Re: [PATCH, PR target/69454] Disable TARGET_STV when stack is not properly aligned

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Feb 2, 2016 at 5:21 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
> 2016-02-02 16:14 GMT+03:00 H.J. Lu <hjl.tools@gmail.com>:
>> On Tue, Feb 2, 2016 at 5:11 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>> 2016-02-02 16:06 GMT+03:00 H.J. Lu <hjl.tools@gmail.com>:
>>>> On Tue, Feb 2, 2016 at 5:03 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>>>> 2016-02-02 15:46 GMT+03:00 H.J. Lu <hjl.tools@gmail.com>:
>>>>>> On Tue, Feb 2, 2016 at 4:30 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>> On Tue, Feb 2, 2016 at 4:29 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>>>>>>>> On Tue, Feb 02, 2016 at 01:24:26PM +0100, Uros Bizjak wrote:
>>>>>>>>> On Tue, Feb 2, 2016 at 12:53 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>>>>>>>>>
>>>>>>>>> >> The bottom line is  ix86_minimum_alignment must return the correct
>>>>>>>>> >> number for DImode or you can just turn off STV.   My suggestion is
>>>>>>>>> >> to use my patch.
>>>>>>>>> >
>>>>>>>>> > Uros, any preferences here?  I mean, it is possible to use
>>>>>>>>> > e.g. the ix86_option_override_internal and have H.J's ix86_minimum_alignment
>>>>>>>>> > change as a safety net, in the usual case for -mpreferred-stack-boundary=2
>>>>>>>>> > we'll just disable TARGET_STV and ix86_minimum_alignment change won't do
>>>>>>>>> > anything, as TARGET_STV will be false, and if for whatever case it gets
>>>>>>>>> > through (target attribute, -mincoming-stack-boundary=, ...)
>>>>>>>>> > ix86_minimum_alignment will be there to ensure enough stack alignment.
>>>>>>>>> > Most of the smaller -mpreferred-stack-boundary= uses are -mno-sse anyway,
>>>>>>>>> > and that is something we don't want to affect.
>>>>>>>>>
>>>>>>>>> IMO, we should disable STV when -mpreferred-stack-boundary < 3, as STV
>>>>>>>>> is only an optimization. Perhaps we can also emit a "sorry" for
>>>>>>>>> explicit -mstv in case stack boundary requirement is not satisfied.
>>>>>>>>> *If* there is a need for -mstv with smaller stack boundary, we can
>>>>>>>>> revisit this decision for later gcc versions.
>>>>>>>>>
>>>>>>>>> I think disabling STV is less surprising option than increasing stack
>>>>>>>>> boundary behind the user's back.
>>>>>>>>
>>>>>>>> So, is http://gcc.gnu.org/ml/gcc-patches/2016-01/msg02129.html
>>>>>>>> ok for trunk then (alone or with additional sorry, incremental or not?)?
>>>>>>>> I believe it does just that.
>>>>>>>
>>>>>>> This patch is WRONG.
>>>>>>>
>>>>>>> --
>>>>>>> H.J.
>>>>>>
>>>>>> You will run into the same ICE with
>>>>>>
>>>>>> -mincoming-stack-boundary=2 -msse2 -O2 -m32
>>>>>>
>>>>>> in a leaf function which needs DImode spill/fill.
>>>>>
>>>>> Why would we need DImode spill/fill having no DImode registers?
>>>>>
>>>>
>>>> Because STV is enabled with
>>>>
>>>>  -mincoming-stack-boundary=2 -msse2 -O2 -m32
>>>
>>> I misread it as -mpreferred-... So why would we fail having a proper
>>> preferred stack alignment? AFAIK leaf function doesn't affect
>>> alignment until we finalize it after RA.
>>>
>>
>> /* Finalize stack_realign_needed flag, which will guide prologue/epilogue
>>    to be generated in correct form.  */
>> static void
>> ix86_finalize_stack_realign_flags (void)
>> {
>>   /* Check if stack realign is really needed after reload, and
>>      stores result in cfun */
>>   unsigned int incoming_stack_boundary
>>     = (crtl->parm_stack_boundary > ix86_incoming_stack_boundary
>>        ? crtl->parm_stack_boundary : ix86_incoming_stack_boundary);
>>   unsigned int stack_realign
>>     = (incoming_stack_boundary
>>        < (crtl->is_leaf && !ix86_current_function_calls_tls_descriptor
>>           ? crtl->max_used_stack_slot_alignment
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> We call it after RA when all spill slots are allocated and check if we
> may relax stack alignment. Don't see any problem here.

Please see

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69454#c26

Why did LRA crash then?


-- 
H.J.