https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107009
--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> # RANGE [irange] size_t [1, +INF]
> size_t n_12(D) = n;
>
> the nonzero bits info on 'n' is gone. DOM2 used to produce that and
> CCP3 elides the __builtin_unreachable () but if DOM2 is disabled that
> doesn't produce the nonzero bits.
>
> DOM2 from trunk no longer sets nonzero bits. CCP doesn't do any
> conditional non-zero bits (or constant equivalence) propagation.
Indeed, there is a missing nonzero mask on n_12(D). Thanks for the analysis,
Richi. It saved me a ton of work.
This is the problematic BB, where the 2->3 edge is unreachable:
<bb 2> [local count: 118111600]:
_1 = n_12(D) & 7;
if (_1 != 0)
goto <bb 3>; [0.00%]
else
goto <bb 4>; [100.00%]
_1 : [irange] long unsigned int [0, 0]
2->3 (T) _1 : [irange] UNDEFINED
2->3 (T) n_12(D) : [irange] UNDEFINED
2->4 (F) _1 : [irange] long unsigned int [0, 0]
2->4 (F) n_12(D) : [irange] size_t [1, 18446744073709551608] <-- BOO
HISS!!!!
Notice there is no nonzero mask on n_12(D) in the 2->4 edge which is
unreachable.
So this is actually our fault, not because of this patch, but because we didn't
track nonzero bits back then. I keep saying, this patch is a red herring :-P.
However, we do track nonzero bits now, and the testcase still fails so...
The culprit is operator_bitwise_and::op1_range() which is not setting the
nonzero mask. This should be quite easy to fix. What we're trying to solve is
n_12 on the 2->4 edge:
_1 = n_12(D) & 7;
which _1 = 0 because we're talking about the 2->4 edge:
0 = n_12(D) & 7;
So the mask should be everything but the lower 3 bits, 0xffff...f8 if my math
is correct.
