https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106474
--- Comment #2 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Richard Biener from comment #1)
> For the failing case we see
>
> <bb 2> :
> if (c_5(D) == s_6(D))
> goto <bb 3>; [INV]
> else
> goto <bb 6>; [INV]
>
> <bb 3> :
> if (c_5(D) != 0)
> goto <bb 6>; [INV]
> else
> goto <bb 4>; [INV]
>
> <bb 4> :
> if (s_6(D) != 0)
> goto <bb 5>; [INV]
> else
> goto <bb 6>; [INV]
>
> and we basically fail to thread 2->3->4->6, eliminating the if (s_6(D) != 0)
> branch when c_5(D) == s_6(D) and c_5(D) == 0.
>
> It looks like relations / equivalences related, but maybe not exactly.
> Andrew?
it is related. Due to the expense, we don't evaluate every equivalence at
every use point. In this case, there was no obvious "trigger" when we see s_6
!= 0 to go query all of the equivalences to see if they have ranges.
However range_from_dom () is now quite efficient and has a "read-only" mode
where it will make a DOM query without filling the cache. I am experimenting
with having the on-entry cache fill also invoke range_from_dom on equivalences
in read-only mode to see if any existing equivalence value can improve the
range.
The initial protoype reduces that test case in EVRP from
<bb 2> :
_1 = ~c_7(D);
_2 = (int) _1;
_3 = ~s_8(D);
_4 = (int) _3;
if (c_7(D) == s_8(D))
goto <bb 3>; [INV]
else
goto <bb 6>; [INV]
<bb 3> :
if (c_7(D) != 0)
goto <bb 6>; [INV]
else
goto <bb 4>; [INV]
<bb 4> :
if (s_8(D) != 0)
goto <bb 5>; [INV]
else
goto <bb 6>; [INV]
<bb 5> :
DCEMarker0_ ();
<bb 6> :
return;
to
<bb 2> :
_1 = ~c_7(D);
_2 = (int) _1;
_3 = ~s_8(D);
_4 = (int) _3;
return;
Let me run some performance numbers and see what the result is to see if there
is any throtteling needed or not.
