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 Wed, Jan 27, 2016 at 8:29 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
> 2016-01-27 19:18 GMT+03:00 H.J. Lu <hjl.tools@gmail.com>:
>> On Wed, Jan 27, 2016 at 8:11 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>> On 27 Jan 16:44, Jakub Jelinek wrote:
>>>> On Wed, Jan 27, 2016 at 06:34:41PM +0300, Ilya Enkovich wrote:
>>>> > @@ -5453,6 +5443,11 @@ ix86_option_override_internal (bool main_args_p,
>>>> >      opts->x_target_flags |= MASK_VZEROUPPER;
>>>> >    if (!(opts_set->x_target_flags & MASK_STV))
>>>> >      opts->x_target_flags |= MASK_STV;
>>>> > +  /* Disable STV if -mpreferred-stack-boundary={2,3} - the needed
>>>>
>>>> The comment doesn't match the code, you disable STV only for
>>>> -mpreferred-stack-boundary=2.
>>>
>>> Thanks, here is an updated version.
>>>
>>> Ilya
>>> --
>>> gcc/
>>>
>>> 2016-01-27  Jakub Jelinek  <jakub@redhat.com>
>>>             Ilya Enkovich  <enkovich.gnu@gmail.com>
>>>
>>>         PR target/69454
>>>         * config/i386/i386.c (convert_scalars_to_vector): Remove
>>>         stack alignment fixes.
>>>         (ix86_option_override_internal): Disable TARGET_STV if stack
>>>         is not properly aligned.
>>>
>>> gcc/testsuite/
>>>
>>> 2016-01-27  Ilya Enkovich  <enkovich.gnu@gmail.com>
>>>
>>>         PR target/69454
>>>         * gcc.target/i386/pr69454-1.c: New test.
>>>         * gcc.target/i386/pr69454-2.c: New test.
>>>
>>>
>>> diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
>>> index 34b57a4..9fb8db8 100644
>>> --- a/gcc/config/i386/i386.c
>>> +++ b/gcc/config/i386/i386.c
>>> @@ -3588,16 +3588,6 @@ convert_scalars_to_vector ()
>>>    bitmap_obstack_release (NULL);
>>>    df_process_deferred_rescans ();
>>>
>>> -  /* Conversion means we may have 128bit register spills/fills
>>> -     which require aligned stack.  */
>>> -  if (converted_insns)
>>> -    {
>>> -      if (crtl->stack_alignment_needed < 128)
>>> -       crtl->stack_alignment_needed = 128;
>>> -      if (crtl->stack_alignment_estimated < 128)
>>> -       crtl->stack_alignment_estimated = 128;
>>> -    }
>>> -
>>>    return 0;
>>>  }
>>>
>>> @@ -5453,6 +5443,11 @@ ix86_option_override_internal (bool main_args_p,
>>>      opts->x_target_flags |= MASK_VZEROUPPER;
>>>    if (!(opts_set->x_target_flags & MASK_STV))
>>>      opts->x_target_flags |= MASK_STV;
>>> +  /* Disable STV if -mpreferred-stack-boundary=2 - the needed
>>> +     stack realignment will be extra cost the pass doesn't take into
>>> +     account and the pass can't realign the stack.  */
>>> +  if (ix86_preferred_stack_boundary < 64)
>>> +    opts->x_target_flags &= ~MASK_STV;
>>>    if (!ix86_tune_features[X86_TUNE_AVX256_UNALIGNED_LOAD_OPTIMAL]
>>>        && !(opts_set->x_target_flags & MASK_AVX256_SPLIT_UNALIGNED_LOAD))
>>>      opts->x_target_flags |= MASK_AVX256_SPLIT_UNALIGNED_LOAD;
>>
>> MINIMUM_ALIGNMENT keeps track stack alignment.  It is OK
>> to disable STV for -mpreferred-stack-boundary=2.  But you should
>> also update ix86_minimum_alignment to make sure that STV is
>> disabled before returning 32 for DImode.
>
> If -mpreferred-stack-boundary=2 then STV is disabled, if STV is enabled then
> -mpreferred-stack-boundary>=3 and this condition in
> ix86_minimum_alignment works:
>
>   if (TARGET_64BIT || align != 64 || ix86_preferred_stack_boundary >= 64)
>     return align;
>

No, you shouldn't make assumptions in ix86_minimum_alignment. You
should check explicitly that STV is disabled in ix86_minimum_alignment.


-- 
H.J.