mirrors/musl
1
0
Fork 0
mirror of https://git.musl-libc.org/git/musl synced 2025-01-05 06:14:25 +03:00
Code Issues Packages Projects Releases Wiki Activity

printf: drastically reduce stack usage without [long] double args

Browse Source 
internally, printf always works with the maximal-size supported
integer and floating point formats. however, the space needed to
format a floating point number is proportional to the mantissa and
exponent ranges. on archs where long double is larger than double,
knowing that the actual value fit in double allows us to use a much
smaller buffer, roughly 1/16 the size.

as a bonus, making the working buffer a VLA whose dimension depends on
the format specifier prevents the compiler from lifting the stack
adjustment to the top of printf_core. this makes it so printf calls
without floating point arguments do not waste even the smaller amount
of stack space needed for double, making it much more practical to use
printf in tightly stack-constrained environments.
This commit is contained in:
Rich Felker 2024-08-26 16:01:11 -04:00
parent c94a0c16f0
commit 572a2e2eb9
1 changed files with 8 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
src/stdio/vfprintf.c
View File

@ -178,10 +178,14 @@ static char *fmt_u(uintmax_t x, char *s)
typedef char compiler_defines_long_double_incorrectly[9-(int)sizeof(long double)];
#endif
static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t, int ps)
{
uint32_t big[(LDBL_MANT_DIG+28)/29 + 1 // mantissa expansion
+ (LDBL_MAX_EXP+LDBL_MANT_DIG+28+8)/9]; // exponent expansion
int bufsize = (ps==BIGLPRE)
? (LDBL_MANT_DIG+28)/29 + 1 + // mantissa expansion
(LDBL_MAX_EXP+LDBL_MANT_DIG+28+8)/9 // exponent expansion
: (DBL_MANT_DIG+28)/29 + 1 +
(DBL_MAX_EXP+DBL_MANT_DIG+28+8)/9;
uint32_t big[bufsize];
uint32_t *a, *d, *r, *z;
int e2=0, e, i, j, l;
char buf[9+LDBL_MANT_DIG/4], *s;
@ -618,7 +622,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
case 'e': case 'f': case 'g': case 'a':
case 'E': case 'F': case 'G': case 'A':
if (xp && p<0) goto overflow;
l = fmt_fp(f, arg.f, w, p, fl, t);
l = fmt_fp(f, arg.f, w, p, fl, t, ps);
if (l<0) goto overflow;
continue;
}