On 09/04/2011 03:44 PM, Tom de Vries wrote:
> On 09/04/2011 11:10 AM, Richard Guenther wrote:
>> On Sun, Sep 4, 2011 at 11:00 AM, Tom de Vries <vr...@codesourcery.com> wrote:
>>> Hi,
>>>
>>> this patch fixes PR50251, which was caused by r178353.
>>>
>>> The patch was bootstrapped and reg-tested on i686 and x86_64.
>>> On i686, the test-cases reported failing in PR50251 pass again.
>>>
>>> The patch selects the DRAP type stack realignment method in case a
>>> stack_restore
>>> is used. If that is not done, the presence of the stack_restore at reload
>>> leaves
>>> FRAME_POINTER without an elimination rule for i386 port.
>>>
>>> OK for trunk?
>>
>> Shouldn't we eventually simply set cfun->calls_alloca when expanding either
>> stack save or restore? Similar to how it is set from
>> allocate_dynamic_stack_space?
>> I'm not sure we encountered the dead stack save/restore pair before
>> this new folding,
>> so, just to make other targets not confused about them as well?
>
> Setting cfun->calls_alloca will work as well, but doesn't seem necessary to
> me.
> AFAIU, since other targets don't define MAX_STACK_ALIGNMENT, they don't need
> to
> do a realign themselves. If I disable MAX_STACK_ALIGNMENT on i386, the
> middle-end handles the realign and the stack_restore causes no problems. So to
> me this seems a problem with the i386-specific implementation of realignment.
>
> I'm more worried about other similar cases not working for i386 than about
> other
> ports. Another, more involved way to fix this, would be in reload to:
> - calculate which registers we cannot use for elimination (which we already
> do)
> - pass that as context to target.can_eliminate. The i386 target can then
> fulfill its requirement to be able to eliminate the frame pointer by
> setting need_drap if the stack pointer is not available.
> I think this way we could remove most if not all of the
> 'crtl->need_drap = true' snippets, and have a completer solution and perhaps
> also more optimal.
>
> Is this simple crtl->need_drap fix ok for now, or should I start testing the
> cfun->calls_alloca fix?
>
Ping.
I have tested the attached patch on both x86_64 and i686 and it fixes all
failures in PR50251 without any regressions.
Since I don't feel like the right person to dig into reload guts to implement
the solution above, I want to check in this fix to i386 backend.
Uros, Richard H., OK for trunk?
Thank you,
- Tom
2011-09-14 Tom de Vries <t...@codesourcery.com>
* explow.c (emit_stack_restore): Set crtl->need_drap if
stack_restore is emitted.
Index: gcc/explow.c
===================================================================
--- gcc/explow.c (revision 178145)
+++ gcc/explow.c (working copy)
@@ -1062,6 +1062,20 @@ emit_stack_restore (enum save_level save
/* The default is that we use a move insn. */
rtx (*fcn) (rtx, rtx) = gen_move_insn;
+ /* If stack_realign_drap, the x86 backend emits a prologue that aligns both
+ STACK_POINTER and HARD_FRAME_POINTER.
+ If stack_realign_fp, the x86 backend emits a prologue that aligns only
+ STACK_POINTER. This renders the HARD_FRAME_POINTER unusable for accessing
+ aligned variables, which is reflected in ix86_can_eliminate.
+ We normally still have the realigned STACK_POINTER that we can use.
+ But if there is a stack restore still present at reload, it can trigger
+ mark_not_eliminable for the STACK_POINTER, leaving no way to eliminate
+ FRAME_POINTER into a hard reg.
+ To prevent this situation, we force need_drap if we emit a stack
+ restore. */
+ if (SUPPORTS_STACK_ALIGNMENT)
+ crtl->need_drap = true;
+
/* See if this machine has anything special to do for this kind of save. */
switch (save_level)
{
