On Mon, Sep 8, 2025 at 3:16 PM Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Mon, Sep 08, 2025 at 03:05:58PM +0200, Richard Biener wrote:
> > is reduced to __builtin_abort () (for C++). That's because it's
> > __builtin_unreachable () at the end. I am not aware of any
> > other "UB" than missed return that we turn into unreachable(),
> > even
> >
> > unsigned int foo (unsigned int i)
> > {
> > if (i < 33)
> > __builtin_abort ();
> > return i << i;
> > }
> >
> > isn't handled this way. But sure we could, in the future.
> > std::... before the return would help in case it is considered
> > a barrier that might not continue to the UB.
>
> What is gimple-ssa-isolate-paths.cc doing then?
Nothing in the above case - we only isolate null pointer dereferences
IIRC.
> I thought it is about obvious UBs like null pointer dereferences etc.
> and the whole paths can be turned into e.g. __builtin_trap() with it
> and for anything dominated by the UB that is just fine from the POV of this
> C++ paper and it only a question if we don't propagate it also backwards
> across calls (or other statements if std::observable_checkpoint would be
> e.g. inline function with nothing in it and no special attributes).
So I've now read the paper and it's motivated by C++ contracts. I have
no idea how they'd be actually implemented, so I can't say if it's sensible
for those. The only "example" is for in(p) in (*p > 5) where the contract
*p > 5 would "elide" the 'p' one. Whatever "eliding" means here.
Richard.
> Jakub
>
