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Partially fix printf hex float numbers/%a rounding (#1286)

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Hexadecimal printing of floating-point numbers in cosmopolitan (that is,
using the the conversion specifier) is improved to have correct rounding
of results in rounding modes other than the default one (ie. FE_NEAREST)

This commit fixes that, and adds tests for the change (note that there's
still some rounding issues with the a conversion specifier in general in
relatively rare cases (that is without non-default rounding modes) where
I've left commented-out tests for anyone interested in improving it more
This commit is contained in:
Gabriel Ravier 2024-09-11 05:42:52 +02:00 committed by GitHub
parent 51c0f44d1c
commit 4d05060aac
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GPG key ID: B5690EEEBB952194
2 changed files with 61 additions and 8 deletions
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35
libc/stdio/fmt.c
View file

@ -690,26 +690,45 @@ static int __fmt_fpiprec(struct FPBits *b) {
// prec1 = incoming precision (after ".")
static int __fmt_bround(struct FPBits *b, int prec, int prec1) {
uint32_t *bits, t;
int i, inc, j, k, m, n;
int i, j, k, m, n;
bool inc = false;
int current_rounding_mode;
m = prec1 - prec;
bits = b->bits;
inc = 0;
k = m - 1;
// The first two ifs here handle cases where rounding is simple, i.e. where we
// always know in which direction we must round because of the current
// rounding mode (note that if the correct value for inc is `false` then it
// doesn't need to be set as we have already done so above)
// The last one handles rounding to nearest
current_rounding_mode = fegetround();
if (current_rounding_mode == FE_TOWARDZERO ||
(current_rounding_mode == FE_UPWARD && b->sign) ||
(current_rounding_mode == FE_DOWNWARD && !b->sign))
goto have_inc;
if ((current_rounding_mode == FE_UPWARD && !b->sign) ||
(current_rounding_mode == FE_DOWNWARD && b->sign)) {
inc = true;
goto have_inc;
}
if ((t = bits[k >> 3] >> (j = (k & 7) * 4)) & 8) {
if (t & 7)
goto inc1;
goto inc_true;
if (j && bits[k >> 3] << (32 - j))
goto inc1;
goto inc_true;
while (k >= 8) {
k -= 8;
if (bits[k >> 3]) {
inc1:
inc = 1;
goto haveinc;
inc_true:
inc = true;
goto have_inc;
}
}
}
haveinc:
have_inc:
b->ex += m * 4;
i = m >> 3;
k = prec1 >> 3;

34
test/libc/stdio/snprintf_test.c
View file

@ -216,3 +216,37 @@ TEST(snprintf, testLongDoubleRounding) {
ASSERT_EQ(0, fesetround(previous_rounding));
}
TEST(snprintf, testAConversionSpecifierRounding) {
int previous_rounding = fegetround();
ASSERT_EQ(0, fesetround(FE_DOWNWARD));
char buf[20];
int i = snprintf(buf, sizeof(buf), "%.1a", 0x1.fffffp+4);
ASSERT_EQ(8, i);
ASSERT_STREQ("0x1.fp+4", buf);
ASSERT_EQ(0, fesetround(FE_UPWARD));
i = snprintf(buf, sizeof(buf), "%.1a", 0x1.f8p+4);
ASSERT_EQ(8, i);
ASSERT_STREQ("0x2.0p+4", buf);
ASSERT_EQ(0, fesetround(previous_rounding));
}
// This test currently fails because of rounding issues
// If that ever gets fixed, uncomment this
/*
TEST(snprintf, testAConversionSpecifier) {
char buf[20];
int i = snprintf(buf, sizeof(buf), "%.1a", 0x1.7800000000001p+4);
ASSERT_EQ(8, i);
ASSERT_STREQ("0x1.8p+4", buf);
memset(buf, 0, sizeof(buf));
i = snprintf(buf, sizeof(buf), "%.1a", 0x1.78p+4);
ASSERT_EQ(8, i);
ASSERT_STREQ("0x1.8p+4", buf);
}
*/