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utils: Use fixed-point arithmetic in qemu_strtosz

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Once we've parsed the fractional value, extract it into an integral
64-bit fraction.  Perform the scaling with integer arithmetic, and
simplify the overflow detection.

Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210315155835.1970210-2-richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2021-03-14 16:49:30 -06:00
parent 51204c2f18
commit 7625a1ed01
2 changed files with 36 additions and 16 deletions
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2
tests/unit/test-cutils.c
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@ -2128,7 +2128,7 @@ static void test_qemu_strtosz_float(void)
str = "12.345M";
err = qemu_strtosz(str, &endptr, &res);
g_assert_cmpint(err, ==, 0);
g_assert_cmpint(res, ==, (uint64_t) (12.345 * MiB));
g_assert_cmpint(res, ==, (uint64_t) (12.345 * MiB + 0.5));
g_assert(endptr == str + 7);
}

50
util/cutils.c
View file

@ -275,10 +275,9 @@ static int do_strtosz(const char *nptr, const char **end,
int retval;
const char *endptr, *f;
unsigned char c;
bool mul_required = false, hex = false;
uint64_t val;
bool hex = false;
uint64_t val, valf = 0;
int64_t mul;
double fraction = 0.0;
/* Parse integral portion as decimal. */
retval = qemu_strtou64(nptr, &endptr, 10, &val);
@ -308,17 +307,19 @@ static int do_strtosz(const char *nptr, const char **end,
* without fractional digits. If we see an exponent, treat
* the entire input as invalid instead.
*/
double fraction;
f = endptr;
retval = qemu_strtod_finite(f, &endptr, &fraction);
if (retval) {
fraction = 0.0;
endptr++;
} else if (memchr(f, 'e', endptr - f) || memchr(f, 'E', endptr - f)) {
endptr = nptr;
retval = -EINVAL;
goto out;
} else if (fraction != 0) {
mul_required = true;
} else {
/* Extract into a 64-bit fixed-point fraction. */
valf = (uint64_t)(fraction * 0x1p64);
}
}
c = *endptr;
@ -333,16 +334,35 @@ static int do_strtosz(const char *nptr, const char **end,
mul = suffix_mul(default_suffix, unit);
assert(mul > 0);
}
if (mul == 1 && mul_required) {
endptr = nptr;
retval = -EINVAL;
goto out;
if (mul == 1) {
/* When a fraction is present, a scale is required. */
if (valf != 0) {
endptr = nptr;
retval = -EINVAL;
goto out;
}
} else {
uint64_t valh, tmp;
/* Compute exact result: 64.64 x 64.0 -> 128.64 fixed point */
mulu64(&val, &valh, val, mul);
mulu64(&valf, &tmp, valf, mul);
val += tmp;
valh += val < tmp;
/* Round 0.5 upward. */
tmp = valf >> 63;
val += tmp;
valh += val < tmp;
/* Report overflow. */
if (valh != 0) {
retval = -ERANGE;
goto out;
}
}
if (val > (UINT64_MAX - ((uint64_t) (fraction * mul))) / mul) {
retval = -ERANGE;
goto out;
}
*result = val * mul + (uint64_t) (fraction * mul);
*result = val;
retval = 0;
out: