Re: RFA bug fix -- x86-64 stabs and deprecated fp register

[Doug Evans <dje at google dot com>]

On Fri, Nov 21, 2014 at 12:58 PM, David Taylor <dtaylor@emc.com> wrote:
> Sometimes when using STABS on x86-64 GNU/Linux, GDB does not know which
> register to use for the frame pointer and as a result offsets from the
> frame pointer are treated as absolute addresses rather than as
> offsets...
>
> This patch provides a default for when the debug information doesn't
> specify which register to use.
>
> We have seen this problem when debugging problems with a previous
> release of our software (I believe it was built with GCC 4.5.x, if that
> matters).
>
> There were no regressions on x86-64 GNU/Linux.
>
> 2014-11-21  David Taylor  <dtaylor@emc.com>
>
>         * amd64-tdep.c (amd64_init_abi): Set default frame pointer.
>
> diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
> index e69da01..5a68c33 100644
> --- a/gdb/amd64-tdep.c
> +++ b/gdb/amd64-tdep.c
> @@ -3006,6 +3006,8 @@ amd64_init_abi (struct gdbarch_info info, struct gdbarch *
> gdbarch)
>    set_gdbarch_ps_regnum (gdbarch, AMD64_EFLAGS_REGNUM); /* %eflags */
>    set_gdbarch_fp0_regnum (gdbarch, AMD64_ST0_REGNUM); /* %st(0) */
>
> +  set_gdbarch_deprecated_fp_regnum (gdbarch, AMD64_RBP_REGNUM); /* %rbp */
> +
>    /* The "default" register numbering scheme for AMD64 is referred to
>       as the "DWARF Register Number Mapping" in the System V psABI.
>       The preferred debugging format for all known AMD64 targets is

Hi.
I haven't dug too deep to understand whether this is a good fix or not,
but I have a request.
Given the discussion, I think it's reasonable to assume a future
reader of the code will ask "Why is this code here?"
One of my pet peeves is having to spend too much time
answering that question, and while I would much rather have
such questions answered in the code, I also don't want to have
to read more than a few sentences (in general - certainly there
are cases where a paragraph or two in the code can be
invaluable).  At any rate ...
I'll let you decide whether to add something to the code or
to the commit message (or both), but at a minimum please add
a full description to the commit message that explains things.

Thanks!