[llvm-bugs] [Bug 33093] New: [ppc] Slow code for reverse bits

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33093

            Bug ID: 33093
           Summary: [ppc] Slow code for reverse bits
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: PowerPC
          Assignee: unassignedbugs at nondot.org
          Reporter: carrot at google.com
                CC: llvm-bugs at lists.llvm.org

The reverse bits source code is:

unsigned int ReverseBits(unsigned int n) {
  n = ((n >> 1) & 0x55555555) | ((n & 0x55555555) << 1);
  n = ((n >> 2) & 0x33333333) | ((n & 0x33333333) << 2);
  n = ((n >> 4) & 0x0F0F0F0F) | ((n & 0x0F0F0F0F) << 4);
  return ((n & 0xff000000u) >> 24) | ((n & 0x00ff0000u) >> 8) | ((n &
0x0000ff00u) << 8) | ((n & 0x000000ffu) << 24);

}

LLVM generates:

# BB#0:                                 # %entry
        rlwinm 4, 3, 1, 0, 31
        rlwimi 4, 3, 3, 30, 30
        rlwimi 4, 3, 5, 29, 29
        rlwimi 4, 3, 7, 28, 28
        rlwimi 4, 3, 9, 27, 27
        rlwimi 4, 3, 11, 26, 26
        rlwimi 4, 3, 13, 25, 25
        rlwimi 4, 3, 15, 24, 24
        rlwimi 4, 3, 17, 23, 23
        rlwimi 4, 3, 19, 22, 22
        rlwimi 4, 3, 21, 21, 21
        rlwimi 4, 3, 23, 20, 20
        rlwimi 4, 3, 25, 19, 19
        rlwimi 4, 3, 27, 18, 18
        rlwimi 4, 3, 29, 17, 17
        rlwimi 4, 3, 31, 16, 16
        rlwimi 4, 3, 3, 14, 14
        rlwimi 4, 3, 5, 13, 13
        rlwimi 4, 3, 7, 12, 12
        rlwimi 4, 3, 9, 11, 11
        rlwimi 4, 3, 11, 10, 10
        rlwimi 4, 3, 13, 9, 9
        rlwimi 4, 3, 15, 8, 8
        rlwimi 4, 3, 17, 7, 7
        rlwimi 4, 3, 19, 6, 6
        rlwimi 4, 3, 21, 5, 5
        rlwimi 4, 3, 23, 4, 4
        rlwimi 4, 3, 25, 3, 3
        rlwimi 4, 3, 27, 2, 2
        rlwimi 4, 3, 29, 1, 1
        rlwimi 4, 3, 31, 0, 0
        mr 3, 4
        blr

All of the instructions are in a dependence chain. It is much slower than the
algorithm shown in the source code.

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