Richard Biener <richard.guent...@gmail.com> writes:
> On Wed, Oct 31, 2012 at 11:43 AM, Richard Sandiford
> <rdsandif...@googlemail.com> wrote:
>> Richard Biener <richard.guent...@gmail.com> writes:
>>> On Thu, Oct 25, 2012 at 12:55 PM, Kenneth Zadeck
>>> <zad...@naturalbridge.com> wrote:
>>>>
>>>> On 10/25/2012 06:42 AM, Richard Biener wrote:
>>>>>
>>>>> On Wed, Oct 24, 2012 at 7:23 PM, Mike Stump <mikest...@comcast.net> wrote:
>>>>>>
>>>>>> On Oct 24, 2012, at 2:43 AM, Richard Biener <richard.guent...@gmail.com>
>>>>>> wrote:
>>>>>>>
>>>>>>> On Tue, Oct 23, 2012 at 6:12 PM, Kenneth Zadeck
>>>>>>> <zad...@naturalbridge.com> wrote:
>>>>>>>>
>>>>>>>> On 10/23/2012 10:12 AM, Richard Biener wrote:
>>>>>>>>>
>>>>>>>>> + HOST_WIDE_INT val[2 * MAX_BITSIZE_MODE_ANY_INT /
>>>>>>>>> HOST_BITS_PER_WIDE_INT];
>>>>>>>>>
>>>>>>>>> are we sure this rounds properly? Consider a port with max byte mode
>>>>>>>>> size 4 on a 64bit host.
>>>>>>>>
>>>>>>>> I do not believe that this can happen. The core compiler includes all
>>>>>>>> modes up to TI mode, so by default we already up to 128 bits.
>>>>>>>
>>>>>>> And mode bitsizes are always power-of-two? I suppose so.
>>>>>>
>>>>>> Actually, no, they are not. Partial int modes can have bit sizes that
>>>>>> are not power of two, and, if there isn't an int mode that is bigger,
>>>>>> we'd
>>>>>> want to round up the partial int bit size. Something like ((2 *
>>>>>> MAX_BITSIZE_MODE_ANY_INT + HOST_BITS_PER_WIDE_INT - 1) /
>>>>>> HOST_BITS_PER_WIDE_INT should do it.
>>>>>>
>>>>>>>>> I still would like to have the ability to provide specializations of
>>>>>>>>> wide_int
>>>>>>>>> for "small" sizes, thus ideally wide_int would be a template templated
>>>>>>>>> on the number of HWIs in val. Interface-wise wide_int<2> should be
>>>>>>>>> identical to double_int, thus we should be able to do
>>>>>>>>>
>>>>>>>>> typedef wide_int<2> double_int;
>>>>>>>>
>>>>>>>> If you want to go down this path after the patches get in, go for it.
>>>>>>>> I
>>>>>>>> see no use at all for this.
>>>>>>>> This was not meant to be a plug in replacement for double int. This
>>>>>>>> goal of
>>>>>>>> this patch is to get the compiler to do the constant math the way that
>>>>>>>> the
>>>>>>>> target does it. Any such instantiation is by definition placing some
>>>>>>>> predefined limit that some target may not want.
>>>>>>>
>>>>>>> Well, what I don't really like is that we now have two implementations
>>>>>>> of functions that perform integer math on two-HWI sized integers. What
>>>>>>> I also don't like too much is that we have two different interfaces to
>>>>>>> operate
>>>>>>> on them! Can't you see how I come to not liking this? Especially the
>>>>>>> latter …
>>>>>>
>>>>>> double_int is logically dead. Reactoring wide-int and double-int is a
>>>>>> waste of time, as the time is better spent removing double-int from the
>>>>>> compiler. All the necessary semantics and code of double-int _has_ been
>>>>>> refactored into wide-int already. Changing wide-int in any way to vend
>>>>>> anything to double-int is wrong, as once double-int is removed,
>>>>>> then all the
>>>>>> api changes to make double-int share from wide-int is wasted and must
>>>>>> then
>>>>>> be removed. The path forward is the complete removal of double-int; it
>>>>>> is
>>>>>> wrong, has been wrong and always will be wrong, nothing can change that.
>>>>>
>>>>> double_int, compared to wide_int, is fast and lean. I doubt we will
>>>>> get rid of it - you
>>>>> will make compile-time math a _lot_ slower. Just profile when you for
>>>>> example
>>>>> change get_inner_reference to use wide_ints.
>>>>>
>>>>> To be able to remove double_int in favor of wide_int requires _at least_
>>>>> templating wide_int on 'len' and providing specializations for 1 and 2.
>>>>>
>>>>> It might be a non-issue for math that operates on trees or RTXen due to
>>>>> the allocation overhead we pay, but in recent years we transitioned
>>>>> important
>>>>> paths away from using tree math to using double_ints _for speed reasons_.
>>>>>
>>>>> Richard.
>>>>
>>>> i do not know why you believe this about the speed. double int always
>>>> does synthetic math since you do everything at 128 bit precision.
>>>>
>>>> the thing about wide int, is that since it does math to the precision's
>>>> size, it almost never does uses synthetic operations since the sizes for
>>>> almost every instance can be done using the native math on the machine.
>>>> almost every call has a check to see if the operation can be done natively.
>>>> I seriously doubt that you are going to do TI mode math much faster than i
>>>> do it and if you do who cares.
>>>>
>>>> the number of calls does not effect the performance in any negative way and
>>>> it fact is more efficient since common things that require more than one
>>>> operation in double in are typically done in a single operation.
>>>
>>> Simple double-int operations like
>>>
>>> inline double_int
>>> double_int::and_not (double_int b) const
>>> {
>>> double_int result;
>>> result.low = low & ~b.low;
>>> result.high = high & ~b.high;
>>> return result;
>>> }
>>>
>>> are always going to be faster than conditionally executing only one
>>> operation
>>> (but inside an offline function).
>>
>> OK, this is really in reply to the 4.8 thing, but it felt more
>> appropriate here.
>>
>> It's interesting that you gave this example, since before you were
>> complaining about too many fused ops. Clearly this one could be
>> removed in favour of separate and() and not() operations, but why
>> not provide a fused one if there are clients who'll make use of it?
>
> I was more concerned about fused operations that use precision
> or bitsize as input. That is for example
>
>>> + bool only_sign_bit_p (unsigned int prec) const;
>>> + bool only_sign_bit_p () const;
>
> The first is construct a wide-int with precision prec (and sign- or
> zero-extend it) and then call only_sign_bit_p on it. Such function
> should not be necessary and existing callers should be questioned
> instead of introducing it.
>
> In fact wide-int seems to have so many "fused" operations that
> we run out of sensible recognizable names for them. Which results
> in a lot of confusion on what the functions actually do (at least for me).
Well, I suppose I can't really say anything useful either way on
that one, since I'm not writing the patch and I'm not reviewing it :-)
>> I think Kenny's API is just taking that to its logical conclusion.
>> There doesn't seem to be anything sacrosanct about the current choice
>> of what's fused and what isn't.
>
> Maybe. I'd rather have seen an initial small wide-int API and fused
> operations introduced separately together with the places they are
> used. In the current way it's way too tedious to go over all of them
> and match them with callers, lookup enough context and then
> make up my mind on whether the caller should do sth different or not.
>
> Thus, consider the big initial API a reason that all this review takes
> so long ...
>
>> The speed problem we had using trees for internal arithmetic isn't
>> IMO a good argument for keeping double_int in preference to wide_int.
>> Allocating and constructing tree objects to hold temporary values,
>> storing an integer representation in it, then calling tree arithmetic
>> routines that pull out the integer representation again and create a
>> tree to hold the result, is going to be much slower than using either
>> double_int or wide_int. I'd be very surprised if we notice any
>> measurable difference between double_int and wide_int here.
>>
>> I still see no reason to keep double_int around. The width of a host
>> wide integer really shouldn't have any significance.
>>
>> Your main complaint seems to be that the wide_int API is different
>> from the double_int one, but we can't literally use the same API, since
>> double_int has an implicit precision and bitsize, and wide_int doesn't.
>> Having a precision that is separate from the underlying representation
>> is IMO the most important feature of wide_int, so:
>>
>> template wide_int<2> double_int;
>>
>> is never going to be a drop-in, API-compatible replacement for double_int.
>
> My reasoning was that if you strip wide-int of precision and bitsize
> you have a double_int<N> class.
But you don't! Because...
> Thus wide-int should have a base
> of that kind and just add precision / bitsize ontop of that. It wouldn't
> be a step forward if we end up replacing double_int uses with
> wide_int uses with precision of 2 * HOST_BITS_PER_WIDE_INT,
> would it?
...the precision and bitsize isn't an optional extra, either conceptually
or in implementation. wide_int happens to use N HOST_WIDE_INTS under
the hood, but the value of N is an internal implementation detail.
No operations are done to N HWIs, they're done to the number of bits
in the operands. Whereas a double_int<N> class does everything to N HWIs.
Richard
