[Bug middle-end/50481] New: builtin to reverse the bit order

[Bug middle-end/50481] New: builtin to reverse the bit order

[marc.glisse at normalesup dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

             Bug #: 50481
           Summary: builtin to reverse the bit order
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: marc.glisse@normalesup.org


It would be useful if gcc provided a builtin to reverse the order of the bits
(turn abcdefgh into hgfedcba) in objects of all sizes from a byte to an
unsigned long long.

According to Joseph: "Various processors have an instruction to reverse the bit
order in a word (ARMv6T2 and later have RBIT, for example, and C6X has BITR on
C64X and above)." This kind of bit manipulation also has various optimizations
depending on the architecture (mirroring a byte can be implemented using a
64bit multiplication). It is thus well suited to a builtin with different
platform-specific implementations.

Such a builtin could be used for instance for Bug 50160.

[Bug middle-end/50481] builtin to reverse the bit order

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-09-22
     Ever Confirmed|0                           |1

[Bug middle-end/50481] builtin to reverse the bit order

[devel@the-user.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Jonathan Schmidt-Dominé <devel@the-user.org> changed:

           What    |Removed                     |Added
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                 CC|                            |devel@the-user.org

--- Comment #1 from Jonathan Schmidt-Dominé <devel@the-user.org> 2011-09-23 15:30:33 UTC ---
Informative web-page listing some methods:
http://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith64BitsDiv

[Bug middle-end/50481] builtin to reverse the bit order

[ptim034 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Tim Parker <ptim034 at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |ptim034 at gmail dot com

--- Comment #2 from Tim Parker <ptim034 at gmail dot com> 2012-03-29 10:46:11 UTC ---
I totally don't understand what you talking about =_=
http://www.buyanessay.com/

[Bug middle-end/50481] builtin to reverse the bit order

[f.boesing at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Frank <f.boesing at gmx dot de> changed:

           What    |Removed                     |Added
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                 CC|                            |f.boesing at gmx dot de

--- Comment #7 from Frank <f.boesing at gmx dot de> ---
Would be really useful to have this.

[Bug middle-end/50481] builtin to reverse the bit order

[carlo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Carlo Wood <carlo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |carlo at gcc dot gnu.org

--- Comment #8 from Carlo Wood <carlo at gcc dot gnu.org> ---
Bump - I need this too ;)

[Bug middle-end/50481] builtin to reverse the bit order

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |xry111 at gcc dot gnu.org

--- Comment #9 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
Useful for LoongArch too.  And now we already have bitreverse RTX code since
r14-1586.

[Bug middle-end/50481] builtin to reverse the bit order

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot gnu.org
           Keywords|                            |missed-optimization
             Status|NEW                         |ASSIGNED

--- Comment #10 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am going to implement this. and add an optab too.

[Bug middle-end/50481] builtin to reverse the bit order

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The builtins I am going to implement to be similar to clang:
__builtin_bitreverse{8,16,32,64,g}

The g one is not part of clang but will be used for _BitInt types.

[Bug middle-end/50481] builtin to reverse the bit order

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

--- Comment #12 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Also will add an internal function which will be used for vectorization.