https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102650
Andrew Macleod <amacleod at redhat dot com> changed:
What |Removed |Added
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CC| |amacleod at redhat dot com
--- Comment #1 from Andrew Macleod <amacleod at redhat dot com> ---
This is a result of the vagaries of the single subrange value-range.
VRP is seeing:
# f_11 = PHI <-1(2), 2(3)>
goto <bb 6>; [100.00%]
<bb 5> [local count: 955630225]:
_3 = (unsigned short) f_11;
_6 = _3 + 2;
e_19 = (short int) _6;
It knows f_11 is [-1, 2] and when that is cast to a ushort, produces ~[3,
65534].
That is all we knew about it in GCC11, so when we calculate_6 = ~[3,65534] + 2
it comes up with [1,4] and the e_19 == 0 later on then can be folded away.
in gcc12, EVRP has figured out that _3 is unsigned short [2, 2][+INF, +INF].
which if we add 2 to it, would come up with [1,1][4,4] which would be perfect.
We save this to the value_range global table in EVRP, but alas it gets
transliterated to a single pair value_range : _3 : unsigned short [2, +INF]
Now when VRP calculates ~[3, 65534] and intersects that with the known global
[2, +INF] the legacy intersect routine has to come up with pair and decides
to keep [2, +INF].
when you add 2 to that, 0 is no longer eliminated.
