[llvm-dev] Invalid llvm-ir and unreachable code

[Roman Lebedev via llvm-dev]

On Mon, Mar 15, 2021 at 12:53 PM Jeroen Dobbelaere
<Jeroen.Dobbelaere at synopsys.com> wrote:
>
> Hi Roman,
>
> [..]
> > >
> > > - the problematic code construct consists of a cycle of 'dead' basic blocks
> > that conditionally jumps into live blocks.
> > >
> > >   That jump introduce a connection of a dead self-referring 'getelementptr'
> > to live code through a PHI node.
> > >
> > > - during SROA, the scoped noalias analysis (full restrict version) does an
> > unbounded (MaxLookup == 0) 'getUnderlyingObject' call on a pointer.
> > >
> > > - 'getUnderlyingObject' also follows the path to the dead blocks and ends up
> > in the self-referring  'getelementpr'.
> > >
> > >
> > >
> > > Does this mean that 'getUnderlyingObject' should also be prepared to be
> > handling 'invalid instructions' in 'dead code' ?
> > I've just adjusted that function in 36f1c3db66f7268ea3183bcf0bbf05b3e1c570b4
> > to assert that no cycles are encountered instead of endlessly looping.
> >
> > But i *think* the problem is in your SROA changes.
> > Can you link me to the particular patch in question?
>
> This is the patch that triggers the unbounded lookup:  https://reviews.llvm.org/D68507
> (llvm/lib/Analysis/ScopedNoAliasAA.cpp, line 269)
>
> I checked again the endless loop: It happens _right after SROA_, in the MemorySSA pass.

> Apparently, your extra check also triggers cases without my patches ? A self-reference is
> likely a mild version of an 'invalid instruction' but what other constructs can we encounter
> in dead blocks ?
Yeah, i fooled myself there.

> I am wondering if we should allow (analysis) passes to follow code into dead blocks at all ?
I wouldn't think so. It's a waste of time i would say.

> Greetings,
>
> Jeroen
>