GNU Compiler Collection (GCC) Internals: Guidelines for using poly_int

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### 10.9 Guidelines for using `poly_int`

One of the main design goals of `poly_int` was to make it easy to write target-independent code that handles variable-sized registers even when the current target has fixed-sized registers. There are two aspects to this:

- The set of `poly_int` operations should be complete enough that the question in most cases becomes “Can we do this operation on these particular `poly_int` values? If not, bail out” rather than “Are these `poly_int` values constant? If so, do the operation, otherwise bail out”.
- If target-independent code compiles and runs correctly on a target with one value of `NUM_POLY_INT_COEFFS`, and if the code does not use asserting functions like `to_constant`, it is reasonable to assume that the code also works on targets with other values of `NUM_POLY_INT_COEFFS`. There is no need to check this during everyday development.

So the general principle is: if target-independent code is dealing with a `poly_int` value, it is better to operate on it as a `poly_int` if at all possible, choosing conservatively-correct behavior if a particular operation fails. For example, the following code handles an index `pos` into a sequence of vectors that each have `nunits` elements:

    /* Calculate which vector contains the result, and which lane of
       that vector we need.  */
    if (!can_div_trunc_p (pos, nunits, &vec_entry, &vec_index))
      {
        if (dump_enabled_p ())
          dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                           "Cannot determine which vector holds the"
                           " final result.\n");
        return false;
      }

However, there are some contexts in which operating on a `poly_int` is not possible or does not make sense. One example is when handling static initializers, since no current target supports the concept of a variable-length static initializer. In these situations, a reasonable fallback is:

    if (poly_value.is_constant (&const_value))
      {
        …
        /* Operate on const_value.  */
        …
      }
    else
      {
        …
        /* Conservatively correct fallback.  */
        …
      }

`poly_int` also provides some asserting functions like `to_constant`. Please only use these functions if there is a good theoretical reason to believe that the assertion cannot fire. For example, if some work is divided into an analysis phase and an implementation phase, the analysis phase might reject inputs that are not `is_constant`, in which case the implementation phase can reasonably use `to_constant` on the remaining inputs. The assertions should not be used to discover whether a condition ever occurs “in the field”; in other words, they should not be used to restrict code to constants at first, with the intention of only implementing a `poly_int` version if a user hits the assertion.

If a particular asserting function like `to_constant` is needed more than once for the same reason, it is probably worth adding a helper function or macro for that situation, so that the justification only needs to be given once. For example:

    /* Return the size of an element in a vector of size SIZE, given that
       the vector has NELTS elements.  The return value is in the same units
       as SIZE (either bits or bytes).

       to_constant () is safe in this situation because vector elements are
       always constant-sized scalars.  */
    #define vector_element_size(SIZE, NELTS) \
      (exact_div (SIZE, NELTS).to_constant ())

Target-specific code in `config/cpu` only needs to handle non-constant `poly_int`s if `NUM_POLY_INT_COEFFS` is greater than one. For other targets, `poly_int` degenerates to a compile-time constant and is often interchangable with a normal scalar integer. There are two main exceptions:

- Sometimes an explicit cast to an integer type might be needed, such as to resolve ambiguities in a `?:` expression, or when passing values through `...` to things like print functions.
- Target macros are included in target-independent code and so do not have access to the implicit conversion to a scalar integer. If this becomes a problem for a particular target macro, the possible solutions, in order of preference, are:
  - Convert the target macro to a target hook (for all targets).
  - Put the target’s implementation of the target macro in its `cpu.c` file and call it from the target macro in the `cpu.h` file.
  - Add `to_constant ()` calls where necessary. The previous option is preferable because it will help with any future conversion of the macro to a hook.

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