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

[Jeroen Dobbelaere via llvm-dev]

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 ?

I am wondering if we should allow (analysis) passes to follow code into dead blocks at all ?

Greetings,

Jeroen