Re: [PATCH] Fix wrong assertions

[Yao Qi <qiyaoltc@gmail.com>]

Jan Kratochvil <jan.kratochvil@redhat.com> writes:

> That '<' and not '<=' was there intentional.  Personally I think it needs more
> investigation why that can happen.  The idea was that if two solutions exist
> neither can be perfect so there have to be some ambiguous enties so there will
> be '<' and not '<=' (to fit the ambiguous entries between).
>
> But creating artifical reproducers is a bit difficult and you haven't provided
> a reproducer so I cannot say anything much specific.
>
> Personally I do not mind, it is up to the maintainers whether the goal is just
> to remove the assertion or verify there aren't some other bugs causing it.

I can reproduce this internal error via Jan's test case, and spend some
time investigating it, but still unable to fully understand the code.

Jan, since you wrote this piece of code, please help me to understand
it.

(gdb) set debug entry-values 1
(gdb) bt
tailcall: initial: 0x40052e(a)
tailcall: compare: 0x400527(a) 0x40052e(a)
tailcall: reduced: | 0x40052e(a)
gdb/git/gdb/dwarf2loc.c:834: internal-error: chain_candidate: Assertion `result->callers + result->callees < result->length' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) y

I don't know why we need do intersection in chain_candidate, as the
comments say:

/* Intersect RESULTP with CHAIN to keep RESULTP unambiguous, keep in RESULTP
   only top callers and bottom callees which are present in both.  GDBARCH is
   used only for ENTRY_VALUES_DEBUG.  RESULTP is NULL after return if there are
   no remaining possibilities to provide unambiguous non-trivial result.
   RESULTP should point to NULL on the first (initialization) call.  Caller is
   responsible for xfree of any RESULTP data.  */

What do you mean by "ambiguous" here?  Is it ambiguous if we can get
more than one call chain path from caller_pc to callee_pc?  For example,
main tail calls a, a tail call b and c, b and c tail call d, when GDB
unwinds from d, there are two chains, main -> a -> b -> d, and main -> a
-> c -> d.  Are they ambiguous by your definition?

Further, what is "partially ambiguous result" in the comments below?

/* Determined tail calls for constructing virtual tail call frames.  */

struct call_site_chain
  {
    /* Initially CALLERS == CALLEES == LENGTH.  For partially ambiguous result
       CALLERS + CALLEES < LENGTH.  */
    int callers, callees, length;

    /* Variably sized array with LENGTH elements.  Later [0..CALLERS-1] contain
       top (GDB "prev") sites and [LENGTH-CALLEES..LENGTH-1] contain bottom
       (GDB "next") sites.  One is interested primarily in the PC field.  */
    struct call_site *call_site[1];
  };

I am confused by the usage of the variable-sized array call_site,
elements from 0 to CALLERS-1 are top sites, and elements from
LENGTH-CALLEES to LENGTH-1 are bottom sites, so I conclude that
CALLERS-1 < LENGTH-CALLEES, then CALLERS + CALLEES < LENGTH + 1,
then CALLERS + CALLEES =< LENGTH.  Is it right?

I still have other questions, but I'd like to stop here, otherwise this
mail will be too long.  Your answers to these questions may help to
answer the rest of my questions.

-- 
Yao (齐尧)