Re: Setting a floating point register to raw hex value

["Grzegorz Cieslewski" <cieslewski@hcs.ufl.edu>]

Thank you all for help.

In the end since there is no "clean" way of addressing my problem I
have merged the ideas into one.

I decided that forcing shared library to load is slightly better than
overwriting a random memory location (I would have to fix that before
continuing to execute the program).  The shared library contains only
one global variable which is a array of chars. Then I use the method
suggested by Thiago:
(gdb) set {long long} ($temp_xyz) = 1
(gdb) set $f0 = {double} ($temp_xyz)




On 10/19/07, Thiago Jung Bauermann <bauerman@br.ibm.com> wrote:
> On Thu, 2007-10-18 at 17:54 -0400, Daniel Jacobowitz wrote:
> > On Thu, Oct 18, 2007 at 06:16:00PM -0200, Thiago Jung Bauermann wrote:
> > > Hi,
> > >
> > > Yes, that's indeed crazy. :-)
> > >
> > > Just found out that this works:
> > >
> > > (gdb) set (void *) $f1 = 0x1
> > > (gdb) info reg f1
> > > f1             4.9406564584124654e-324  (raw 0x0000000000000001)
> >
> > I think this only works by an accident involving
> > gdbarch_convert_register_p on PowerPC.  We should find some proper way
> > to do it, document that, and then make this not work :-)
>
> You're just spoiling the fun. :-)
>
> You mean that rs6000_convert_register_p should return true for every
> type that's not double, and then GDB would try to convert a void * to a
> double and fail?
>
> I see in value_assign that $f1 in the expression I gave above is being
> represented as a value struct in register $f1 with void * type. Wouldn't
> it be reasonable for value_assign to see that this register is being
> used with a type different than it's default (double) and figure that
> the user wants to use the register in a different way and just let him
> do that?
>
> > > On Thu, 2007-10-18 at 15:38 -0400, Grzegorz Cieslewski wrote:
> > > > I have figured out a workaround but it is really ugly.
> > > >
> > > > Step 1:  Create shared object containing a union as follows
> > > > union longlong2double
> > > > {
> > > > long long int hexNum;
> > > > double floatNum;
> > > > } temp;
> > > >
> > > > Step 2: Force load the library at runtime into your program using
> > > > LD_PRELOAD environmental variable
> > > >
> > > > Step 3: When debugging the program store 0x1 into temp.hexNum;
> > > > (one would think that I could use the following now 'set $f0 =
> > > > temp.floatNum; but that yields incorrect result)
> > > >
> > > > Step4:  Overwrite the current instruction with 'load float'
> > > >
> > > > Step5: Using stepping command 'stepi 1' to write the value to the $f0
> > > >
> > > > Step6: Undo all the changes required by steps 4 and 5
> > > >
> > > > Does any one know of a simpler way because this workaround is crazy?
> >
> > Sure, here's a much easier way that ought to work:
> >
> > (gdb) set {long long} ($sp - 8) = 1
> > (gdb) set $f0 = {double} ($sp - 8)
> >
> > Doesn't have to be $sp - 8, any memory address will do.
>
> $sp - 8 will mess with the function's frame if he happens to be stepping
> through the prologue. Just telling to make sure Grzegorz is aware of
> it...
>
> > Oddly, this does not work on PowerPC.  I can't figure out why
> > not.  It certainly ought to work; perhaps it requires a current
> > version of GDB, since 0x1 is a denormal.  It does work for normals,
> > though, so I know the approach is sound.
>
> It worked for me in GDB 6.7 but not in HEAD.
> --
> []'s
> Thiago Jung Bauermann
> Software Engineer
> IBM Linux Technology Center
>
>


-- 
=====================================================
Grzegorz Cieslewski
Research Assistant
High-performance Computing & Simulation (HCS) Research Laboratory
University of Florida, Dept. of Electrical and Computer Engineering
330 Benton Hall, Gainesville, FL, 32611-6200
Phone: (352) 392-9041
Email: cieslewski@hcs.ufl.edu
Web: www.hcs.ufl.edu
=====================================================