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


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.

Uros.


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