Fix stack checking emulation in Ada

[Eric Botcazou <ebotcazou@adacore.com>]

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When stack checking cannot be done by means of probes, typically because of 
the lack of MMU, the Ada compiler emulates it in the runtime (that's required 
if you want to pass the ACATS testsuite) through calls to a specific routine.

Now calls to this routine are emitted as libcalls and libcalls are ECF_NOTHROW 
by default:

  /* By default, library functions cannot throw.  */
  flags = ECF_NOTHROW;

so this is problematic with optimization enabled because the compiler needs to 
catch the Storage_Error exception for ACATS.  That's why the attached patch 
changes probe_stack_range to use a LCT_THROW libcall instead and also tweaks 
emit_library_call_value_1 not to set ECF_NORETURN for it (the stack checking 
routine obviously returns most of the time and throws only exceptionally).

This only affects the Ada compiler since stack_check_libfunc is set only in 
Ada and LCT_THROW has been unused since 2009.  That's an old issue which 
recently resurfaced under PR ada/70017 because s390/Linux was still using 
stack checking emulation; the PR was addressed for GCC 6 by switching to real 
stack checking but I have applied the patch on the mainline for the other 
platforms still using emulation, after testing it on x86-64/Linux.


2016-03-12  Eric Botcazou  <ebotcazou@adacore.com>

	PR ada/70017
	* calls.c (emit_library_call_value_1): Clear the ECF_NOTHROW flag if
	the libcall is LCT_THROW.
	* explow.c (probe_stack_range): Pass LCT_THROW to emit_library_call
	for the checking routine.

-- 
Eric Botcazou

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Index: calls.c
===================================================================
--- calls.c	(revision 234098)
+++ calls.c	(working copy)
@@ -3852,7 +3852,7 @@ emit_library_call_value_1 (int retval, r
   reg_parm_stack_space = REG_PARM_STACK_SPACE ((tree) 0);
 #endif
 
-  /* By default, library functions can not throw.  */
+  /* By default, library functions cannot throw.  */
   flags = ECF_NOTHROW;
 
   switch (fn_type)
@@ -3869,7 +3869,7 @@ emit_library_call_value_1 (int retval, r
       flags |= ECF_NORETURN;
       break;
     case LCT_THROW:
-      flags = ECF_NORETURN;
+      flags &= ~ECF_NOTHROW;
       break;
     case LCT_RETURNS_TWICE:
       flags = ECF_RETURNS_TWICE;
Index: explow.c
===================================================================
--- explow.c	(revision 234098)
+++ explow.c	(working copy)
@@ -1566,7 +1566,7 @@ probe_stack_range (HOST_WIDE_INT first,
 					         stack_pointer_rtx,
 					         plus_constant (Pmode,
 								size, first)));
-      emit_library_call (stack_check_libfunc, LCT_NORMAL, VOIDmode, 1, addr,
+      emit_library_call (stack_check_libfunc, LCT_THROW, VOIDmode, 1, addr,
 			 Pmode);
     }