Re: PATCH: PR target/40838: gcc shouldn't assume that the stack is aligned

[Richard Guenther <richard.guenther@gmail.com>]

On Sat, Oct 17, 2009 at 9:02 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Sat, Oct 17, 2009 at 11:51 AM, Richard Guenther
> <richard.guenther@gmail.com> wrote:
>> On Sat, Oct 17, 2009 at 8:14 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Fri, Oct 16, 2009 at 5:59 PM, Ian Lance Taylor <iant@google.com> wrote:
>>>> "H.J. Lu" <hjl.tools@gmail.com> writes:
>>>>
>>>>> @@ -947,6 +949,12 @@ vect_verify_datarefs_alignment (loop_vec_info loop_vinfo, bb_vec_info bb_vinfo)
>>>>>              }
>>>>>            return false;
>>>>>          }
>>>>> +
>>>>> +      mode = TYPE_MODE (STMT_VINFO_VECTYPE (stmt_info));
>>>>> +      align = GET_MODE_ALIGNMENT (mode);
>>>>> +      if (cfun->hard_stack_alignment < align)
>>>>> +     cfun->hard_stack_alignment = align;
>>>>> +
>>>>>        if (supportable_dr_alignment != dr_aligned
>>>>>            && vect_print_dump_info (REPORT_ALIGNMENT))
>>>>>          fprintf (vect_dump, "Vectorizing an unaligned access.");
>>>>
>>>> I do not understand why this is the right place to set your new cfun
>>>> field.  Your patch is about a misaligned stack.  The code you are
>>>> changing here does not appear to have anything to do with a misaligned
>>>> stack.  This code is about the correct alignment of data.  The data
>>>> may be on the stack but it need not be.  If the data is on the stack,
>>>> then the compiler should be able to determine the alignment available
>>>> on the stack.
>>>
>>> The issue here is to set proper incoming stack alignment. We have
>>> to do it before RTL expansion which uses this information to check
>>> if stack alignment is needed. To do that, we need to know what the
>>> minimum stack alignment required by hardware. We collect hard
>>> stack alignment when we put variables on stack and check the alignment
>>> generated by vectorizer.
>>
>> It's surely not the right function to do this.  Nor is it a proper
>> interface IMHO.
>> Why don't you need to check whether the access is to stack memory at all?
>>
>> I think you want to instead see if any user-alignment on automatic
>> variables requires stack realignment.  Thus, wherever the vectorizer
>> sets DECL_USER_ALIGN on automatic storage.
>>
>
> RTL expansion may generate automatic variables from vectorizer
> statements while vectorizer doesn't generate any automatic variables
> at all. How should I handle this case?

The same way you would handle spills or you would handle user code
using intrinsics.  I don't see how the vectorizer is special.

Richard.

>
> --
> H.J.
>