Thread: Decouple C++ support in Meson's PGXS from LLVM enablement
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)
Attachment
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?
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