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From: "marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/50481] New: builtin to reverse the bit order
Date: Thu, 22 Sep 2011 10:33:00 -0000
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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.