[llvm-bugs] [Bug 40058] New: Failure to detect bswap at the end of a bitreverse sequence

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40058

            Bug ID: 40058
           Summary: Failure to detect bswap at the end of a bitreverse
                    sequence
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: craig.topper at gmail.com
                CC: llvm-bugs at lists.llvm.org

We should recognize that these two sequences end in a bswap.


unsigned long lreverse(unsigned long x)    // swap all bits
{
    x = ((x &  0xAAAAAAAAUL) >> 1)
      | ((x & ~0xAAAAAAAAUL) << 1);
    x = ((x &  0xCCCCCCCCUL) >> 2)
      | ((x & ~0xCCCCCCCCUL) << 2);
    x = ((x &  0xF0F0F0F0UL) >> 4)
      | ((x & ~0xF0F0F0F0UL) << 4);
    x = ((x &  0xFF00FF00UL) >> 8)
      | ((x & ~0xFF00FF00UL) << 8);
    x = ((x &  0xFFFF0000UL) >> 16)
      | ((x & ~0xFFFF0000UL) << 16);

    return x;
}

unsigned long long llreverse(unsigned long long x)
{
    x = ((x & 0xAAAAAAAAAAAAAAAAULL) >> 1)
      | ((x & ~0xAAAAAAAAAAAAAAAAULL) << 1);
    x = ((x & 0xCCCCCCCCCCCCCCCCULL) >> 2)
      | ((x & ~0xCCCCCCCCCCCCCCCCULL) << 2);
    x = ((x & 0xF0F0F0F0F0F0F0F0ULL) >> 4)
      | ((x & ~0xF0F0F0F0F0F0F0F0ULL) << 4);
    x = ((x & 0xFF00FF00FF00FF00ULL) >> 8)
      | ((x & ~0xFF00FF00FF00FF00ULL) << 8);
    x = ((x & 0xFFFF0000FFFF0000ULL) >> 16)
      | ((x & ~0xFFFF0000FFFF0000ULL) << 16);
    x = ((x & 0xFFFFFFFF00000000ULL) >> 32)
      | ((x & ~0xFFFFFFFF00000000ULL) << 32);

    return x;
}


https://godbolt.org/z/K0FuOg

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20181218/c9564ee4/attachment.html>