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[PULL 1/1] utils: Use fixed-point arithmetic in qemu_strtosz
From: |
Richard Henderson |
Subject: |
[PULL 1/1] utils: Use fixed-point arithmetic in qemu_strtosz |
Date: |
Mon, 15 Mar 2021 12:51:17 -0600 |
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>
---
tests/unit/test-cutils.c | 2 +-
util/cutils.c | 50 ++++++++++++++++++++++++++++------------
2 files changed, 36 insertions(+), 16 deletions(-)
diff --git a/tests/unit/test-cutils.c b/tests/unit/test-cutils.c
index bad3a60993..e025b54c05 100644
--- a/tests/unit/test-cutils.c
+++ b/tests/unit/test-cutils.c
@@ -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);
}
diff --git a/util/cutils.c b/util/cutils.c
index d89a40a8c3..c442882b88 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -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:
--
2.25.1