[Bug ada/46943] New: Unnecessary ZERO_EXTEND

[Bug ada/46943] New: Unnecessary ZERO_EXTEND

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46943

           Summary: Unnecessary ZERO_EXTEND
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: ada
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jakub@gcc.gnu.org
            Target: x86_64-linux


In http://blog.regehr.org/archives/320 Example 4
unsigned long long v;
unsigned short
foo (signed char x, unsigned short y)
{
  v = (unsigned long long) y;
  return (unsigned short) ((int) y / 3);
}
we emit a redundant zero-extension:
        movzwl  %si, %eax
        movq    %rax, v(%rip)
        movzwl  %si, %eax
        imull   $43691, %eax, %eax
        shrl    $17, %eax
        ret
The reason why the second movzwl %si, %eax wasn't CSEd with the first one is
because the first one is (set (reg:DI reg1) (zero_extend:DI (reg:HI reg2))
while the second one is (set (reg:SI reg3) (zero_extend:SI (reg:HI reg2))
Wonder if we can't teach CSE to optimize it (say that reg3 is actually
(subreg:SI (reg:DI reg1) 0), or if e.g. one of the zee/see passes (implicit-zee
e.g.) couldn't handle such cases.  Combiner can't do anything here, as there is
no data dependency, so try_combine won't see them together.

[Bug rtl-optimization/46943] Unnecessary ZERO_EXTEND

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46943

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-16 15:10:12 UTC ---
Hmm.  I think at least postreload CSE should handle it as we then know that
zero-extends extend to the full HW register.  Pre-regalloc would be difficult
as we'd need some new form of canonicalization with an associated cost
(think of a zero_extend:TI).

[Bug rtl-optimization/46943] Unnecessary ZERO_EXTEND

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46943

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-02-02
     Ever Confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-02-02 08:51:50 UTC ---
Confirmed.

[Bug rtl-optimization/46943] Unnecessary ZERO_EXTEND

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46943

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2018-04-22 00:00:00         |2023-5-19
           Severity|normal                      |enhancement