GCC mixing C and C++ with -flto

[lilvinz]

Hi Giovanni,

it seems that there is an issue with GCC when using lto and accessing the same union from C as well as C++ code.
The background is that i am running unit tests using google test (C++) against some custom hal drivers and a custom posix port. This means that the public ch structure is being used from compile units in C as well as C++. Now if -flto is enabled, this leads to the following Error:

Code: Select all

gcc (Ubuntu 5.4.0-6ubuntu1~16.04.2) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO

os/rt/include/chschd.h:535:20: error: type of ‘ch’ does not match original declaration [-Werror]
 extern ch_system_t ch;
                    ^
os/rt/src/chschd.c:42:13: note: previously declared here
 ch_system_t ch;
             ^
lto1: all warnings being treated as errors
lto-wrapper: fatal error: g++ returned 1 exit status
compilation terminated.
/usr/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status


The issue seems to be that the anonymous union in struct ch_thread is being translated differently for C++ than for C language. If i change that union to be a struct (took quite some time to get to that point), the issue is "fixed". I am not sure if this is a gcc bug.
I assume that this issue can also be hit when using c++ and lto on arm or so.
If found a similar bug record:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44871

Any thoughts on that?

Cheers

Vinz

PS: Seeing this on ChibiOS 16.1.5 Stable

[Giovanni]

Him

Not sure, it looks like a GCC bug because it is triggered by LTO, we could implement a workaround if it does not mess too much with the code, suggestions?

Giovanni

[lilvinz]

Currently i am using this in chschd.h within struct ch_thread:

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#ifdef PORT_ARCHITECTURE_SIMIA32
  /* This is to work around an issue with gcc when accessing a union from both
   * C as well as C++ compile units in combination with -flto. */
  struct {
#else
  union {
#endif


But this really isn't a workaround. Also it will only help for my custom stuff and my unit tests.
Making that union into a struct wastes memory. Currently i have no idea how to fix this without using a union. I have tried all kinds of __attribute__() things to get that C and C++ types match without any success.

Cheers

Vinz

[lilvinz]

Not sure, it looks like a GCC bug because it is triggered by LTO

I am very hesitant with attributing issues to GCC because very often LTO just makes hidden bugs surface.
In this case, the type of the ch structure doesn't match between C and C++ - at least by means of how gcc compares them to each others.

[Giovanni]

GCC for ARM also uses LTO and it works.

If there is a bug in the code then I don't know what rule I am violating.

Giovanni