Re: [PATCH v1] lib/vsprintf.c: Even faster decimal conversion

From: Jeff Epler
Date: Thu Mar 12 2015 - 20:08:53 EST

Since you asked about big-endian systems I also built your test program
for the armeb architecture -- which involved hacking up the test harness
fairly heavily to not require libc -- and ran the result in qemu.

Actually, it hasn't finished after 2 hours of (qemu) CPU time, but I can
tell from the output that the first phase has completed successfully but
the second hasn't finished (because it's printed 1 and 2 but not yet 3).
I am pretty sure this is because it's just rather slow in emulation, and
obviously performance figures are going to be useless.

Anyway, I built this on my armhf machine with
gcc-4.8 -O -o testeb linux32.c rv32.c verify.c -nostdlib -mbig-endian
copied it to my fastest x86_64 desktop and ran it with
qemu-armeb ./a.out || echo fail

#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#include <pthread.h>
#include <string.h>

#include "common.h"

#define NTHR 1

#define LO_START 10ULL
#define LO_STOP 10000000000ULL

static unsigned long long lenfreq[NTHR][32];

#include <unistd.h>
#include <sys/syscall.h>
static void do_exit(int i) {
register int i0 asm("r0") = i;
register int ss asm("r7") = SYS_exit;
__asm__ __volatile__("swi 0x0" : : "r"(ss), "r"(i0));

static void do_write(int fd, const char *buf, size_t n) {
register int i0 asm("r0") = fd;
register const char *i1 asm("r1") = buf;
register size_t i2 asm("r2") = n;
register int ss asm("r7") = SYS_write;
__asm__ __volatile__("swi 0x0" : : "r"(ss), "r"(i0), "r"(i1), "r"(i2));

int memcmp(const void * va, const void * vb, size_t sz) {
unsigned char *a, *b, aa, bb;
while(sz--) {
int diff = *a++ - *b++;
if(diff) return diff;
return 0;

static int do_check(unsigned long long n, unsigned idx)
char buf1[24];
char buf2[24];
int len1, len2;

len1 = linux_put_dec(buf1, n) - buf1;
len2 = rv_put_dec(buf2, n) - buf2;
if (len1 != len2 || memcmp(buf1, buf2, len1)) {
return -1;
return 0;

static void *check(void *arg)
unsigned long idx = (unsigned long)arg;
unsigned long long n;

do_write(1, "1\n", 2);
for (n = LO_START; n % NTHR != idx; ++n)
for (; n <= LO_STOP; n += NTHR) {
if (do_check(n, idx))
return (void*) -1;

do_write(1, "2\n", 2);
for (n = HI_START; n % NTHR != idx; --n)
for (; n >= HI_STOP; n -= NTHR) {
if (do_check(n, idx))
return (void*) -1;

* This will also visit a few one-digit numbers, but both the
* old and new code actually handle that just fine for
* non-zero n (it's just irrelevant because all callers of
* put_dec take a shortcut for n < 10).
do_write(1, "3\n", 2);
n = 2*idx + 1;
do {
if (do_check(n, idx))
return (void*) -1;
n *= 17179869185ull;
} while (n != 2*idx + 1);

return NULL;

int _start(void)
do_write(1, ".\n", 2);
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