Re: Setting a floating point register to raw hex value

[Thiago Jung Bauermann <bauerman@br.ibm.com>]

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