[001/nnn] poly_int: add poly-int.h

[Jeff Law]

On 11/13/2017 05:04 PM, Richard Sandiford wrote:
> Richard Sandiford <richard.sandiford@linaro.org> writes:
>> Richard Sandiford <richard.sandiford@linaro.org> writes:
>>> This patch adds a new "poly_int" class to represent polynomial integers
>>> of the form:
>>>
>>>   C0 + C1*X1 + C2*X2 ... + Cn*Xn
>>>
>>> It also adds poly_int-based typedefs for offsets and sizes of various
>>> precisions.  In these typedefs, the Ci coefficients are compile-time
>>> constants and the Xi indeterminates are run-time invariants.  The number
>>> of coefficients is controlled by the target and is initially 1 for all
>>> ports.
>>>
>>> Most routines can handle general coefficient counts, but for now a few
>>> are specific to one or two coefficients.  Support for other coefficient
>>> counts can be added when needed.
>>>
>>> The patch also adds a new macro, IN_TARGET_CODE, that can be
>>> set to indicate that a TU contains target-specific rather than
>>> target-independent code.  When this macro is set and the number of
>>> coefficients is 1, the poly-int.h classes define a conversion operator
>>> to a constant.  This allows most existing target code to work without
>>> modification.  The main exceptions are:
>>>
>>> - values passed through ..., which need an explicit conversion to a
>>>   constant
>>>
>>> - ?: expression in which one arm ends up being a polynomial and the
>>>   other remains a constant.  In these cases it would be valid to convert
>>>   the constant to a polynomial and the polynomial to a constant, so a
>>>   cast is needed to break the ambiguity.
>>>
>>> The patch also adds a new target hook to return the estimated
>>> value of a polynomial for costing purposes.
>>>
>>> The patch also adds operator<< on wide_ints (it was already defined
>>> for offset_int and widest_int).  I think this was originally excluded
>>> because >> is ambiguous for wide_int, but << is useful for converting
>>> bytes to bits, etc., so is worth defining on its own.  The patch also
>>> adds operator% and operator/ for offset_int and widest_int, since those
>>> types are always signed.  These changes allow the poly_int interface to
>>> be more predictable.
>>>
>>> I'd originally tried adding the tests as selftests, but that ended up
>>> bloating cc1 by at least a third.  It also took a while to build them
>>> at -O2.  The patch therefore uses plugin tests instead, where we can
>>> force the tests to be built at -O0.  They still run in negligible time
>>> when built that way.
>>
>> Changes in v2:
>>
>> - Drop the controversial known_zero etc. wrapper functions.
>> - Fix the operator<<= bug that Martin found.
>> - Switch from "t" to "type" in SFINAE classes (requested by Martin).
>>
>> Not changed in v2:
>>
>> - Default constructors are still empty.  I agree it makes sense to use
>>   "= default" when we switch to C++11, but it would be dangerous for
>>   that to make "poly_int64 x;" less defined than it is now.
> 
> After talking about this a bit more internally, it was obvious that
> the choice of "must" and "may" for the predicate names was a common
> sticking point.  The idea was to match the names of alias predicates,
> but given my track record with names ("too_empty_p" being a recently
> questioned example :-)), I'd be happy to rename them to something else.
> Some alternatives we came up with were:
I didn't find the must vs may naming problematical as I was going
through the changes.  What I did find much more difficult was
determining if the behavior was correct when we used a "may" predicate.
It really relies a good deal on knowing the surrounding code.

In places where I knew the code reasonably well could tell without much
surrounding context.  In other places I had to look at the code and
deduce proper behavior in the "may" cases -- and often I resorted to
spot checking and relying on your reputation & testing to DTRT.


> 
> - known_eq / maybe_eq / known_lt / maybe_lt etc.
> 
>   Some functions already use "known" and "maybe", so this would arguably
>   be more consistent than using "must" and "may".
> 
> - always_eq / sometimes_eq / always_lt / sometimes_lt
> 
>   Similar to the previous one in intent.  It's just a question of which
>   wordng is clearer.
> 
> - forall_eq / exists_eq / forall_lt / exists_lt etc.
> 
>   Matches the usual logic quantifiers.  This seems quite appealing,
>   as long as it's obvious that in:
> 
>     forall_eq (v0, v1)
> 
>   v0 and v1 themselves are already bound: if vi == ai + bi*X then
>   what we really saying is:
> 
>     forall X, a0 + b0*X == a1 + b1*X 
> 
> Which of those sounds best?  Any other suggestions?
I can live with any of them.  I tend to prefer one of the first two, but
it's not a major concern for me.  So if you or others have a clear
preference, go with it.


jeff