ceil(log2(x)) implementation for HiFive1 RevB

log2_ceil_hifive1_revb.c

int log2_ceil(uint32_t v)
{
	if ( v == 0 ) {
		return -1;
	} else if ( v == 1 ) {
		return 0;
	} else if (((int32_t)(v-1)) < 0) {
		return 32;
	} else {
		for ( ;; ) {
			uint32_t mcycle_beg, mcycle_end;
			uint32_t v_result;
			uint32_t v_mid;
			uint32_t mcycle_delta;
			asm volatile (
				"divu %[v_mid], %[vnum], %[vden]\n"
				"csrr %[mcycle_beg], mcycle\n"
				"mv %[v_result], %[v_mid]\n"
				"csrr %[mcycle_end], mcycle\n"
				: [mcycle_beg] "=&r"(mcycle_beg)
				, [mcycle_end] "=&r"(mcycle_end)
				, [v_result] "=&r"(v_result)
				, [v_mid] "=&r"(v_mid)

				: [vnum] "r"(v-1)
				, [vden] "r"(1)

				:
			);
			(void)v_mid;
			(void)v_result;

			mcycle_delta = mcycle_end - mcycle_beg;

			// If the cycle count jumped up, we probably hit an interrupt
			if ( mcycle_delta < 42 ) {
				return (mcycle_delta+1)-5;
			}
		}
	}
}