The measured mcycle is less than minstret in Spike - riscv

Question

I use spike to run the test program in "riscv-tools/riscv-tests/build/benchmarks":
$ spike multiply.riscv
And the output shows:
mcycle = 24096
minstret = 24103
Why mcycle is less than minstret?
Dose it means that spike can run more than one instructions in one cycle?
(I tried to trace spike code but cannot find how mcycle is counted.)

Answer 1

The printing of mcycle and minstret values are not from Spike in this case, it is from the test (benchmark). There is the code:
https://github.com/ucb-bar/riscv-benchmarks/blob/master/common/syscalls.c
#define NUM_COUNTERS 2
static uintptr_t counters[NUM_COUNTERS];
static char* counter_names[NUM_COUNTERS];
static int handle_stats(int enable)
{
int i = 0;
#define READ_CTR(name) do { \
while (i >= NUM_COUNTERS) ; \
uintptr_t csr = read_csr(name); \
if (!enable) { csr -= counters[i]; counter_names[i] = #name; } \
counters[i++] = csr; \
} while (0)
READ_CTR(mcycle);
READ_CTR(minstret);
#undef READ_CTR
return 0;
}
There is some code between reading mcycle & minstret. And now you know how much code is there between readings.
In Spike mcycle & minstret are always equal by definition (they are handled by the same code): https://github.com/riscv/riscv-isa-sim/blob/9e012462f53113dc9ed00d7fbb89aeafeb9b89e9/riscv/processor.cc#L347
case CSR_MINSTRET:
case CSR_MCYCLE:
if (xlen == 32)
state.minstret = (state.minstret >> 32 << 32) | (val & 0xffffffffU);
else
state.minstret = val;
break;
The syscalls.c was linked into multiply.riscv binary from https://github.com/ucb-bar/riscv-benchmarks/blob/master/multiply/bmark.mk - multiply_riscv_bin = multiply.riscv
$(multiply_riscv_bin): ... $(patsubst %.c, %.o, ... syscalls.c ... )
There is _init in syscalls.c function which calls main of the test and prints values, recorded on SYS_stats "syscall" with handle_stats.
void _init(int cid, int nc)
{
init_tls();
thread_entry(cid, nc);
// only single-threaded programs should ever get here.
int ret = main(0, 0);
char buf[NUM_COUNTERS * 32] __attribute__((aligned(64)));
char* pbuf = buf;
for (int i = 0; i < NUM_COUNTERS; i++)
if (counters[i])
pbuf += sprintf(pbuf, "%s = %d\n", counter_names[i], counters[i]);
if (pbuf != buf)
printstr(buf);
exit(ret);
}