Re: PATCH: PR rtl-optimization/54157: [x32] -maddress-mode=long failures

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

On Wed, Aug 8, 2012 at 6:43 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Wed, Aug 8, 2012 at 3:40 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>
>> <rdsandiford@googlemail.com> wrote:
>>> "H.J. Lu" <hjl.tools@gmail.com> writes:
>>>> diff --git a/gcc/combine.c b/gcc/combine.c
>>>> index a07c046..b9a3589 100644
>>>> --- a/gcc/combine.c
>>>> +++ b/gcc/combine.c
>>>> @@ -10784,12 +10784,30 @@ gen_lowpart_for_combine (enum machine_mode omode, rtx
>>>> x)
>>>>    if (omode == imode)
>>>>      return x;
>>>>
>>>> -  /* Return identity if this is a CONST or symbolic reference.  */
>>>> -  if (omode == Pmode
>>>> -      && (GET_CODE (x) == CONST
>>>> -       || GET_CODE (x) == SYMBOL_REF
>>>> -       || GET_CODE (x) == LABEL_REF))
>>>> -    return x;
>>>> +  if (omode == Pmode)
>>>> +    {
>>>> +      /* Return identity if this is a symbolic reference.  */
>>>> +      if (GET_CODE (x) == SYMBOL_REF || GET_CODE (x) == LABEL_REF)
>>>> +     return x;
>>>> +
>>>> +      /* Return identity for CONST unless this is a PLUS of 2 constant
>>>> +      operands.  */
>>>> +      if (GET_CODE (x) == CONST)
>>>> +     {
>>>> +       rtx y = XEXP (x, 0);
>>>> +       if (GET_CODE (y) == PLUS
>>>> +           && ((CONST_INT_P (XEXP (y, 1))
>>>> +                && (GET_CODE (XEXP (y, 0)) == CONST
>>>> +                    || GET_CODE (XEXP (y, 0)) == SYMBOL_REF
>>>> +                    || GET_CODE (XEXP (y, 0)) == LABEL_REF))
>>>> +               || (CONST_INT_P (XEXP (y, 1))
>>>> +                   && (GET_CODE (XEXP (y, 0)) == CONST
>>>> +                       || GET_CODE (XEXP (y, 0)) == SYMBOL_REF
>>>> +                       || GET_CODE (XEXP (y, 0)) == LABEL_REF))))
>>>> +         goto fail;
>>>> +       return x;
>>>> +     }
>>>> +    }
>>>>
>>>>    /* We can only support MODE being wider than a word if X is a
>>>>       constant integer or has a mode the same size.  */
>>>>
>>>> works for the testcase.
>>>
>>> My point was that the "return x" is always wrong.  Whenever we return x
>>> here, we know we're returning something in a different mode from the one
>>> that the caller wanted.  Returning a Pmode LABEL_REF might not trigger
>>> that plus_constant assert, but it's still wrong.
>>>
>>> It looks like this came from the mips-rewrite branch:
>>>
>>> 2003-03-13  Richard Sandiford  <rsandifo@redhat.com>
>>>
>>>         * combine.c (gen_lowpart_for_combine): Treat the lowpart Pmode of
>>>         a CONST as identity.  Check the return value of gen_lowpart_common.
>>>
>>> so I can categorically confirm that the person who wrote it didn't
>>> know what they were doing.  It also means that this case was motivated
>>> by an experiment to make Pmode == DImode for n32, which we ended up
>>> discarding because it produced worse code.
>>>
>>> If this case really is important, we might consider using
>>> convert_memory_address (Pmode, x) instead.  I'm not sure whether
>>> that would be right for targets with address spaces though, because
>>> we don't know which address space is associated with the address.
>>> Hopefully someone who knows address spaces can comment.
>>>
>>> If it is correct, then it should probably go in gen_lowpart_common
>>> rather than gen_lowpart_for_combine.
>>>
>>> But given how few people have hit this, it doesn't look like a
>>> particularly important attempted optimisation.  I'll pre-approve
>>> a patch that undoes my mistake and simply removes:
>>>
>>>   /* Return identity if this is a CONST or symbolic reference.  */
>>>   if (omode == Pmode
>>>       && (GET_CODE (x) == CONST
>>>           || GET_CODE (x) == SYMBOL_REF
>>>           || GET_CODE (x) == LABEL_REF))
>>>     return x;
>>>
>>> Richard
>>
>> This is the patch I checked in.
>
> Probably we need to backport this patch to 4.7, where x32 is
> -maddress-mode=long by default.
>

It doesn't fail on 4.7 branch since checking mode on PLUS CONST
is new on trunk.  However, I think it is a correctness issue.  Is this
OK to backport to 4.7?

Thanks.

-- 
H.J.