[llvm-bugs] [Bug 42197] New: Canonicalize OR with operands which do not have shared bits to ADD

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42197

            Bug ID: 42197
           Summary: Canonicalize OR with operands which do not have shared
                    bits to ADD
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

typedef unsigned char byte;

byte add_no_common_bits(byte x, byte y) {
    // hint, no common bits
    __builtin_assume((x & y) == 0);
    return x | y;
}

byte or_no_common_bits(byte x, byte y) {
    // hint, no common bits
    __builtin_assume((x & y) == 0);
    return x + y;
}

add_no_common_bits(unsigned char, unsigned char):
  mov eax, edi
  or eax, esi
  ret
or_no_common_bits(unsigned char, unsigned char):
  lea eax, [rsi + rdi]
  ret


If we can prove that there are no shared bits between X and Y, maybe we could
turn OR to ADD.

Or ADD to OR.. not sure which form is better.

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