On 4/24/16 9:17 PM, Xinok wrote:
I modified David's solution a bit to (hopefully) eliminate the branch: bool powerOf2(uint x){ return !x ^ !(x & (x - 1)); }0000000000000000 <_D4test8powerOf2FkZb>: 0: 50 push %rax 1: 53 push %rbx 2: 48 89 fa mov %rdi,%rdx 5: 85 d2 test %edx,%edx 7: 0f 94 c0 sete %al a: 8d 8a ff ff ff ff lea -0x1(%rdx),%ecx 10: 85 ca test %ecx,%edx 12: 0f 94 c3 sete %bl 15: 32 c3 xor %bl,%al 17: 5b pop %rbx 18: 59 pop %rcx 19: c3 retq 1a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) The branches are gone but I'm really not sure if this is better or worse.
With gdc https://godbolt.org/g/jcU4np isPow2B is the winner (shortest code, simplest instructions). -- Andrei
