Re: bitset<32> and bitset<64> efficiency
On 28.11.2012 09:58, Ninds wrote:
I would like to know whether using bitset<32> for bit operations on a 32bit machine would generally be as efficient as using a 32bit int.
Moreover, if the answer is yes would it also hold for bitset<64> and 64bit int on a 64 bit arch.
I realise the standard says nothing about the implementation so there is no definitive answer but what is 'likely' to be the case ?
Why don't you try it out for your constellation? I just tried:
#include <bitset>
#include <string>
#include <iostream>
int main(int argc, char ** argv) {
std::bitset<64> i, j;
std::cerr << sizeof(i) << std::endl;
__asm__ __volatile__("nop");
i.set(argc);
__asm__ __volatile__("nop");
std::cerr << i << std::endl;
__asm__ __volatile__("nop");
j.set(64 - argc);
__asm__ __volatile__("nop");
std::cerr << j << std::endl;
__asm__ __volatile__("nop");
i ^= j;
__asm__ __volatile__("nop");
std::cerr << i << std::endl;
return 0;
}
on Linux x86_64 with g++ 4.6.2. The "set" always results in a call, but
the xor is done as if it were a int, i.e.:
// First set
400ce0: ba 01 00 00 00 mov $0x1,%edx
400ce5: 48 63 f3 movslq %ebx,%rsi
400ce8: 48 89 e7 mov %rsp,%rdi
400ceb: e8 60 01 00 00 callq 400e50
<std::bitset<64ul>::set(unsigned long, bool)>
[...]
// Second set
400d07: be 40 00 00 00 mov $0x40,%esi
400d0c: 48 8d 7c 24 10 lea 0x10(%rsp),%rdi
400d11: ba 01 00 00 00 mov $0x1,%edx
400d16: 29 de sub %ebx,%esi
400d18: 48 63 f6 movslq %esi,%rsi
400d1b: e8 30 01 00 00 callq 400e50
<std::bitset<64ul>::set(unsigned long, bool)>
[...]
// XOR operation
400d39: 48 8b 44 24 10 mov 0x10(%rsp),%rax
400d3e: 48 31 04 24 xor %rax,(%rsp)
A bit curious, I'd have thought "set" would get inlined/optimized away,
but YMMV.
Best regards,
Johannes
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