== Introduction ==

Ada makes it possible to define fixed point types. In order to specify how
these will be represented at runtime, the compiler selects a size in bytes and
a "scale factor", which is a strictly positive rational number.  Assuming type
T is associated with the S scale factor, then values V will be represented at
runtime by R so that V = S * R.


== Fixed points in DWARF today ==

Since DWARFv3, there are several ways to describe fixed point types, all of
these require to use DW_TAG_base_type DIEs.  The encoding must be
DW_ATE_signed_fixed or DW_ATE_unsigned_fixed and the scale factor must be
encoded using one of the following attributes:

* DW_AT_decimal_scale, which makes it possible to represent scale factors of
  the form 10**N (with N any signed integer).

* DW_AT_binary_scale, which makes it possible to represent scale factors of
  the form 2**N (with N any signed integer).

* DW_AT_small, which is a reference to a DW_TAG_constant entry providing the
  scale factor itself.


== Limitations ==

While DW_AT_small enables one to describe any scale factor the two other
attributes could, there are still some scale factors that are not encodable as
of today.

In Ada, it is possible to define fixed point types with scale factors such as
1/3.  For instance:

    type T is delta 0.1 range 0.0 .. 1.0
       with Small => 1.0/3.0;

Trying to represent 1/3 with a DW_TAG_constant leads to a chicken an egg problem
today: the constant is encoded in the DW_AT_const_value attribute, which must
be "the actual constant value [...] represented as it would be on the target
architecture". 1/3 can be represented on the target architecture, but only with
a fixed point type... whose scalar factor is 1/3, or 1/30, etc.


== Proposed extension ==

One new tag and two new attributes are proposed to solve this problem:

* DW_TAG_GNU_rational_constant, whose purpose is to describe any rational
  constant.  It must have exactly two attributes:

* DW_AT_GNU_numerator and DW_AT_GNU_denominator, whose forms are any constant
  form (DW_FORM_*data*) and which represent an integer (unsigned, unless the
  form represents a signed one).

The rational constant represented by DW_TAG_GNU_rational_constant is N/D where:

* N is the integer represented by the DW_AT_GNU_numerator attribute;
* D is the integer represented by the DW_AT_GNU_denominator attribute;

With this new tag, it is possible to represent arbitrary rational scale factors
for fixed point types, enabling one to describe all possible Ada fixed point
types.