Hi all,

Since I use the last version of Fedora 26 with gcc-7.1.1, I have these warnings (see output.txt file)

We could add -Wimplicit-fallthrough=0 to Makefile ? it will disable the fallthrough of the coed.

What do you think about that ? What's your feedback on this option and can we use it in the case of CPython ?

Thank you

It seems to me that by default the compiler recognizes a wide variety of "falls through" comments. Thus we need just add missed comments.

There are warnings emitted when compile imported third-party code. We should ask Stefan for adding "falls through" comments in his libmpdec. And either compile expat with -Wimplicit-fallthrough=0, or wait until warnings will be fixed in upstream, or remove the bundled copy of expat.

It's a useful warning, but I find it annoying to add 20 "fall through" comments. I may add a pragma at some point.

New changeset 72b543308ee3087e3fa247981f5cb4be1138c515 by Stefan Krah in branch 'master':
bpo-30923: Suppress fall-through warnings in libmpdec. (#2698)
https://github.com/python/cpython/commit/72b543308ee3087e3fa247981f5cb4be1138c515

Hmm, that took about 20 min to commit a 3 line diff.  Now I'm watching the buildbots...

Stéphane, if you want the libmpdec change cherry picked and are willing to do the (significant) work of backporting, I'll merge it.

@stefan

In fact I could create the PR for the backports, but I have again the same issues for the rest of CPython.

I have checked your code, and in my case 'gcc' is in the cc string, 'gcc -pthread' and you have fixed for libmpdec.

But for the other modules ?

Hum, I don't think that it's worth it to backport changes which only fix warnings. Usually, we first focus on fixing all warnings on master.

yep, currently, 3.6 and 3.5 are in 'bug fix' mode, and in this case, it's not a bug.

Well, it's not a bug, but perhaps it is annoying for users of this gcc version if they compile 3.6 often.


Actually, I think gcc should not include this warning in -Wextra.  It's something that could be run manually before a release.


I'd vote for making -Wno-implicit-fallthrough global.

Stefan, ask on python-dev ml, or we have to ask to Ned Deily for this version ?

I'm ok to kill warnings, but I would suggest to first fix most GCC7 warnings because starting to discuss backports. I expect that it will require multiple changes.

I am not for a backport, it's not a security fix or a bug fix.

for my case, I just want to "kill" the warnings, maybe we could check the version of gcc and add "-Wimplicit-fallthrough=0".

Here is a good explanation of the 'fallthrough' in gcc7 
https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/

Yeah the warnings are quit annoying when compiling the master and 3.6 branch.

Fedora 26 currently includes gcc 7 which emits those warnings, other distros will follow up when they update gcc, so I'd think it would be better to fix it.

We can check for the version, but all versions of gcc that I tested
accept and ignore -Wno-implicit-fallthrough, even though they don't 
actually have -Wimplicit-fallthrough.

Of course they choke on -Wimplicit-fallthrough=0.

I think the fall-through blog notes are slightly overstated. :-)

"The switch fallthrough has been widely considered a design defect in C."


It's an important feature.

The following commit broke _decimal compilation on the "x86 Tiger 3.x" buildbot:

commit 72b543308ee3087e3fa247981f5cb4be1138c515
Author: Stefan Krah <skrah@bytereef.org>
Date:   Thu Jul 13 20:54:20 2017 +0200

    bpo-30923: Suppress fall-through warnings in libmpdec. (#2698)

http://buildbot.python.org/all/builders/x86%20Tiger%203.x/builds/1068/steps/compile/logs/stdio

building '_decimal' extension
gcc -fno-strict-aliasing -Wsign-compare -g -O0 -Wall -Wstrict-prototypes -std=c99 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -DUNIVERSAL=1 -I/Users/db3l/buildarea/3.x.bolen-tiger/build/Modules/_decimal/libmpdec -I./Include -I. -I/usr/local/include -I/Users/db3l/buildarea/3.x.bolen-tiger/build/Include -I/Users/db3l/buildarea/3.x.bolen-tiger/build -c /Users/db3l/buildarea/3.x.bolen-tiger/build/Modules/_decimal/_decimal.c -o build/temp.macosx-10.4-i386-3.7-pydebug/Users/db3l/buildarea/3.x.bolen-tiger/build/Modules/_decimal/_decimal.o -Wno-implicit-fallthrough
cc1: error: unrecognized command line option "-Wno-implicit-fallthrough"

Thanks. I tried to revert it, but got:

remote: Resolving deltas: 100% (2/2), completed with 2 local objects.
remote: error: GH006: Protected branch update failed for refs/heads/master.
remote: error: 2 of 2 required status checks are expected.
To https://github.com/python/cpython.git
 ! [remote rejected]       master -> master (protected branch hook declined)
error: failed to push some refs to 'https://github.com/python/cpython.git'


So I'm supposed to spend 20 min creating an irrelevant PR to revert a 3 line diff?!

I would never have finished _decimal under these conditions.

So I installed gcc-7.2.0 from source. Hilariously compiling gcc *itself*
emits fallthrough warnings!

Then I tried to be a good open source drone and add 20 /* fall through */
comments to libmpdec.

gcc is too stupid to recognize the /* fall through */ at the #endif
position.


Perhaps the author of this gcc warning wants to submit a patch.

New changeset d73a960c575207539c3f9765cff26d4fff400b45 by Stefan Krah in branch 'master':
bpo-30923: Disable warning that has been part of -Wextra since gcc-7.0. (#3142)
https://github.com/python/cpython/commit/d73a960c575207539c3f9765cff26d4fff400b45

PR 3157 addresses everything apart from expat and

  https://github.com/python/cpython/blob/master/Modules/cjkcodecs/_codecs_iso2022.c#L816


I'm not sure about that one. It looks harmless but a bit odd.

New changeset f432a3234f9f2ee09bd40be03e06bf72865ee375 by Stefan Krah in branch 'master':
bpo-30923: Silence fall-through warnings included in -Wextra since gcc-7.0. (#3157)
https://github.com/python/cpython/commit/f432a3234f9f2ee09bd40be03e06bf72865ee375

Cherry picking has too many conflicts, I'm not backporting this myself.

New changeset 9e1e6f528f3fec16b9bd99f5ee38048ffec04a81 by Stefan Krah in branch 'master':
bpo-30923: Silence fall-through warnings in libexpat build. (#3205)
https://github.com/python/cpython/commit/9e1e6f528f3fec16b9bd99f5ee38048ffec04a81

All warnings except for #31275 are dealt with.

New changeset c0e77364ca29df6cfb311e79892955c92bd8e595 by Victor Stinner in branch '3.6':
[3.6] bpo-30923: Silence fall-through warnings included in -Wextra since gcc-7.0 (#3518)
https://github.com/python/cpython/commit/c0e77364ca29df6cfb311e79892955c92bd8e595

New changeset dedcbee04cd52790027ecfb46cb3aa33efebdc84 by Victor Stinner in branch '3.6':
[3.6] bpo-30923, bpo-31279: Fix GCC warnings (#4620)
https://github.com/python/cpython/commit/dedcbee04cd52790027ecfb46cb3aa33efebdc84