[Bug tree-optimization/102540] [12 Regression] Dead Code Elimination Regression at -O3 since r12-476-gd846f225c25c5885

["amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102540

--- Comment #6 from Andrew Macleod <amacleod at redhat dot com> ---

> 
> >  It removes a
> > relationship between c_10 and _2. The reason ranger no longer can fold _2 == 0
> > is because the sequence is now:
> > 
> >     a.0_1 = a;
> >     _2 = (unsigned int) a.0_1;
> >     b = _2;
> >     _6 = a.0_1 & 4294967295;
> >     c_10 = _6;
> >     if (c_10 != 0)
> >       goto <bb 3>; [INV]
> > 
> > We do not find _2 is non-zero on the outgoing edge because _2 is not related to
> > the calculation in the condition.  (ie c_10 no longer has a dependency on _2)
> > 
> > We do recalculate _2 based on the outgoing range of a.0_1, but with it being a
> > 64 bit value and _2 being 32 bits, we only know the outgoing range of a.0_1 is
> > non-zero.. we dont track any of the upper bits... 
> >  2->3  (T) a.0_1 :       long int [-INF, -1][1, +INF]
> > And when we recalculate _2 using that value, we still get varying because
> > 0xFFFF0000 in not zero, but can still produce a zero in _2.
> > 
> > The problem is that the condition c_10 != 0 no longer related to the value of
> > _2 in the IL... so ranger never sees it. and we cant represent the 2^16
> > subranges that end in [1,0xFFFF].
> > 
> > Before that transformation, 
> >   _2 = (unsigned int) a.0_1;
> >    b = _2;
> >   c_10 = (long int) _2;
> > The relationship is obvious, and ranger would relate the c_10 != 0 to _2 no
> > problem.
> 
> I see - too bad.  Note the transform made the dependence chain of _6
> one instruction shorter without increasing the number of instructions
> so it's a profitable transform.
> 
> Btw, the relation is still there but only indirectly via a.0_1.  The
> old (E)VRP had this find_asserts(?) that produced assertions based
> on the definitions - sth that now range-ops does(?), so it would
> eventually have built assertions for a.0_1 for both conditions and
> allow relations based on that?  I can't seem to find my way around
> the VRP code now - pieces moved all over the place and so my mind
> fails me on the searching task :/

We do know that a.0_1 is non-zero on that edge:
2->3  (T) a.0_1 :       long int [-INF, -1][1, +INF]

the problem is that we can't currently represent that the bitmask operation
causes all patterns ending in 0x00000000 to not occur.. we just leave it at
~[0,0].  which isn't sufficient for this use case. 

we don't currently track any equivalences between values of different
precision.. (even though ranger once did).   Handling it as a general
equivalence was fraught with issues. 

We might be able to add a new equivalence class "slice" or something.. I had
considered it but hadn't seen a great need case.   This would make _6 a 32 bit
slice of a.0_1 with range [1, 0xffffffff].
Then when we are querying for the cast
  _2 = (unsigned int) a.0_1;
we could also query the 32 bit equivalence slices of a.0_1, find _6, and get
the outgoing range of [1,0xffffffff].. and apply that value.

It would probably resolve an entire class of things where we don't recognize an
equivalence between a cast and a bitmask of equivalent precision.

This would also mean the reverse would apply.. ie if we instead branched on _2
!= 0 we would also understand that _6 will be non-zero.