armsbc.w
LBT intrinsic · group: ARM Integer
Synopsis
Instruction: armsbc.w rj, rk, cond
CPU Flags: LBT
Description
ARM-style subtract with borrow (32-bit): conditionally subtract value in rk from rj with borrow (C in ARMFLAGS). Update ARMFLAGS (C, V, N, Z). Only ARMFLAGS (LBT4) is updated; the GPR is not modified.
Operation
if (arm_cond_holds(ARMFLAGS, cond)) {
uint32_t lhs = (uint32_t)a;
uint32_t rhs = (uint32_t)b;
uint32_t borrow_in = ARMFLAGS.C ? 0 : 1;
uint64_t subtrahend = (uint64_t)rhs + borrow_in;
uint32_t r = (uint32_t)(lhs - subtrahend);
ARMFLAGS.C = lhs < subtrahend;
ARMFLAGS.V = ((lhs ^ (uint32_t)subtrahend) & (lhs ^ r) & 0x80000000) != 0;
ARMFLAGS.N = (r >> 31) & 1;
ARMFLAGS.Z = r == 0;
}
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) |