> This piece of code along doesn't tell me exactly why the frame pointer
> is needed. I was looking for an explicit use, but I now guess that if
> you have multiple adjusts of the [stack] pointer you can't easily undo
> them in the error case (the function behaves as-if using alloca). Is
> that it?
Yes, exactly, the analogy with alloca is correct.
> And without exceptions I assume you just get a call to abort so
> it doesn't matter? If I understood all that right, then this is ok.
If you don't care about exceptions on stack overflow, then the signal will
very likely terminate the program instead of overwriting stack contents, which
is good enough. In Ada, the language requires you to exit gracefully or even
to resume regular execution (at least in theory for the latter).
> In i386.c I see a code block with a similar condition,
>
> /* If the only reason for frame_pointer_needed is that we conservatively
> assumed stack realignment might be needed, but in the end nothing that
> needed the stack alignment had been spilled, clear
> frame_pointer_needed
> and say we don't need stack realignment. */
>
> and the condition has
>
> && !(flag_stack_check && STACK_CHECK_MOVING_SP)
>
> Should that be changed too?
Yes, it probably should, thanks for spotting it, revised patch attached.
PR target/67265
* ira.c (ira_setup_eliminable_regset): Do not necessarily create the
frame pointer for stack checking if non-call exceptions aren't used.
* config/i386/i386.c (ix86_finalize_stack_realign_flags): Likewise.
--
Eric BotcazouIndex: ira.c
===================================================================
--- ira.c (revision 230146)
+++ ira.c (working copy)
@@ -2259,9 +2259,12 @@ ira_setup_eliminable_regset (void)
frame_pointer_needed
= (! flag_omit_frame_pointer
|| (cfun->calls_alloca && EXIT_IGNORE_STACK)
- /* We need the frame pointer to catch stack overflow exceptions
- if the stack pointer is moving. */
- || (flag_stack_check && STACK_CHECK_MOVING_SP)
+ /* We need the frame pointer to catch stack overflow exceptions if
+ the stack pointer is moving (as for the alloca case just above). */
+ || (STACK_CHECK_MOVING_SP
+ && flag_stack_check
+ && flag_exceptions
+ && cfun->can_throw_non_call_exceptions)
|| crtl->accesses_prior_frames
|| (SUPPORTS_STACK_ALIGNMENT && crtl->stack_realign_needed)
/* We need a frame pointer for all Cilk Plus functions that use
Index: config/i386/i386.c
===================================================================
--- config/i386/i386.c (revision 230146)
+++ config/i386/i386.c (working copy)
@@ -12470,7 +12466,11 @@ ix86_finalize_stack_realign_flags (void)
&& !crtl->accesses_prior_frames
&& !cfun->calls_alloca
&& !crtl->calls_eh_return
- && !(flag_stack_check && STACK_CHECK_MOVING_SP)
+ /* See ira_setup_eliminable_regset for the rationale. */
+ && !(STACK_CHECK_MOVING_SP
+ && flag_stack_check
+ && flag_exceptions
+ && cfun->can_throw_non_call_exceptions)
&& !ix86_frame_pointer_required ()
&& get_frame_size () == 0
&& ix86_nsaved_sseregs () == 0
