[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 #4 from Andrew Macleod <amacleod at redhat dot com> ---


(In reply to Richard Biener from comment #2)
> FRE1 has the following difference, simplifying the (unsigned int) truncation.
> 
>    <bb 2> :
>    a.0_1 = a;
>    _2 = (unsigned int) a.0_1;
>    b = _2;
> -  c_10 = (long int) _2;
> +  _6 = a.0_1 & 4294967295;
> +  c_10 = _6;
>    if (c_10 != 0)
>      goto <bb 3>; [INV]
>    else
> 

Why does FRE make this transformation/simplification?  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.