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  1. Decouple C++ support in Meson's PGXS from LLVM enablement

  2. ci: Configure g++ with 32-bit for 32-bit build

  3. meson: Rename cpp variable to cxx

  4. Refactor to avoid code duplication in transformPLAssignStmt.

  1. Decouple C++ support in Meson's PGXS from LLVM enablement

    Tristan Partin <tristan@partin.io> — 2025-04-17T01:57:26Z

    Howdy folks,

While playing around with pg_duckdb[0], a Postgres extension written in 
C++ which uses PGXS, I came across a strange build error:

std=c++17  -Wno-sign-compare...
/bin/sh: line 1: -Wno-sign-compare: command not found

I was very confused by the error, but reading the command line, it made 
sense. After talking to Jelte off-list, he told me to try a Postgres 
installation that had been built with autotools. Today, I finally had 
a chance to try that tip, and building pg_duckdb succeeded.

I spent some time exploring the Meson build a bit, and I realized that 
C++ support in PGXS is tied to LLVM enablement. Checking the autotools 
build in the configure.ac script indicates that that is not the case for 
it.

On master, C++ support looks like:

llvmopt = get_option('llvm')
llvm = not_found_dep
if add_languages('cpp', required: llvmopt, native: false)
  llvm = dependency('llvm', version: '>=14', method: 'config-tool', required: llvmopt)
    if llvm.found()

      cdata.set('USE_LLVM', 1)

      cpp = meson.get_compiler('cpp')

By default, the `llvm` option is disabled, which Meson takes to mean, 
"do not check for C++ support". Thusly, add_languages() returns false. 
In addition, every check for adding to cxxflags, et. al. is gated on 
llvm.found(), which is always false for the `not_found_dep`. All this 
considered, the Makefile.global of a Postgres build roughly looked like:

CXX =
CXXFLAGS = 
...

This then accounts for the original pg_duckdb command line looking the 
way that it did.

Attached is a patch which decouples C++ support in PGXS from LLVM for 
a Meson-compiled Postgres.

[0]: https://github.com/duckdb/pg_duckdb

-- 
Tristan Partin
Neon (https://neon.tech)

  2. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    Jelte Fennema-Nio <me@jeltef.nl> — 2025-04-17T07:15:41Z

    On Thu, 17 Apr 2025 at 03:57, Tristan Partin <tristan@partin.io> wrote:
> I spent some time exploring the Meson build a bit, and I realized that
> C++ support in PGXS is tied to LLVM enablement. Checking the autotools
> build in the configure.ac script indicates that that is not the case for
> it.

Thank you for looking into this. I didn't try the patch, but just
reading it it all looks sensible (although my meson knowledge is quite
limited). I assume you tried to build pg_duckdb with a Meson based PG
build containing this patch and pg_duckdb built correctly?

  3. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    Tristan Partin <tristan@partin.io> — 2025-04-17T13:18:40Z

    On Thu Apr 17, 2025 at 2:15 AM CDT, Jelte Fennema-Nio wrote:
> On Thu, 17 Apr 2025 at 03:57, Tristan Partin <tristan@partin.io> wrote:
>> I spent some time exploring the Meson build a bit, and I realized that
>> C++ support in PGXS is tied to LLVM enablement. Checking the autotools
>> build in the configure.ac script indicates that that is not the case for
>> it.
>
> Thank you for looking into this. I didn't try the patch, but just
> reading it it all looks sensible (although my meson knowledge is quite
> limited). I assume you tried to build pg_duckdb with a Meson based PG
> build containing this patch and pg_duckdb built correctly?

I took some time to backpatch to 17 on my machine so that I could build 
pg_duckdb, and indeed with the backpatch applied, pg_duckdb compiles 
successfully.

I do however hit linker errors later, but that is unrelated to the 
current patch discussion. I'll discuss them with you in a different 
forum.

-- 
Tristan Partin
https://tristan.partin.io

  4. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    Tristan Partin <tristan@partin.io> — 2025-09-28T21:16:52Z

    On Wed Apr 16, 2025 at 8:57 PM CDT, Tristan Partin wrote:
> Howdy folks,
>
> While playing around with pg_duckdb[0], a Postgres extension written in 
> C++ which uses PGXS, I came across a strange build error:
>
> std=c++17  -Wno-sign-compare...
> /bin/sh: line 1: -Wno-sign-compare: command not found
>
> I was very confused by the error, but reading the command line, it made 
> sense. After talking to Jelte off-list, he told me to try a Postgres 
> installation that had been built with autotools. Today, I finally had 
> a chance to try that tip, and building pg_duckdb succeeded.
>
> I spent some time exploring the Meson build a bit, and I realized that 
> C++ support in PGXS is tied to LLVM enablement. Checking the autotools 
> build in the configure.ac script indicates that that is not the case for 
> it.
>
> On master, C++ support looks like:
>
> llvmopt = get_option('llvm')
> llvm = not_found_dep
> if add_languages('cpp', required: llvmopt, native: false)
>   llvm = dependency('llvm', version: '>=14', method: 'config-tool', required: llvmopt)
>     if llvm.found()
>
>       cdata.set('USE_LLVM', 1)
>
>       cpp = meson.get_compiler('cpp')
>
> By default, the `llvm` option is disabled, which Meson takes to mean, 
> "do not check for C++ support". Thusly, add_languages() returns false. 
> In addition, every check for adding to cxxflags, et. al. is gated on 
> llvm.found(), which is always false for the `not_found_dep`. All this 
> considered, the Makefile.global of a Postgres build roughly looked like:
>
> CXX =
> CXXFLAGS = 
> ...
>
> This then accounts for the original pg_duckdb command line looking the 
> way that it did.
>
> Attached is a patch which decouples C++ support in PGXS from LLVM for 
> a Meson-compiled Postgres.
>
> [0]: https://github.com/duckdb/pg_duckdb

With PGConf NYC around the corner, I thought I would rebase the original 
patch. Please find v2 attached, which applies on top of 
b0fb2c6aa5a485e28210e13ae5536c1231b1261f[0] :D.

GitHub branch: https://github.com/tristan957/postgres/tree/meson-cpp

[0]: https://github.com/tristan957/postgres/commit/b0fb2c6aa5a485e28210e13ae5536c1231b1261f

-- 
Tristan Partin
https://tristan.partin.io

  5. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    Josef Šimánek <josef.simanek@gmail.com> — 2025-11-02T17:05:09Z

    ne 2. 11. 2025 v 17:21 odesílatel Tristan Partin <tristan@partin.io> napsal:
>
> Howdy folks,
>
> While playing around with pg_duckdb[0], a Postgres extension written in
> C++ which uses PGXS, I came across a strange build error:
>
> std=c++17  -Wno-sign-compare...
> /bin/sh: line 1: -Wno-sign-compare: command not found
>
> I was very confused by the error, but reading the command line, it made
> sense. After talking to Jelte off-list, he told me to try a Postgres
> installation that had been built with autotools. Today, I finally had
> a chance to try that tip, and building pg_duckdb succeeded.
>
> I spent some time exploring the Meson build a bit, and I realized that
> C++ support in PGXS is tied to LLVM enablement. Checking the autotools
> build in the configure.ac script indicates that that is not the case for
> it.
>
> On master, C++ support looks like:
>
> llvmopt = get_option('llvm')
> llvm = not_found_dep
> if add_languages('cpp', required: llvmopt, native: false)
>   llvm = dependency('llvm', version: '>=14', method: 'config-tool', required: llvmopt)
>     if llvm.found()
>
>       cdata.set('USE_LLVM', 1)
>
>       cpp = meson.get_compiler('cpp')
>
> By default, the `llvm` option is disabled, which Meson takes to mean,
> "do not check for C++ support". Thusly, add_languages() returns false.
> In addition, every check for adding to cxxflags, et. al. is gated on
> llvm.found(), which is always false for the `not_found_dep`. All this
> considered, the Makefile.global of a Postgres build roughly looked like:
>
> CXX =
> CXXFLAGS =
> ...

Did local build and review. I can confirm it detects CXX properly with
llvm disabled. Tested with

meson setup "$BUILD_DIR" \
    --prefix="$INSTALL_DIR" \
    --buildtype=debug \
    -Dcassert=true \
    -Dllvm=disabled

pre-patch (resulting into empty CXX) and post-patch (properly assigned
"ccache c++" to CXX).

> This then accounts for the original pg_duckdb command line looking the
> way that it did.
>
> Attached is a patch which decouples C++ support in PGXS from LLVM for
> a Meson-compiled Postgres.
>
> [0]: https://github.com/duckdb/pg_duckdb
>
> --
> Tristan Partin
> Neon (https://neon.tech)

  6. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    BharatDB <bharatdbpg@gmail.com> — 2025-11-05T07:04:22Z

    Hi Tristan, Andres, Jelte, and pgsql-hackers,

Thank you for the excellent work on decoupling C++ support in Meson from
LLVM

SUMMARY: v2 PATCH TESTED AND WORKS PERFECTLY

- Built PostgreSQL 19devel (current master) with Meson

- Config: -Dllvm=disabled -Dcpp_support=auto

- Before patch: CXX = (empty) → extension fails with shell error

- After patch: CXX = ccache c++ → extension builds successfully

- Tested with minimal C++ extension → cpp_test.so created

 REPRODUCTION & FIX STEPS

1. git remote add tristan https://github.com/tristan957/postgres.git

   git fetch tristan

   git checkout -b meson-cpp tristan/meson-cpp

2. meson setup build-fixed .. -Dllvm=disabled -Dcpp_support=auto

   meson compile && meson install

3. Built test C++ extension:

    cd ~/cpp_test_ext && make clean && make


VERIFIED OUTPUT
$ grep -E "^(CXX|CXXFLAGS) =" build-fixed/src/Makefile.global

CXX = ccache c++

CXXFLAGS = -fno-strict-aliasing -fwrapv -Wall -g -O0 ...

$ ls -l ~/cpp_test_ext/*.so

  -rwxr-xr-x 1 boss boss 21568 Nov  4 12:18
/home/boss/cpp_test_ext/cpp_test.so

Patch applies cleanly and works perfectly on 19devel.

While testing the C++ PGXS patch, I noticed a usability gap:

> If no C++ compiler is installed (e.g., `g++` missing), Meson silently

> proceeds with C++ disabled — users only discover the issue when extensions

> like pg_duckdb fail with confusing build errors.

This patch adds a clear, single warning during configuration:

WARNING: No C++ compiler found on your system.

Extensions using C++ (e.g. pg_duckdb) will FAIL to build.

Install g++ or clang++ to enable C++ support.


- Only shown when `add_languages('cpp')` returns `false`

- No warning when C++ is available

- Tested on PostgreSQL 19devel (with and without `g++`)

Attached: `0001-meson-warn-when-no-C-compiler-is-found.patch`

Happy to revise if there are better ways to surface this — open to
suggestions!

Thanks again for the great work on Meson C++ support.

Best regards,

Lakshmi

  7. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    Peter Eisentraut <peter@eisentraut.org> — 2025-11-07T15:39:42Z

    On 28.09.25 23:16, Tristan Partin wrote:
>> Attached is a patch which decouples C++ support in PGXS from LLVM for
>> a Meson-compiled Postgres.
>>
>> [0]: https://github.com/duckdb/pg_duckdb
> 
> With PGConf NYC around the corner, I thought I would rebase the original
> patch. Please find v2 attached, which applies on top of
> b0fb2c6aa5a485e28210e13ae5536c1231b1261f[0] :D.

This patch makes sense to me.  But I wonder what the behavior should be 
if no C++ compiler is found.  Because then you're back to the state 
you're started from, with the confusing make error.  If you imagine a 
packager building postgresql in a minimal environment, and without llvm 
for some reason, then they would ship PGXS in that state without noticing.

The AC_PROG_CXX macro sets CXX to g++ if no C++ compiler is found.  Then 
a PGXS user would at least get an error message about g++ not found and 
they can fix it by installing it.  Maybe we should do that under Meson too.

An alternative would be to add some kind of build option or feature to 
enable C++ support explicitly.

  8. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    Jelte Fennema-Nio <me@jeltef.nl> — 2025-11-07T16:19:56Z

    On Fri, 7 Nov 2025 at 16:39, Peter Eisentraut <peter@eisentraut.org> wrote:
> The AC_PROG_CXX macro sets CXX to g++ if no C++ compiler is found.  Then
> a PGXS user would at least get an error message about g++ not found and
> they can fix it by installing it.  Maybe we should do that under Meson too.

I think that sounds reasonable.

(although I think using c++ instead of g++ would make more sense
though, but that seems like a separate thread as it would also apply
to that autoconf code)

  9. Re: Decouple C++ support in Meson's PGXS from LLVM enablement

    Tristan Partin <tristan@partin.io> — 2025-11-07T16:29:50Z

    On Fri Nov 7, 2025 at 9:39 AM CST, Peter Eisentraut wrote:
> On 28.09.25 23:16, Tristan Partin wrote:
>>> Attached is a patch which decouples C++ support in PGXS from LLVM for
>>> a Meson-compiled Postgres.
>>>
>>> [0]: https://github.com/duckdb/pg_duckdb
>> 
>> With PGConf NYC around the corner, I thought I would rebase the original
>> patch. Please find v2 attached, which applies on top of
>> b0fb2c6aa5a485e28210e13ae5536c1231b1261f[0] :D.
>
> This patch makes sense to me.  But I wonder what the behavior should be 
> if no C++ compiler is found.  Because then you're back to the state 
> you're started from, with the confusing make error.  If you imagine a 
> packager building postgresql in a minimal environment, and without llvm 
> for some reason, then they would ship PGXS in that state without noticing.
>
> The AC_PROG_CXX macro sets CXX to g++ if no C++ compiler is found.  Then 
> a PGXS user would at least get an error message about g++ not found and 
> they can fix it by installing it.  Maybe we should do that under Meson too.
>
> An alternative would be to add some kind of build option or feature to 
> enable C++ support explicitly.

Great idea. I need to re-spin the patch anyway after a discussion with 
Jelte at NYC. I'll be sure to add this as well.

-- 
Tristan Partin
Databricks (https://databricks.com)