RFR: 8317132: Prepare HotSpot for permissive- [v9]

[Kim Barrett]

On Tue, 3 Oct 2023 07:45:42 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Is Thomas around? Would like to get his opinion of what to do with this particular snippet
>
> I know he was away until October but not exactly when in October.

Ick! I hadn't followed https://github.com/openjdk/jdk/pull/15096 closely, so
hadn't noticed the discussion there.  Not sure why this is using a literal 0
rather than `(T) '\n'` as in that PR?

So the literal "0" (or previously, NUL char constant) is either an integral
zero or a null pointer constant (because one of the types used for T is
HMODULE). imprint_sentinal uses `(T)'X'`. (Double ick! The value is either an
integral constant or a HMODULE with a weird value.)

An approach to being type safe/consistent would be to provide a traits utility
for specifying the terminator and the sentinal on a type-specific basis. Don't
know if that's worth doing in this PR. We've been living (dangerously?) with
the existing mechanism for a long time.

Something like

template<typename T>
struct SimpleBufferSpecialValues;

template<>
struct SimpleBufferSpecialValues<char> {
  char terminator() const { return '\0'; }
  char sentinal() const { return '\X'; }
};

template<>
struct SimpleBufferSpecialValues<HMODULE> {
  HMODULE terminator() const { return nullptr; }
  HMODULE sentinal() const {
    // Untested, might be completely wrong.
    alignas(HMODULE) static const char sentinal_data[1];
    return reinterpret_cast<HMODULE>(&sentinal_data);
  }
};

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PR Review Comment: https://git.openjdk.org/jdk/pull/15955#discussion_r1377160121