https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101240
Aldy Hernandez <aldyh at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |aldyh at gcc dot gnu.org
--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Andrew Macleod from comment #3)
> in VRP2 we see:
>
> <bb 2> [local count: 1073741824]:
> _1 = (*a_6(D))[0];
> _2 = (*a_6(D))[1];
> pretmp_8 = (*a_6(D))[2];
> if (_1 < _2)
> goto <bb 3>; [50.00%]
> else
> goto <bb 5>; [50.00%]
>
> <bb 3> [local count: 536870913]:
> if (_2 < pretmp_8)
> goto <bb 4>; [0.00%]
> else
> goto <bb 5>; [100.00%]
>
> <bb 4> [count: 0]:
> __builtin_unreachable ();
>
> <bb 5> [local count: 1073741824]:
> _7 = _1 < pretmp_8;
> return _7;
>
> There isnt much VRP can do about this because if we take the path 2->5, we
> have no way of resolving _1 < pretmp_8 because the comparison has not been
> made yet.
>
> interestingly, if this were to be threaded,
> 2->3 would register _1 < _2
> 3->5 would register _2 >= ptrtmp_8
> then in bb5' _1 < pretmp_8 should be answered by the path oracle as TRUE
> and then be folded to return true. at least it should :-)
>
> I guess the next question would be... why doesn't the threader think this is
> worth threading?
The threader only looks at paths that end in a block with multiple exits:
FOR_EACH_BB_FN (bb, m_fun)
if (EDGE_COUNT (bb->succs) > 1)
maybe_thread_block (bb);
so...a switch or a cond.
