Re: [PATCH] i386_skip_prologue.

[Mark Kettenis <mark dot kettenis at xs4all dot nl>]

> Date: Sat, 30 Dec 2006 15:48:26 -0500
> From: Daniel Jacobowitz <drow@false.org>
> 
> On Sat, Dec 09, 2006 at 08:32:00PM +0000, Pedro Alves wrote:
> > The i386 targets currently don't look at line number markers
> > or the symbol table at all in i386_skip_prologue.
> > 
> > I used the attached patch to test the gcc side of the fix,
> > (http://gcc.gnu.org/ml/gcc-patches/2006-12/msg00633.html)
> > 
> > With the gcc patch above applied, this patch fixes all the runto_main issues
> > on Cygwin. There are a few other FAILs related to breakpoints and main,
> > but those are testsuite bugs, unrelated to this. I will send patches for 
> > those shortly.
> 
> As this is an i386-specific change, I'd been hoping Mark Kettenis could
> take a look at it.  Mark, any opinion on this?

I'm sorry.  I tend to suffer from packet loss at my side.

> We've been going back and forth on prologue skipping choices for a long
> time now.  We have at least four options, each with at least two users:
> 
>   - skip_prologue_using_sal
>   - refine_prologue_limit
>   - this simple approach using line numbers, i.e. what you did
>   - sticking to analysis of the instructions
> 
> Aside from how difficult the analysis is, there's very little target
> dependent about this.  It's a shame we've got so many different ways
> to do it.

Yes, that's my general feeling.  But last time I brought it up, we
didn't reach a real conclusion.

> Is there a general philosophy we could adopt that would apply to most
> or all targets?
> 
> Here's a proposal to get us started: when skipping the prologue to
> place a breakpoint or finish single stepping (skip_prologue), try to
> skip to the end of the first sal.  But some targets may optionally run
> their prologue analyzer and make sure it doesn't see anything it
> objects to - like jumps.  That would mean the i386 prologue analyzer
> would need to know about this new call to __main.  When scanning the
> prologue to build frame unwind information, ignore sals entirely.
> Scan until we hit the saved PC or until we believe we understand
> the entire frame.

The last time I tried using sals on i386, I simply encountered too
many cases where the line number information couldn't be trusted and
putting a breakpoint on a function that was defenitely called never
hit.