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.
