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4302. Problematic vector_sum_of_squares wording

Section: 29.9.13.8 [linalg.algs.blas1.ssq] Status: LEWG Submitter: Mark Hoemmen Opened: 2025-07-23 Last modified: 2025-10-27

Priority: 1

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Discussion:

The current wording for vector_sum_of_squares 29.9.13.8 [linalg.algs.blas1.ssq] has three problems with its specification of the value of result.scaling_factor.

  1. The function permits InVec::value_type and Scalar to be any linear algebra value types. However, computing result.scaling_factor that satisfies both (3.1) and (3.2) requires more operations, such as division. Even if those operations are defined, they might not make result.scaling_factor satisfy the required properties. For example, integers have division, but integer division won't help here.

  2. LAPACK's xLASSQ (the algorithm to which Note 1 in 29.9.13.8 [linalg.algs.blas1.ssq] refers) changed its algorithm recently (see Reference-LAPACK/lapack/pull/#494) so that the scaling factor is no longer necessarily the maximum of the input scaling factor and the absolute value of all the input elements. It's a better algorithm and we would like to be able to use it.

  3. Both members of sum_of_squares_result<Scalar> have the same type, Scalar. If the input mdspan's value_type represents a quantity with units, this would not be correct. For example, if value_type has units of distance (say [m]), the sum of squares should have units of area ([m2]), while the scaling factor should have units of distance ([m]).

Problem (1) means that the current wording is broken. I suggest two different ways to fix this.

  1. Remove vector_sum_of_squares entirely (both overloads from 29.9.2 [linalg.syn], and the entire 29.9.13.8 [linalg.algs.blas1.ssq]). That way, we won't be baking an old, less good algorithm into the Standard. Remove Note 3 from 29.9.13.9 [linalg.algs.blas1.nrm2], which is the only other reference to vector_sum_of_squares in the Standard.

  2. Fix 29.9.13.8 [linalg.algs.blas1.ssq] by adding to the Mandates element (para 2) that InVec::value_type and Scalar are both floating-point types (so that we could fix this later if we want), and remove 29.9.13.8 [linalg.algs.blas1.ssq] 3.1. Optionally add Recommended Practice, though Note 1 already suggests the intent.

I prefer just removing vector_sum_of_squares. Implementers who care about QoI of vector_two_norm should already know what to do. If somebody cares sufficiently, they can propose it back for C++29 and think about how to make it work for generic number types.

[2025-10-17; Reflector poll. Status changed: New → LEWG.]

Set priority to 1 after reflector poll. Send to LEWG.

This is the subject of NB comment 169-276. LWG took a poll in the 2025-10-10 telecon and recommends that LEWG confirms this resolution.

[2025-10-24; Resolves US 169-276]

Proposed resolution:

This wording is relative to N5014.

[Drafting note: The wording below implements option 1 of the issue discussion]

  1. Modify 29.9.2 [linalg.syn], header <linalg> synopsis, as indicated:

    namespace std::linalg {
  […]
  // 29.9.13.8 [linalg.algs.blas1.ssq], scaled sum of squares of a vector's elements
  template<class Scalar>
    struct sum_of_squares_result {
      Scalar scaling_factor;
    };
  template<in-vector InVec, class Scalar>
    sum_of_squares_result<Scalar>
      vector_sum_of_squares(InVec v, sum_of_squares_result<Scalar> init);
  template<class ExecutionPolicy, in-vector InVec, class Scalar>
    sum_of_squares_result<Scalar>
      vector_sum_of_squares(ExecutionPolicy&& exec,
                            InVec v, sum_of_squares_result<Scalar> init); 
  […]
}

  2. Delete the entire 29.9.13.8 [linalg.algs.blas1.ssq] as indicated:

    29.9.13.8 Scaled sum of squares of a vector's elements [linalg.algs.blas1.ssq] 

    template<in-vector InVec, class Scalar>
  sum_of_squares_result<Scalar> vector_sum_of_squares(InVec v, sum_of_squares_result<Scalar> init);
template<class ExecutionPolicy, in-vector InVec, class Scalar>
  sum_of_squares_result<Scalar> vector_sum_of_squares(ExecutionPolicy&& exec,
                                                            InVec v, sum_of_squares_result<Scalar> init);

    -1- [Note 1: These functions correspond to the LAPACK function xLASSQ[20]. — end note]

    -2- Mandates: decltype(abs-if-needed(declval<typename InVec::value_type>())) is convertible to Scalar.

    -3- Effects: Returns a value result such that

    1. (3.1) — result.scaling_factor is the maximum of init.scaling_factor and abs-if-needed(x[i]) for all i in the domain of v; and

    2. (3.2) — let s2init be

      init.scaling_factor * init.scaling_factor * init.scaled_sum_of_squares

      then result.scaling_factor * result.scaling_factor * result.scaled_sum_of_squares equals the sum of s2init and the squares of abs-if-needed(x[i]) for all i in the domain of v.

    -4- Remarks: If InVec::value_type, and Scalar are all floating-point types or specializations of complex, and if Scalar has higher precision than InVec::value_type, then intermediate terms in the sum use Scalar's precision or greater.

  3. Modify 29.9.13.9 [linalg.algs.blas1.nrm2] as indicated:

    template<in-vector InVec, class Scalar>
  Scalar vector_two_norm(InVec v, Scalar init);
template<class ExecutionPolicy, in-vector InVec, class Scalar>
  Scalar vector_two_norm(ExecutionPolicy&& exec, InVec v, Scalar init);

    -1- [Note 1: […] ]

    -2- Mandates: […]

    -3- Returns: […]

    [Note 2: […] ]

    -4- Remarks: […]

    [Note 3: An implementation of this function for floating-point types T can use the scaled_sum_of_squares result from vector_sum_of_squares(x, {.scaling_factor=1.0, .scaled_sum_of_squares=init}). — end note]