acos_test.c

// acos_test.c
#include <stdio.h>
#include <openlibm.h>

// Fill the stack with 0x55, 0x55, 0x55...., which is a big double no matter where you start
void prepare_stack() {
        volatile unsigned char stack[256];
        for( unsigned int i=0; i<256; ++i )
                stack[i] = 0x55;
}

int main(void) {
        // Do this once so that we don't have to bother with the dl loading stuff
        volatile double x = 0.8;
        acos(x);

        prepare_stack();
        double y = acos(x);
        printf("%.16f\n", y);
        return 0;
}

shell output

[root@47a48e9c363b openlibm-1581174c85f7b645b15ba1ac1c3a98fb601f0fe7]# gcc -O1 -o acos_test -march=pentium4 -m32 -fno-gnu89-inline -fno-builtin -std=c99 -fPIC -I src -I include ./acos_test.c ./src/e_acos.c ./src/e_sqrt.c && ./acos_test
0.6435011087932843
[root@47a48e9c363b openlibm-1581174c85f7b645b15ba1ac1c3a98fb601f0fe7]# gcc -O1 -fpeephole2 -o acos_test -march=pentium4 -m32 -fno-gnu89-inline -fno-builtin -std=c99 -fPIC -I src -I include ./acos_test.c ./src/e_acos.c ./src/e_sqrt.c && ./acos_test
6066969536937485543883473977334376971721498622912628554206287716489800464666960761304196338155520.0000000000000000