Richard Sandiford <richard.sandif...@linaro.org> writes:
> Richard Sandiford <richard.sandif...@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:
- 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?
Thanks,
Richard
