Why does musl printf() use so much more stack when printfing floating point numbers?

_musl_printf_stack_usage.md

Why does musl printf() use so much more stack when printfing floating point numbers?

How to test?

• Create a small test program and run it with different configurations.

Run the test program on Ubuntu with glibc

$ gcc           -O0 -o svnprintf svnprintf.c && ./svnprintf
- main() starting
- found paint smudge 1008 bytes into stack
- foo
- found paint smudge 168 bytes into stack
- foo sp=0x7ffd37682110
- found paint smudge 1800 bytes into stack
- foo sp=0x7ffd37682110 sp=0x7ffd37682110
- found paint smudge 1800 bytes into stack
- foo5(): !!1 <= !!2 (=0x7ffd37682126) <= !!3; 1.00% 2
- found paint smudge 2240 bytes into stack
- foo6(): 1.000000
- found paint smudge 2208 bytes into stack
- main() ending

Run the test program on Ubuntu with mpaland printf

$ gcc -DMPALAND -O0 -o svnprintf svnprintf.c && ./svnprintf
- main() starting
- found paint smudge 1008 bytes into stack
- foo
- found paint smudge 648 bytes into stack
- foo sp=00007FFC1C7B7680
- found paint smudge 912 bytes into stack
- foo sp=00007FFC1C7B7680 sp=00007FFC1C7B7680
- found paint smudge 912 bytes into stack
- foo5(): !!1 <= !!2 (=00007FFC1C7B7696) <= !!3; 1.00% 2
- found paint smudge 976 bytes into stack
- foo6(): 1.000000
- found paint smudge 800 bytes into stack
- main() ending

Run test test program on Alpine Linux with musl

$ gcc           -O0 -o svnprintf svnprintf.c && ./svnprintf
- main() starting
- found paint smudge 1008 bytes into stack
- foo
- found paint smudge 168 bytes into stack
- foo sp=0x7ffd47ba8e40
- found paint smudge 976 bytes into stack
- foo sp=0x7ffd47ba8e40 sp=0x7ffd47ba8e40
- found paint smudge 976 bytes into stack
- foo5(): !!1 <= !!2 (=0x7ffd47ba8e56) <= !!3; 1.00% 2
- found paint smudge 8792 bytes into stack
- foo6(): 1.000000
- found paint smudge 8728 bytes into stack
- main() ending

Run test test program on Alpine Linux with mpaland printf

$ gcc -DMPALAND -O0 -o svnprintf svnprintf.c && ./svnprintf
- main() starting
- found paint smudge 1008 bytes into stack
- foo
- found paint smudge 592 bytes into stack
- foo sp=00007FFCB19D4E90
- found paint smudge 840 bytes into stack
- foo sp=00007FFCB19D4E90 sp=00007FFCB19D4E90
- found paint smudge 840 bytes into stack
- foo5(): !!1 <= !!2 (=00007FFCB19D4EA6) <= !!3; 1.00% 2
- found paint smudge 904 bytes into stack
- foo6(): 1.000000
- found paint smudge 728 bytes into stack
- main() ending

Results

• Stack usage printf()ing one float:

| OS     |  printf |  size |
| ------ | ------- | ----- |
| Ubuntu |   glibc | 2,208 |
| Ubuntu | mpaland |   800 |
| Alpine |    musl | 8,728 |
| Alpine | mpaland |   728 |

• Why does musl-libc use so much more stack space than the other printf() implementations tested?

svnprintf.c

#include <stdio.h>
#include <alloca.h>
#include <stdlib.h>

// gcc           -O0 -o svnprintf svnprintf.c && ./svnprintf
// gcc -DMPALAND -O0 -o svnprintf svnprintf.c && ./svnprintf

#ifdef MPALAND
void _putchar(char character) {
    putchar(character);
}

#include "printf/printf.c" // see https://github.com/mpaland/printf
#endif

char * sp;

// see idea from: https://stackoverflow.com/questions/389219/how-to-determine-maximum-stack-usage
void paint_stack(void) {
    sp = alloca(1) - 0;
    for (int i = 0; i < 32768; i ++) {
        sp[-i] = 0x55;
    }
}

void paint_analysis(void) {
    int i_hi = 0;
    for (int i = 0; i < 32768; i ++) {
        if (0x55 != sp[-i]) {
            i_hi = i;
        }
    }
    printf("- found paint smudge %d bytes into stack\n", i_hi);
}

void foo1(void) {
    int my_size = 1000;
    char * p = alloca(my_size);
    for (int i = 0; i < my_size; i ++) {
        p[i] = 1;
    }
}

void foo2(void) {
    printf("- foo\n");
}

void foo3(void) {
    printf("- foo sp=%p\n", sp);
}

void foo4(void) {
    printf("- foo sp=%p sp=%p\n", sp, sp);
}

void foo5(void) {
    char s[] = "!";
#ifdef MPALAND
    // see https://github.com/mpaland/printf/issues/71
    printf("- foo5(): %s%s%u <= %s%s%u (=%p) <= %s%s%u; %.2f%% %.0f\n", s, s, 1, s, s, 2, s, s, s, 3, 1.0, 2.0);
#else
    printf("- foo5(): %s%s%'u <= %s%s%'u (=%p) <= %s%s%'u; %'.2f%% %'.0f\n", s, s, 1, s, s, 2, s, s, s, 3, 1.0, 2.0);
#endif
}

void foo6(void) {
    printf("- foo6(): %f\n", 1.0);
}

void main(void) {
    printf("- main() starting\n");
    paint_stack(); foo1(); paint_analysis();
    paint_stack(); foo2(); paint_analysis();
    paint_stack(); foo3(); paint_analysis();
    paint_stack(); foo4(); paint_analysis();
    paint_stack(); foo5(); paint_analysis();
    paint_stack(); foo6(); paint_analysis();
    printf("- main() ending\n");
}