Re: ignore helper no longer works?

["Jafa" <jafa at silicondust dot com>]

Hi Andrew,

>- When stepping, the need to identify and single step through
>trampolines / helpers.

I have updated the backtrace code so that if it detects that it is in an
epilogue stub then return a frame with no known return address (0) and no
known FP (0).

I can probably work out the FP of the previous function if need be, but have
no hope (without some simulation) of figuring out the return address.

Step-over at the end of a fucntion results in gdb running without stopping
(and it doesn't call ignore_helper).

One way to address this problem would be for gdb to call ignore_helper, and
then know not attempt a stack-backtrace.

Another solution (ok, crude hack) would be for the tdep to cache the last
known PC so that if it steps into a stub then it knows where it came from.

>- When stopped, the need to back trace out of a trampoline / helper.

I think it is accceptable not being able to do a backtrace if you are in an
epilogue helper.... the code now returns that the FP and PC are unknown to
abort the backtrace.

I am open to ideas (I figure this must be a rare but general problem). If
all else fails I will implement the crude hack above because I need to get
this working.

Thanks

Nick

----- Original Message -----
From: "Andrew Cagney" <ac131313@redhat.com>
To: "Jafa" <jafa@silicondust.com>
Cc: <gdb@sources.redhat.com>
Sent: Thursday, July 03, 2003 8:10 AM
Subject: Re: ignore helper no longer works?



> 1) Say it is my responsibility to determine the return address anywhere in
> code, even if it means writing a whole lot of code in gdb to simulate the
> execution of these stubs.
>
> 2) Fix gdb at a higher level so that it doesn't try to obtain a return
> address from a helper function - instead keep stepping (like it gdb used
to
> do).

Sounds like you've got two problems:

- When stepping, the need to identify and single step through
trampolines / helpers.

- When stopped, the need to back trace out of a trampoline / helper.

Given a core file, it isn't possible to `step'.  Your unwinder is going
to need to identify and dig itself out of that hole.  Main thing to do
is to not lie - from what I understand of your case, given a callee that
jumped to a helper, you'll end up displaying:
<helper>
caller

Andrew


> I can figure out the FP ok (all be it that it is painful without being
able
> to see the prologue) - the old system only asked for the FP and after
> detecting that it was the same frame, kept stepping.
>
> BTW - I have also noticed that gdb now does not call ignore_helper for any
> step-over operations, instead relying on the tdep to supply the return
> address for the stub. It think it always used to call ignore_helper and if
> it was a helper then it stepped rather that setting a breakpoint as if it
> was a function.
>
> I don't want to be too quick to butcher infrun - I would appreciate your
> advice.