Re: IA32: printing FP register variables

[Jeffrey A Law <law@cygnus.com>]

  In message < 199907090356.WAA01337@zwingli.cygnus.com >you write:
  > On IA32 processors, how should GDB find the values of variables which
  > live in floating-point registers?  At the moment, it can't do this
  > reliably, which must be a royal pain for people doing numeric work.
To say the least :-)  It's not just folks doing numeric work -- it's a 
pain for those of us who have to analyze bug reports on ia32 which perform
FP computations ;-)


  > What does the register number which GCC emits now mean?  If an N_RSYM
  > stab has a value of 8, what does that mean?  ST(0)?  When?  Every
  > variable is ST(0) when it's just been pushed.
I have some guesses about what it might mean ;-)

gcc works by initially pretending it has a flat register file with 8 FP regs.
It assigns user variables, temporaries, etc to those 8 FP regs.  I suspect
the number gcc emits corresponds to the index into the flat register file.

After allocation is finished, gcc converts the flat register file into a 
stacked register file.  I do not think it tries to update any debug info
when converting to a stacked register file.

I believe gcc will have to emit more debug info for gdb to be able to find
FP variables on the regstack.


  > Should GDB derive this info on its own?  It could disassemble the
  > function, starting from the end of the prologue, and count pushes and
  > pops, building a table mapping PC values onto stack depths.  (This
  > assumes that the stack depth is constant at a given PC.)  That would
  > require no debug info, but would be a pain to implement.
Not only would it be a pain, I believe gcc already has this information handy,
just not in a good place for emitting debugging symbols.
jeff