question about assembly code

[Ian Lance Taylor]

Olivier Maury <Olivier_maury@mentor.com> writes:

> I was wondering, is the following assembly code correct:
>      c94:       e8 fc ff ff ff          call   c95
> <load_param_def_given+0x1d1>
>      c99:       84 c0                   test   %al,%al
>      c9b:       dd 45 d0                fldl   -0x30(%ebp)
>      c9e:       dd 45 d8                fldl   -0x28(%ebp)
>      ca1:       da c9                   fcmove %st(1),%st
>      ca3:       dd d9                   fstp   %st(1)
>      ca5:       dd 5d d0                fstpl  -0x30(%ebp)
>      ca8:       89 f8                   mov    %edi,%eax
>
> This code is some optimized assembly code extracted from the object
> file using objdump. My concern here is that I have a test instruction
> without the corresponding jump !

The test instruction is setting the flags for the fcmove instruction.


> That piece of assembly comes from a code that look like:
> if (my_function(param1, &out_param))
>     value = out_param;
>
> with :
> out_param a double value that is not assigned with a default value
> and
> char my_function(long param1, double *out)
> {
>    char ret = 0;
>     ... do some stuff ...
>     if (some_property)
>    {
>        ... do some stuff ...
>        *out = a_value_computed;
>        ret = 1;
>    }
>    ....
>    return ret;
> }
>
> And taking that optimized code in a debugger it crash with a FPE from
> time to time because even if the my_function returns 0 it seems to do
> the assignement (value = out_param) with an unitialized out_param
> value !
>
> What do you think ? Am I doing something wrong or is it a gcc bug ?

It's pretty hard to say anything about an incomplete example.  However,
fcmove is a conditional move, so the code will only do the move if the
value returned is non-zero.  I don't see anything wrong in what you have
shown us.

Ian