x86rcl.b
LBT intrinsic · group: x86 Shift
Synopsis
Instruction: x86rcl.b rj, rk
CPU Flags: LBT
Description
x86-style rotate left through carry: rotate 8-bit value in rj and CF together as a 9-bit ring left by the amount specified in rk. Update CF, OF (for shift count of 1). Only EFLAGS (LBT4) is updated; the GPR is not modified.
Operation
uint8_t v = (uint8_t)a;
unsigned c = (unsigned)(b & 0x1f);
unsigned n = c % 9;
if (n != 0) {
unsigned carry_out = ((v >> (8 - n)) & 1);
EFLAGS.CF = carry_out;
if (c == 1) {
uint8_t r = (uint8_t)(v << 1);
EFLAGS.OF = ((r & 0x80) != 0) != carry_out;
}
}
Tested on real machine.
Latency and Throughput
| CPU | µarch | Latency | Throughput (IPC) |
|---|---|---|---|
| 3C5000 | LA464 | 2 | 0.5(1/2) |
| 3C6000 | LA664 | 2 | 0.5(1/2) |