifying other parts of target-specific code.  [...]

Here, note: "if exceptions are disabled" (meaning: '-fno-exceptions' etc.) may
"return @code{UI_NONE}".  That's what other back ends do with code like:

    /* For simplicity elsewhere in this file, indicate that all unwind
       info is disabled if we're not emitting unwind tables.  */
    if (!opts->x_flag_exceptions && !opts->x_flag_unwind_tables)
      return UI_NONE;
    else
      return UI_TARGET;

The corresponding "simplifying other parts of target-specific code"/
"simplicity elsewhere" would then be the early returns from
'TARGET_ASM_UNWIND_EMIT', 'ARM_OUTPUT_FN_UNWIND', etc. for
'TARGET_EXCEPT_UNWIND_INFO != UI_TARGET' (that is, for 'UI_NONE').

From the documentation (and implementation), however, it does *not* follow that
if a target doesn't implement support for exception handling, it may just set
'UI_NONE' for 'TARGET_EXCEPT_UNWIND_INFO'.

Therefore, switch to 'UI_TARGET', allocating a "fake" 'exception_section'.

With that, all these 'internal compiler error: in get_personality_function'
test cases turn into PASS, or UNSUPPORTED ('exception handling not supported'),
or re-classify into a few other, already known issues.  And, this change also
happens to resolve the class of errors identified in GCC PR113331
"AMDGCN: Compilation failure due to duplicate .LEHB<n>/.LEHE<n> symbols".

(In case that use of 'UI_NONE' like originally intended really makes sense, and
is preferable over this 'UI_TARGET' solution, then more work will be necessary
for implementing the missing parts, where 'UI_NONE' currently isn't handled.)

	PR target/94282
	PR target/113331
	gcc/
	* common/config/gcn/gcn-common.cc (gcn_except_unwind_info): 'return UI_TARGET;'.
	* config/gcn/gcn.cc (gcn_asm_init_sections): New function.
	(TARGET_ASM_INIT_SECTIONS): '#define'.