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stdio/format: fix printing sign for long double #487

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stdio/format: fix printing sign for long double #487

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20 changes: 14 additions & 6 deletions stdio/format.c
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Original file line number Diff line number Diff line change
Expand Up @@ -621,7 +621,7 @@ static int format_sprintfScientificForm(struct buffer *buff, struct bigdouble *b
}


static int format_sprintfDouble(void *ctx, feedfunc feed, double d, uint32_t flags, int minFieldWidth, int precision, char format)
static int format_sprintfDouble(void *ctx, feedfunc feed, double d, uint32_t flags, int minFieldWidth, int precision, char format, uint8_t msb)
{
struct buffer buff;
char sign = 0;
Expand All @@ -640,7 +640,7 @@ static int format_sprintfDouble(void *ctx, feedfunc feed, double d, uint32_t fla
}

/* check sign */
if (((num64 >> 63) & 1) != 0) {
if (msb != 0) {
sign = '-';
d = -d;
}
Expand All @@ -654,6 +654,7 @@ static int format_sprintfDouble(void *ctx, feedfunc feed, double d, uint32_t fla
sign = '\0';
}
num64 = format_u64FromDouble(d);
num64 |= ((uint64_t)msb << 63);

CHECK_FAIL(ret, format_bufferInit(&buff, 2 * precision + 6));

Expand Down Expand Up @@ -1178,19 +1179,26 @@ int format_parse(void *ctx, feedfunc feed, const char *format, va_list args)
case 'f':
case 'g': {
#ifndef IO_NO_FLOAT
double doubleNumber;
long double doubleNumber;
uint8_t msb = 0;

if (fmt == 'g' || fmt == 'G') {
flags |= FLAG_NO_TRAILING_ZEROS;
}
if ((flags & FLAG_LONG_DOUBLE) != 0) {
/* NOTE: support for long double is incomplete */
doubleNumber = (double)va_arg((args), long double);
doubleNumber = va_arg((args), long double);
}
else {
doubleNumber = (double)va_arg((args), double);
doubleNumber = (long double)va_arg((args), double);
}

/* preserve sign bit during down-conversion to double */
if (__builtin_signbit(doubleNumber) != 0) {
msb = 1;
}

CHECK_FAIL(ret, format_sprintfDouble(ctx, feed, doubleNumber, flags, minFieldWidth, precision, tolower(fmt)));
CHECK_FAIL(ret, format_sprintfDouble(ctx, feed, (double)doubleNumber, flags, minFieldWidth, precision, tolower(fmt), msb));
Comment on lines +1182 to +1201

@gemini-code-assist gemini-code-assist Bot Jul 3, 2026

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high

Casting standard double values to long double and then back to double introduces significant performance overhead on platforms without hardware 128-bit float support (such as RISC-V), as it forces the use of soft-float emulation routines (__extenddftf2 and __trunctfdf2) for every standard double formatting.

Additionally, this unnecessarily subjects standard double formatting to the buggy __trunctfdf2 emulation routine, which could lead to incorrect sign behavior even for standard double values.

We can avoid this overhead and risk by keeping doubleNumber as a standard double and only using long double locally when FLAG_LONG_DOUBLE is set.

double doubleNumber;
				uint8_t msb = 0;

				if (fmt == 'g' || fmt == 'G') {
					flags |= FLAG_NO_TRAILING_ZEROS;
				}
				if ((flags & FLAG_LONG_DOUBLE) != 0) {
					/* NOTE: support for long double is incomplete */
					long double ld = va_arg((args), long double);
					if (__builtin_signbit(ld) != 0) {
						msb = 1;
					}
					doubleNumber = (double)ld;
				}
				else {
					doubleNumber = va_arg((args), double);
					if (__builtin_signbit(doubleNumber) != 0) {
						msb = 1;
					}
				}

				CHECK_FAIL(ret, format_sprintfDouble(ctx, feed, doubleNumber, flags, minFieldWidth, precision, tolower(fmt), msb));
References
  1. Avoid unnecessary type promotion/demotion overhead (such as double to long double) on soft-float platforms to prevent performance degradation and potential emulation bugs.

break;
#else
if ((flags & FLAG_LONG_DOUBLE) != 0) {
Expand Down
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