Laszlo Ersek 63417f6a57 utils/small_vector: refactor expansion condition in reserve*() ...

Rewrite

  _begin + n > _capacity_end

as

  n > _capacity_end - _begin

and then as

  n > capacity()

for two reasons:

- The last form is easier to read than the first form.

- Per N4950 (the final C++23 working draft), [expr.add] paragraph 4, the
  expression

    _begin + n                            (i.e., P + J)

  is defined only if

    0 ≤ 0 + n ≤ _capacity_end - _begin    (i.e., 0 ≤ i + j ≤ n)

  equivalently, only if

    _begin ≤ _begin + n ≤ _capacity_end

  Therefore, the expression

    _begin + n

  invokes undefined behavior exactly when we'd expect our check

    _begin + n > _capacity_end

  to evaluate to true.

gcc and clang have been aggressively equating undefined behavior to "never
happens"; let's prevent that here.

Signed-off-by: Laszlo Ersek <laszlo.ersek@scylladb.com>

Closes scylladb/scylladb#21213

2024-10-22 17:12:11 +03:00

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