RFR: 8345159: RISCV: Fix -Wzero-as-null-pointer-constant warning in emit_static_call_stub

[Kim Barrett]

On Thu, 28 Nov 2024 15:37:01 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

> Change is okay and I sanity tested, all ok.

Thanks.

> But I must object to keep the underlying type of address. For all our purposes it is an integer value. I.e. `address dest_end = dest->_total_start + dest->_total_size;` Furthermore address 0 is a valid _address_, changing that to nullptr make little sense.

0 is only barely a valid address, and only if very careful.

All supported platforms use 0 to represent all null pointers (including null
pointer constants). The standards don't guarantee that (neither C nor C++.)
(There exist or have existed platforms that used a different representation,
but we don't support any of those.) There are certainly places in our code
that assume that representation. Trying to avoid that is effectively
impossible. It tends to uglify code. And we can't expect testing to uncover
any of those places that exist now, or might accidentally be introduced in the
future.

And tools like ubsan and asan won't help, since on the platforms we support,
those tools make the same assumption about the representation of null pointers
being 0, and indeed complain about arithmetic involving pointers with a value
of 0.

> 
> So I believe the correct fix is to change the type of address.

I'm not sure what you mean by this.

Do you mean something like "change the type of the argument involved here"?

Or do you mean something like "change the alias type `address` to be an
integral type (say, `intptr_t`) instead of `char*`"?

Neither of those seem like a good idea to me. But maybe you mean something
else that I haven't thought of?

-------------

PR Comment: https://git.openjdk.org/jdk/pull/22435#issuecomment-2507214458