Re: GCC 8.3.0, -flto and violation of C++ One Definition Rule

[Tom Kacvinsky <tkacvins@gmail.com>]

On Wed, Dec 29, 2021 at 7:55 AM Xi Ruoyao <xry111@mengyan1223.wang> wrote:
>
> On Wed, 2021-12-29 at 06:44 -0500, Tom Kacvinsky via Gcc-help wrote:
> > Hi,
> >
> > First, using GCC 8.3.0 and binutils 2.37.I am trying to increase
> > performance of linking our product, so I thought I'd give LTO a try.  So
> > I am compiling all object files with -flto, and passing -flto to g++
> > (which we use as our link driver).  However, what I have found is that
> > some of our code violates the C++ One Definition Rule (-Werror=odr). This
> > only happens when building with LTO - without LTO, the C++ rule is
> > not violated.
>
> No, it's violated all the time.  -Wodr only works with LTO because it's
> impossible to detect ODR volations (across multiple TUs) just by linking
> normal (non-LTO) object files.
>
> > The problem exists with LTO using both the BFD and gold
> > linkers.
> >
> > So, my question is, since the LTO object files are now such that one
> > needs to use gcc-nm to examine them (which I know is a wrapper around nm,
> > and passes an option to load the LTO plugin). how can I leverage that to
> > see if there are other translation units that define the class that ODR
> > violation is complaining about?
> > I did do a fairly thorough analysis of
> > the object files and did not see there the particular class and methods
> > would be multiply defined, but that was just based on symbol names from
> > gcc-nm output.
>
> Generally you can't find ODR violations this way.  ODR violation does
> not mean multiple definitions of a symbol.  For example:
>
> // in 1.cpp
>
> enum A
> {
>         a = 1,
> };
>
> // in 2.cpp
>
> enum A
> {
>         a = 2,
> };
>
> If you link the objects compiled from these TUs together, you will
> violate ODR.  But these TUs won't produce any symbols for A or A::a at
> all.
>
> > I suspect there is more to this since the object files
> > have LTO information now, and that is what I'd like to examine.
> >
> > Any hints on how to move forward with diagnosing LTO link errors?
>
> With GCC-11, the diagnostic shows the exact location of ODR violation,
> like:
>
> > t-1.cc:1:6: warning: type 'A' violates the C++ One Definition Rule [-Wodr]
> >     1 | enum A
> >       |      ^
> > t-2.cc:3:6: note: an enum with different value name is defined in another translation unit
> >     3 | enum A
> >       |      ^
> > t-1.cc:3:9: note: name 'a' is defined to 1 while another translation unit defines it as 2
> >     3 |         a = 1,
> >       |         ^
> > t-2.cc:5:9: note: mismatching definition
> >     5 |         a = 2,
> >       |         ^
> >
>
> I'm not sure how GCC 8 behaves.
>

that's the problem.  It is _not_ defined in another translation unit.
Only defined in one
C++ file