[Bug target/106420] New: Missed optimization for comparisons

[Bug target/106420] New: Missed optimization for comparisons

[zero at smallinteger dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106420

            Bug ID: 106420
           Summary: Missed optimization for comparisons
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zero at smallinteger dot com
  Target Milestone: ---

Created attachment 53339
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53339&action=edit
Sample code

When comparing different variables to the same constants, in some cases the
compiler could first combine the variables and then do a single compare.  In
the sample given, two variables are compared against 7.  In the slow path, GCC
produces the following with -O2.

        cmp     edi, 7
        setg    al
        cmp     esi, 7
        setg    dl
        or      eax, edx
        movzx   eax, al
        ret

In the fast path, GCC produces this instead.

        or      edi, esi
        xor     eax, eax
        cmp     edi, 7
        setg    al
        ret

Although the expression a > 7 || b > 7 is the same as (a | b) > 7, the latter
is better because it results in fewer instructions.  A quick experiment shows
the latter also runs quite faster.

Verified with Godbolt for GCC trunk.  Clang, ICC, and MSVC latest versions also
miss this opportunity as per Godbolt.

[Bug target/106420] Missed optimization for comparisons

[zero at smallinteger dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106420

--- Comment #1 from zero at smallinteger dot com ---
(it should be possible to massage the output further to use test and setne,
which ICC prefers to e.g. cmp and seta)

[Bug tree-optimization/106420] Missed optimization for comparisons

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106420

Hongtao.liu <crazylht at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |crazylht at gmail dot com

--- Comment #2 from Hongtao.liu <crazylht at gmail dot com> ---

> Although the expression a > 7 || b > 7 is the same as (a | b) > 7, the
Shoudn't the optimization be available for unsigned type only?

[Bug tree-optimization/106420] Missed optimization for comparisons

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106420

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This can only be done for unsigned types as if either one was negative, then
the check becames invalid.
It is already done since GCC 11 (most likely the patch which fixed PR 95731).

So this is invalid.

[Bug tree-optimization/106420] Missed optimization for comparisons

[zero at smallinteger dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106420

--- Comment #4 from zero at smallinteger dot com ---
Sorry about that :/.