Re: Question about sibling call epilogues & registers

[Daniel Santos <daniel dot santos at pobox dot com>]

On 10/15/2016 08:41 PM, Segher Boessenkool wrote:
> Hi Daniel,
>
> On Sat, Oct 15, 2016 at 01:45:12AM -0500, Daniel Santos wrote:
> > The insn that's getting deleted is 75, where RCX is set.  I'm starting
> > to think that maybe df_analyze() presumes that my call (to the stub) is
> > invalidating RCX, although it does not.
> It looks like you don't use add_function_usage_to on the call insn?
>
>
> Segher

Oddly enough, I had forgotten to call add_function_usage_to() on my save 
stub (which I didn't post), but not the restore stub. So thanks for that 
psychic intervention. :) But if you look carefully, it's there, although 
it's hard to read because of all the line wrapping:

    (expr_list (use (reg:DI 4 si))
        (nil)))

Right now I'm using gdb and gradually walking through df_analyze() and 
learning how the data flow and problems work. I presume that I can find 
my answer somewhere in the "live register" data flow problem code . This 
eventually runs "dead code elimination" on this insn (during the 
"finalize" phase), so I believe that I'm getting closer to discovering 
the root cause. So the bright side is that I'm learning a lot more about 
gcc internals! Which is probably my main goal of this exercise.

Daniel