https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114277
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |11.5
Last reconfirmed| |2024-03-08
Summary|Missed optimization: |[11/12/13/14 Regression]
|x*(x||b) => x |Missed optimization:
| |x*(x||b) => x
Known to fail| |8.1.0
Known to work| |7.5.0
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
In GCC 7, before PRE we had:
```
# iftmp.0_3 = PHI <_8(3), 1(5)>
_2 = iftmp.0_3 * x_4(D);
```
Which was able to optimized by PRE to:
```
_9 = x_4(D) * _8;
<bb 4> [100.00%]:
# iftmp.0_3 = PHI <_8(3), 1(2)>
# prephitmp_10 = PHI <_9(3), x_4(D)(2)>
a = prephitmp_10;
```
Which is how GCC 7 was able to optimize it.
In GCC8+, reassociatation takes:
```
<bb 2> [local count: 1073741825]:
if (x_4(D) != 0)
goto <bb 4>; [50.00%]
else
goto <bb 3>; [50.00%]
<bb 3> [local count: 536870912]:
b.1_1 = b;
_7 = b.1_1 != 0;
_8 = (int) _7;
<bb 4> [local count: 1073741825]:
# iftmp.0_3 = PHI <_8(3), 1(2)>
_2 = iftmp.0_3 * x_4(D);
```
Into:
```
_9 = x_4(D) | b.1_1;
_10 = _9 != 0;
_7 = b.1_1 != 0;
_11 = (int) _10;
_8 = (int) _7;
_2 = x_4(D) * _11;
```
Which no longer can be optimized to just x_4(D).
Maybe:
(simplify
(mult:c @0 (convert? (ne (bit_ior:c @0 @1) integer_zero_p)))
@0)
Like similarly was done for `a * !a` just recently. Note this won't catch all
cases though as @0 be deep in the bit_ior chain but it will catch some.
