Re: remove wrong code in immed_double_const

[Richard Sandiford <rdsandiford@googlemail.com>]

Richard Guenther <richard.guenther@gmail.com> writes:
> On Tue, Mar 20, 2012 at 12:31 AM, Mike Stump <mikestump@comcast.net> wrote:
>> On Mar 19, 2012, at 2:44 PM, Richard Sandiford wrote:
>>> Mike Stump <mikestump@comcast.net> writes:
>>>>> If we're going to remove the assert, we need to define stuff like
>>>>> that.
>>>>
>>>> Orthogonal.  The rest of the compiler defines what happens, it either
>>>> is inconsistent, in which case it is by fiat, undefined, or it is
>>>> consistent, in which case that consistency defines it.  The compiler
>>>> is free to document this in a nice way, or do, what is usually done,
>>>> which is to assume everybody just knows what it does.  Anyway, my
>>>> point is, this routine doesn't define the data structure, and is
>>>> _completely_ orthogonal to your concern.  It doesn't matter if it zero
>>>> extends or sign extends or is inconsistent, has bugs, doesn't have
>>>> bugs, is documented, or isn't documented.  In every single one of
>>>> these cases, the code in the routine I am fixing, doesn't change.
>>>> That is _why_ it is orthogonal.  If it weren't, you'd be able to state
>>>> a value for which is mattered.  You can't, which is why you are wrong.
>>>> If you think you are not wrong, please state a value for which it
>>>> matters how it is defined.
>>>
>>> immed_double_const and CONST_DOUBLE are currently
>>> only defined for 2 HOST_WIDE_INTs.
>>
>> I don't happen to share your view.  The routine is defined by documentation.  The documentation might exist in a .texi file, in this case there is no texi file for immed_double_const I don't think, next up, it is defined by the comments before the routine.  In this case, it isn't so defined.
>>
>> The current definition reads:
>>
>> /* Return a CONST_DOUBLE or CONST_INT for a value specified as a pair
>>   of ints: I0 is the low-order word and I1 is the high-order word.
>>   Do not use this routine for non-integer modes; convert to
>>   REAL_VALUE_TYPE and use CONST_DOUBLE_FROM_REAL_VALUE.  */
>>
>> which, is is fine, and I don't _want_ to change that definition of the routine.  I can't fix it, because it isn't broken.  If it were, you would be able to state a case where the new code behaves in a manor inconsistent with the definition, since there is none you cannot state one, and this is _why_ you have failed to state such a case.  If you disagree, please state the case.
>>
>> Now, if you review comment is, could you please update the comments in the routine, I would just say, oh, sure:
>>
>> Index: emit-rtl.c
>> ===================================================================
>> --- emit-rtl.c  (revision 184563)
>> +++ emit-rtl.c  (working copy)
>> @@ -525,10 +525,9 @@ immed_double_const (HOST_WIDE_INT i0, HO
>>
>>      1) If GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT, then we use
>>        gen_int_mode.
>> -     2) GET_MODE_BITSIZE (mode) == 2 * HOST_BITS_PER_WIDE_INT, but the value of
>> -       the integer fits into HOST_WIDE_INT anyway (i.e., i1 consists only
>> -       from copies of the sign bit, and sign of i0 and i1 are the same),  then
>> -       we return a CONST_INT for i0.
>> +     2) If the value of the integer fits into HOST_WIDE_INT anyway
>> +       (i.e., i1 consists only from copies of the sign bit, and sign
>> +       of i0 and i1 are the same), then we return a CONST_INT for i0.
>>      3) Otherwise, we create a CONST_DOUBLE for i0 and i1.  */
>>   if (mode != VOIDmode)
>>     {
>> @@ -540,8 +539,6 @@ immed_double_const (HOST_WIDE_INT i0, HO
>>
>>       if (GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT)
>>        return gen_int_mode (i0, mode);
>> -
>> -      gcc_assert (GET_MODE_BITSIZE (mode) == 2 * HOST_BITS_PER_WIDE_INT);
>>     }
>>
>>   /* If this integer fits in one word, return a CONST_INT.  */
>>
>>
>> Sorry I missed it.  Now, on to CONST_DOUBLE.  It does appear in a texi file:
>>
>>
>> @findex const_double
>> @item (const_double:@var{m} @var{i0} @var{i1} @dots{})
>> Represents either a floating-point constant of mode @var{m} or an
>> integer constant too large to fit into @code{HOST_BITS_PER_WIDE_INT}
>> bits but small enough to fit within twice that number of bits (GCC
>> does not provide a mechanism to represent even larger constants).  In
>> the latter case, @var{m} will be @code{VOIDmode}.
>>
>> @findex CONST_DOUBLE_LOW
>> If @var{m} is @code{VOIDmode}, the bits of the value are stored in
>> @var{i0} and @var{i1}.  @var{i0} is customarily accessed with the macro
>> @code{CONST_DOUBLE_LOW} and @var{i1} with @code{CONST_DOUBLE_HIGH}.
>>
>>
>> Here again, I don't want to change the definition.  The current definition applies and I am merely making the code conform to it.  It says that CONST_DOUBLE is used when the _value_ of the constant is too large to fit into HOST_BITS_PER_WIDE_INT bits.
>>
>> So, if you disagree with me, you will necessarily have to quote the definition you are using, explain what the words mean to you _and_ state a specific case in which the code post modification doesn't not conform with the existing definition.  You have failed yet again to do that.
>>
>>
>>> So, as good functions do, immed_double_const asserts that it is not being used out of spec.
>>
>> This does not follow from the definition.  0 is a value that fits into HOST_BITS_PER_WIDE_INT bits.  It is representable in 0 bits.  HOST_BITS_PER_WIDE_INT is zero or more, and by induction, is representable by HOST_BITS_PER_WIDE_INT bits.
>>
>>> You want to remove that restriction on immed_double_const and CONST_DOUBLE.
>>> That is, you want to change their spec.  We should only do that if we define
>>> what the new semantics are.
>>
>> You're assuming a definition for CONST_DOUBLE that only exists in your mind, instead, please refer to the actual definition in the .texi file.
>
> Btw, I agree with Mike here (quite obvious if you followed the old
> e-mail thread).

I've no objection to moving the assert down to after the GEN_INT.
But it sounds like I'm on my own with the whole CONST_DOUBLE sign thing.
(That is, if we remove the assert altogether, we effectively treat the
number as sign-extended if it happens to fit in a CONST_INT, and
zero-extended otherwise.  That kind of inconsistency seems wrong,
and could turn what is now an ICE into a wrong code bug.)

> But as there is some disagreement here I leave approval of the patch with the
> comment change to someone to break that tie ;)

No need for that.  Clearly it's just me :-)  Please go ahead and approve
whatever you think is right.

Richard