It's an operation that is required in some cases, and it can easily be emulated by the compiler for targets which lack it. However deducing that you're trying to do bit rotation and emitting the CPU instruction when present is not easy for the compiler.
Uh, no. a rotate_left b is equivalent to a << b | a >>> (sizeof_in_bits(a) - b). As far as common instructions not present in language standards go, regular rotates are the easiest for the compiler to recognize.
edit: For example, VS2017 x86 target does not output rol instruction for your code with /O1.
For the record, here was what I was testing:
#include <iostream>
uint8_t rol(uint8_t x, uint8_t c) {
return (x << c | x >> (sizeof(x) * 8 - c));
}
int main()
{
volatile uint8_t a = 0b01010000;
volatile uint8_t b = 3;
uint8_t c = rol(a, b);
std::cout << static_cast<unsigned int>(0b10000010) << std::endl;
std::cout << static_cast<unsigned int>(c) << std::endl;
return 0;
}Weirdly enough, it does have rotate instructions. Even weirder, there is no logical shift right instruction on the PDP-11. The original c compiler must have been emulating one by clearing the carry bit beteen rotates.
unsigned int rotv(unsigned int x, int c) {
return (x << c) | (x >> (32 - c));
}
Relevant to the original post here of course, as the original spec was a processor that could interpret BBC BASIC as fast as hand optimised 6502 assembler.