Re: Warning -Wclobbered in PG_TRY(...)

On Thu, 29 May 2025 at 14:14, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> -Wclobbered would be really useful if it worked --- but sadly,
> on practically all gcc and clang versions, those warnings have
> nearly nothing to do with reality. We typically ignore them.

Yes, in the vast majority of cases. But I believe this is not one of them.
Actually, we do
not adhere to the C language standard for a setjump/logjump. Notice how the
non-volatile local variable "bool dorethrow##" utilized in the
setjump/logjump block.
This topic reminds me of [0].

In fact, we do not receive a warning for a C-lang build because the
compiler does not
appear to be "smart" enough to put this variable in a register. I ran a
synthetic test
equivalent to PG_TRY using godbolt.org, and the final code was the same
whether
the volatile qualifier was used or not. This is not the case with the C++
build. If the
volatile qualifier is not used, the compiler optimizes this line and puts
it in a register,
and we get a warning. And to get rid of this warning, you need to either
turn it off via
the pragma directive, write your own version of PG_TRY with a volatile type
qualifier,
or patch Postgres. None of these solutions sound good to me. You may be
wondering
who will write Postgres extensions in C++. Having two different runtimes is
absolute
misery, and I completely agree. However, such extensions already exist; for
example,
pg_duckdb. AFAICS, it is quite popular in our area.

So, to be safe, we should follow the standard and include a volatile
qualifier here. It
really costs us nothing; the resulting code will be identical for C, and
we'll avoid any
future issues. In the case of C++, it will also resolve the warning.

Don't consider me as something of a formalist or someone who wants to make
mountains out of molehills. I just want to make things better. PFA trivial
patch.

[0]
https://www.postgresql.org/message-id/flat/2eda015b-7dff-47fd-d5e2-f1a9899b90a6%40postgrespro.ru

--
Best regards,
Maxim Orlov.