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Hopefully completely fix printf-family %a rounding (#1287)

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The a conversion specifier to printf had some issues w.r.t. rounding, in
particular in edge cases w.r.t. "to nearest, ties to even" rounding (for
instance, "%.1a" with 0x1.78p+4 outputted 0x1.7p+4 instead of 0x1.8p+4).

This patch fixes this and adds several tests w.r.t ties to even rounding
This commit is contained in:
Gabriel Ravier 2024-09-15 21:11:27 +02:00 committed by GitHub
parent e65fe614b7
commit b55e4d61a9
No known key found for this signature in database
GPG key ID: B5690EEEBB952194
2 changed files with 45 additions and 42 deletions
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45
libc/stdio/fmt.c
View file

@ -565,12 +565,12 @@ static int __fmt_stoa(int out(const char *, void *, size_t), void *arg,
static void __fmt_dfpbits(union U *u, struct FPBits *b) {
int ex, i;
b->fpi = kFpiDbl;
// Uncomment this if needed in the future - we currently do not need it, as
// the only reason we need it in __fmt_ldfpbits is because gdtoa reads
// fpi.rounding to determine rounding (which dtoa does not need as it directly
// reads FLT_ROUNDS)
// if (FLT_ROUNDS != -1)
// b->fpi.rounding = FLT_ROUNDS;
// dtoa doesn't need this, unlike gdtoa, but we use it for __fmt_bround
i = FLT_ROUNDS;
if (i != -1)
b->fpi.rounding = i;
b->sign = u->ui[1] & 0x80000000L;
b->bits[1] = u->ui[1] & 0xfffff;
b->bits[0] = u->ui[0];
@ -616,10 +616,14 @@ static void __fmt_ldfpbits(union U *u, struct FPBits *b) {
#error "unsupported architecture"
#endif
b->fpi = kFpiLdbl;
// gdtoa doesn't check for FLT_ROUNDS but for fpi.rounding (which has the
// same valid values as FLT_ROUNDS), so handle this here
if (FLT_ROUNDS != -1)
b->fpi.rounding = FLT_ROUNDS;
// (we also use this in __fmt_bround now)
i = FLT_ROUNDS;
if (i != -1)
b->fpi.rounding = i;
b->sign = sex & 0x8000;
if ((ex = sex & 0x7fff) != 0) {
if (ex != 0x7fff) {
@ -692,7 +696,6 @@ static int __fmt_bround(struct FPBits *b, int prec, int prec1) {
uint32_t *bits, t;
int i, j, k, m, n;
bool inc = false;
int current_rounding_mode;
m = prec1 - prec;
bits = b->bits;
k = m - 1;
@ -701,22 +704,24 @@ static int __fmt_bround(struct FPBits *b, int prec, int prec1) {
// 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))
// They use the FLT_ROUNDS value, which are the same as gdtoa's FPI_Round_*
// enum values
if (b->fpi.rounding == FPI_Round_zero ||
(b->fpi.rounding == FPI_Round_up && b->sign) ||
(b->fpi.rounding == FPI_Round_down && !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 ((b->fpi.rounding == FPI_Round_up && !b->sign) ||
(b->fpi.rounding == FPI_Round_down && b->sign))
goto inc_true;
if ((t = bits[k >> 3] >> (j = (k & 7) * 4)) & 8) {
if (t & 7)
goto inc_true;
if (j && bits[k >> 3] << (32 - j))
// ((1 << (j * 4)) - 1) will mask appropriately for the lower bits
if ((bits[k >> 3] & ((1 << (j * 4)) - 1)) != 0)
goto inc_true;
// If exactly halfway and all lower bits are zero (tie), round to even
if ((bits[k >> 3] >> (j + 1) * 4) & 1)
goto inc_true;
while (k >= 8) {
k -= 8;

42
test/libc/stdio/snprintf_test.c
View file

@ -217,39 +217,37 @@ TEST(snprintf, testLongDoubleRounding) {
ASSERT_EQ(0, fesetround(previous_rounding));
}
void check_a_conversion_specifier_prec_1(const char *result_str, double value) {
char buf[30] = {0};
int i = snprintf(buf, sizeof(buf), "%.1a", value);
ASSERT_EQ(strlen(result_str), i);
ASSERT_STREQ(result_str, buf);
}
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_DOWNWARD));
check_a_conversion_specifier_prec_1("0x1.fp+4", 0x1.fffffp+4);
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);
check_a_conversion_specifier_prec_1("0x2.0p+4", 0x1.f8p+4);
ASSERT_EQ(0, fesetround(previous_rounding));
}
// This test currently fails because of rounding issues
// If that ever gets fixed, uncomment this
/*
// This test specifically checks that we round to even, accordingly to IEEE
// rules
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);
check_a_conversion_specifier_prec_1("0x1.8p+4", 0x1.7800000000001p+4);
check_a_conversion_specifier_prec_1("0x1.8p+4", 0x1.78p+4);
check_a_conversion_specifier_prec_1("0x1.8p+4", 0x1.88p+4);
check_a_conversion_specifier_prec_1("0x1.6p+4", 0x1.58p+4);
check_a_conversion_specifier_prec_1("0x1.6p+4", 0x1.68p+4);
check_a_conversion_specifier_prec_1("0x1.ap+4", 0x1.98p+4);
check_a_conversion_specifier_prec_1("0x1.ap+4", 0x1.a8p+4);
}
*/
TEST(snprintf, apostropheFlag) {
char buf[20];