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assemblycompilationcompiler-construction

Segmentation fault (core dumped) when making compliation


I was solving assignment that involve creating small Compiler. My compiler produce the following assembly code for simple assignment statement

// global declaration
int k;

float x,y,z;
// inside main func
k=10;
x = y = k;

Assembly code (I'm getting Segmentation fault (core dumped) :

 #   19
                    # b_push_ext_addr (x)
        subl    $8, %esp
        movl    $x, (%esp)
                    # b_push_ext_addr (y)
        subl    $8, %esp
        movl    $y, (%esp)
                    # b_push_ext_addr (k)
        subl    $8, %esp
        movl    $k, (%esp)
                    # b_deref (signed int)
        movl    (%esp), %eax

# Proplm on next Line ... why?
         movl (%eax), %edx


        movl    %edx, (%esp)
                    # b_convert (signed int -> float)
        fildl   (%esp)
        fstps   (%esp)
                    # b_assign (float)
        movl    (%esp), %edx
        addl    $8, %esp
        movl    (%esp), %eax
        movl    %edx, (%eax)
        movl    %edx, (%esp)
                    # b_deref (float)
        movl    (%esp), %eax
        movl    (%eax), %edx
        movl    %edx, (%esp)
                    # b_convert (float -> double)
        flds    (%esp)
        fstpl   (%esp)
                    # b_convert (double -> float)
        fldl    (%esp)
        fstps   (%esp)
                    # b_assign (float)
        movl    (%esp), %edx
        addl    $8, %esp
        movl    (%esp), %eax
        movl    %edx, (%eax)
        movl    %edx, (%esp)
                    # b_pop ()
        addl    $8, %esp

When I used gdb: it point to this line:

(gdb) x/i $eip
=> 0x80485bd <main+336>:        mov    (%eax),%edx

Solution

  • The actual fault is at a similar instruction but further down:

        fstps   (%esp)       # stack has some float
                    # b_assign (float)
        movl    (%esp), %edx # edx has a float now
        addl    $8, %esp
        movl    (%esp), %eax
        movl    %edx, (%eax)
        movl    %edx, (%esp) # write the float onto stack
                    # b_deref (float)
        movl    (%esp), %eax # load the float from stack
        movl    (%eax), %edx # try to use float as pointer => segfault