[Bug debug/48220] DW_OP_GNU_entry_value/DW_TAG_GNU_call_site_parameter vs. register window targets

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48220

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-03-23 20:30:53 UTC ---
(In reply to comment #4)
> Yes, end of the prologue.  I was under the impression that, with your alternate
> scheme, the opposite situation would arise, i.e. it wouldn't be possible to use
> DW_OP_GNU_entry_value after the end of the prologue.  Given how GDB works, this
> sounded far less appealing than the current scheme.  Sorry if I misunderstood.

If say reference to first parameter's original value is
DW_OP_GNU_entry_value 1 <DW_OP_reg8> (i.e. %o0), then it will work just fine
anywhere in the function.  The debugger either looks up matching entry
for %o0 in DW_TAG_GNU_call_site, or a debugger could put a breakpoint on the
first insn in a function and remember the %o0 value there, then just use the
remembered value anywhere where DW_OP_GNU_entry_value appears.
And for SPARC, using %o0 is also much easier for the debugger, it doesn't have
to translate anything.

On IA-64 things are different because there it is mainly the call insn which
moves the register window, not some insn executed somewhere in function's
prologue.