扩展的内联汇编GCC-编译时表达式错误后，错误的寄存器名称和垃圾内容“完成”

Question

I'm writing a program using assembly code to write a program that calculate one of the quadratic equation roots. I've written all of my code, but I have the following error:

main.c:37: Error: bad register name `%qword'
main.c:39: Error: junk `done' after expression

How do I correct those error, please?

My codes is:

// function for checking that assembly code is computing the correct result
double quadraticRootC(double a, double b, double c)
{
    return (-b + sqrt(b * b - 4 * a * c)) / (2 * a);
}

double quadraticRoot(double a, double b, double c)
{
    double root;

    asm(
        "fld        %1              \n"
        "fadd       %%ST            \n"
        "fld        %1              \n"
        "fld        %3              \n"
        "fmulp      %%ST(1)         \n"
        "fadd       %%ST            \n"
        "fadd       %%ST            \n"
        "fchs                       \n"
        "fld        %2              \n"
        "fld        %2              \n"
        "fmulp      %%ST(1)         \n"
        "faddp      %%ST(1)         \n"
        "ftst                       \n"
        "fstsw      %%1X            \n"
        "sahf                       \n"
        "fsqrt                      \n"
        "fld        %2              \n"
        "fchs                       \n"
        "fdivp      %%ST(1)         \n"
        "mov        %0, %%eax    \n"
        "fstp       %%qword, %%eax  \n"
        "mov        $1, %%eax       \n"
        "jmp        short done      \n"
        "done:                      \n"
        :"=g"(root)
        :"g"(a), "g"(b), "g"(c)
        :"eax"
        );
    return(root);
}

int main(int argc, char **argv)
{
    double  a, b, c;
    double  root, rootC;

    if (argc != 4) {
        printf("need 3 arguments: a, b, c\n");
        return -1;
    }
    a = atof(argv[1]);
    b = atof(argv[2]);
    c = atof(argv[3]);
    root = quadraticRoot(a, b, c);
    rootC = quadraticRootC(a, b, c);

    printf("quadraticRoot(%.3f, %.3f, %.3f) = %.3f, %.3f\n", a, b, c, root, rootC);

    return 0;
}

The line where the error occurs:

"mov        %0, %%eax    \n"
"fstp       %%qword, %%eax  \n"
"mov        $1, %%eax       \n"

Any help would be appreciated.

Answer 1

Apart from the errors you listed, there were some other issues. The main one being that the default operand size for fld is single precision, while you have doubles. So you need to explicitly use fldl . Second, if you ever find yourself using g constraint, think twice, because chances are you are doing something wrong. In this case, the input operands should be memory references ( m constraint), and the output should be left in the FPU stack ( t constraint). I have removed the unused code dealing with the status word for now, I suppose that is a placeholder to detect taking the square root of a negative number.

You should always comment your code, but especially if you ask others for help. Since you haven't done that, I have no idea what you wanted to do with eax at the end. I removed that too.

Here is a working version :

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

// function for checking that assembly code is computing the correct result
double quadraticRootC(double a, double b, double c)
{
    return (-b + sqrt(b * b - 4 * a * c)) / (2 * a);
}

double quadraticRoot(double a, double b, double c)
{
    double root;

    asm(
        "fldl       %1       # a                   \n"
        "fadd       %%ST     # 2a                  \n"
        "fldl       %1       # a 2a                \n"
        "fldl       %3       # c a 2a              \n"
        "fmulp      %%ST(1)  # ac 2a               \n"
        "fadd       %%ST     # 2ac 2a              \n"
        "fadd       %%ST     # 4ac 2a              \n"
        "fldl       %2       # b 4ac 2a            \n"
        "fldl       %2       # b b 4ac 2a          \n"
        "fmulp      %%ST(1)  # b^2 4ac 2a          \n"
        "fsubp      %%ST(1)  # b^2-4ac 2a          \n"
        "fsqrt               # sqrt(b^2-4ac) 2a    \n"
        "fldl       %2       # b sqrt(b^2-4ac) 2a  \n"
        "fchs                # -b sqrt(b^2-4ac) 2a \n"
        "faddp      %%ST(1)  # -b+sqrt(b^2-4ac) 2a \n"
        "fdivp      %%ST(1)  # -b+sqrt(b^2-4ac)/2a \n"
        :"=t"(root)
        :"m"(a), "m"(b), "m"(c)
        );
    return(root);
}

int main(int argc, char **argv)
{
    double  a, b, c;
    double  root, rootC;

    if (argc != 4) {
        printf("need 3 arguments: a, b, c\n");
        return -1;
    }
    a = atof(argv[1]);
    b = atof(argv[2]);
    c = atof(argv[3]);
    root = quadraticRoot(a, b, c);
    rootC = quadraticRootC(a, b, c);

    printf("quadraticRoot(%.3f, %.3f, %.3f) = %.3f, %.3f\n", a, b, c, root, rootC);

    return 0;
}