Re: [RFC] Using One Definition Rule for types during LTO devirtualizatoin?

[Richard Biener <rguenther at suse dot de>]

On Mon, 17 Jun 2013, Jan Hubicka wrote:

> > On 06/17/2013 09:35 AM, Jan Hubicka wrote:
> > >To get meaningful warnings, we need to know what decls/types are subject
> > >to ODR. Do you think you can make C++ FE to drop a flag so middle-end know?
> > >We can LTO ODR and non-ODR languages together.
> > 
> > Basically everything in C++ is subject to the ODR.  There are two
> > cases: either there can be only one definition anywhere (e.g. normal
> > variables and functions, anything local to such a function) or all
> > definitions need to be identical.
> 
> Does ODR hold also for extern "C" declarations in C++?
> 
> > 
> > In C, you can treat all structurally identical types as the same, right?
> 
> Sort of. Currently we have two notions of equivalency of types
> 
> 1) TYPE_CANONICAL that is strictly merged by structural equivalence.
>    The equivalency is defined by gimple_canonical_types_compatible_p
>    and is very conservative (i.e. ignores type names for example)
> 
>    This is what drives most of backend and aliasing machinery.
> 
>    Having false equivalences leads to worse TBAA, false nonequivalences
>    leads to wrong code due to aliasing.

That's 100% accurate.

> 2) Richard's type (and now tree) merging. This merging is conservative
>    on what is considered to be equivalent type and compare pretty
>    much everything in the in-memory type representaion (i.e. names, modifiers,
>    BINFOs, stuff that matters for debug or devirt machinery)
> 
>    false equivalences leads to loss of debug info quality and false
>    devirtualizations, false nonequivalences leads to explossions of memory
>    use and streaming time for Firefox or any other huge LTO build.

There should not be false equivalencies - false equivalencies can
lead to random bugs including wrong-code.  Basically we consider
two tree SCCs equivalent if all values inside them are equal and
the tree SCC is structurally equivalent.

>    At least this is my understanding, Richard know more ;)
> 
> ODR adds third notion of equivalency that should sit strictly in between
> and in fact perhaps for ODR types we can make 1)=2)=ODR unless ODR is
> obviously violated.

The question is when will two types be "ODR equivalent" but not
"tree equivalent".  Certainly there is no such thing as "ODR" for C,
so given C++ manually implemented in C you cannot expect same
"vtable" contents when the types have the same name.

Richard.