[Bug tree-optimization/113487] New: Missed optimization:simplify demanded bits on multi-use instructions like select

[Bug tree-optimization/113487] New: Missed optimization:simplify demanded bits on multi-use instructions like select

[xxs_chy at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113487

            Bug ID: 113487
           Summary: Missed optimization:simplify demanded bits on
                    multi-use instructions like select
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xxs_chy at outlook dot com
  Target Milestone: ---

Godbolt example: https://godbolt.org/z/EfKrTbK77

Based on common demanded bits from s&8 and s&16,
```
void src(bool c, unsigned a, unsigned b) {
    unsigned s = c ? a & 24 : b & 25;
    use(s & 8);
    use(s & 16);
}
```

can be folded to:

```
void tgt(bool c, unsigned a, unsigned b) {
    unsigned s = c ? a : b ;
    use(s & 8);
    use(s & 16);
}
```

Both LLVM and GCC missed this optimization opportunity.

[Bug tree-optimization/113487] Missed optimization:simplify demanded bits on multi-use instructions like select

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113487

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
           Severity|normal                      |enhancement

[Bug tree-optimization/113487] Missed optimization:simplify demanded bits on multi-use instructions like select

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113487

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-01-18
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

I don't know which pass this would be best implemented in though. Maybe phiopt
but I am not 100% sure there ...

[Bug tree-optimization/113487] Missed optimization:simplify demanded bits on multi-use instructions like select

[xxs_chy at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113487

--- Comment #2 from XChy <xxs_chy at outlook dot com> ---
I may miss something here... Apart from this one, it seems that GCC doesn't
simplify **one-use** instruction based on demanded bits too:
https://godbolt.org/z/67bYxd8hY

But LLVM indeed handle the one-use case.

[Bug tree-optimization/113487] Missed optimization:simplify demanded bits on multi-use instructions like select

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113487

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Feels more like reassoc to me.  You could also view it as "bit-DCE", eliding
defs of dead bits.  Which might make it suitable for backprop (currently
doing sth similar for the sign "bit").