Insights on error when moving from c99 to c17 in scikit-learn meson.build?

[lesteve]

I seem to be able to do emcc -std=c17 test.c with emscripten 3.1.45 so not sure why meson is complaining about this:

../meson.build:1:0: ERROR: None of values ['c17'] are supported by the C compiler. Possible values are ['none', 'c89', 'c99', 'c11', 'gnu89', 'gnu99', 'gnu11']

Could it be an issue in pywasmcross.py somehow?

See scikit-learn/scikit-learn#29013 for more details about how it manifests itself in scikit-learn.

[rgommers]

This is almost certainly a problem in Meson itself. The supported C standard versions for Emscripten are inherited from Clang, but the Emscripten version numbers are much lower than the Clang ones. Clang 6.0 is the minimum version for C17 support (https://github.com/mesonbuild/meson/blob/cfd57180eef9036c7167c5682b9f3055a540fccc/mesonbuild/compilers/c.py#L112), but Emscripten is only at version 3.x so it'll fail that check if it's not special-cased.

I can open a PR for that tonight probably. Just need to figure out what the correct minimum version is for declaring C17 (and C18/C20) support.

[rgommers]

@lesteve I'm traveling and have no install at hand. It looks like there is no public list of Emscripten <--> Clang versions, but it can be obtained from emcc -v and ./emsdk list --old (says https://emscripten.org/docs/tools_reference/emsdk.html#how-do-i-install-and-activate-old-emscripten-sdks-and-tools). If that works, would you be able to check the versions of Emscripten that correspond to Clang 6.0.0, 8.0.0, 9.0.0, 18.0.0?

This should fix the problem I believe (again, I can't check right now): https://github.com/rgommers/meson/tree/emscripten-c17

[lesteve]

• clang >= 18.0.0 corresponds to emscripten >= 3.0.0
• for the other clang >= 6.0.0, clang >= 8.0.0 and clang >= 9.0.0, it's not clear or they correspond to older versions that what is available through emsdk? Maybe @hoodmane or @ryanking13, you know a better way to have the relationship between Emscripten and Clang versions than downloading a particular version through emsdk and doing emcc -v?

Those were the particular clang versions that you care about If I understand your commit mesonbuild/meson@master...rgommers:meson:emscripten-c17

_C17_VERSION = '>=3.0.0'  # '>=6.0.0'
_C18_VERSION = '>=3.0.0'  # '>=8.0.0'
_C2X_VERSION = '>=3.0.0'  # '>=9.0.0'
_C23_VERSION = '>=3.0.0'  # '>=18.0.0'

Here is a table for a few versions (1.38.33 is the older version available through emsdk):

| emsdk version |         clang version |
|---------------+-----------------------|
|    1.38.33-34 | error getting version |
|       1.38.35 |                 9.0.0 |
|        1.39.0 |                10.0.0 |
|         2.0.0 |                12.0.0 |
|        2.0.34 |                14.0.0 |
|         3.0.0 |                18.0.0 |

I got them through a hacky script like this:

set -e

func() {
    version=$1
    echo installing $version
    ./emsdk install $version > /dev/null 2>&1
    echo done installing
    ./emsdk activate $version > /dev/null 2>&1
    source emsdk_env.sh > /dev/null 2>&1
    echo emcc version $version
    echo clang version $(emcc -v 2>&1 | grep 'clang version' || echo error getting version)
}

# for version in 1.38.40 1.39.0 2.0.0 3.0.0; do
# for version in 1.38.34 1.38.35 1.38.36 1.38.37 1.38.38 1.38.39; do
#     func $version
# done

func $1

[rgommers]

Great, thanks @lesteve. I opened mesonbuild/meson#13221 and added you as a co-author.