[Bug symtab/18392] ../../gdb/dwarf2loc.c:834: internal-error: chain_candidate: Assertion `result->callers + result->callees < result->length' failed

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18392

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jan Kratochvil <jkratoch@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=e0619de699ae6e86d8b93fa96a7668aef2e9636a

commit e0619de699ae6e86d8b93fa96a7668aef2e9636a
Author: Jan Kratochvil <jan.kratochvil@redhat.com>
Date:   Mon Jun 1 14:02:34 2015 +0200

    PR symtab/18392

    Initially there is some chain (let's say the longest one
    but that doe snot matter).  Consequently its elements from the middle are
    being removed and there remains only some few unambiguous top and bottom
ones.

    The original idea why the comparison should be sharp ("<") was that if
there
    are multiple chains like (0xaddr show jmp instruction address):
            main(0x100) -> a(0x200) -> d(0x400)
            main(0x100) -> a(0x200) -> c(0x300) -> d(0x400)
    then - such situation cannot exist - if two jmp instructions in "a" have
the
    same address they must also jump to the same address (*).

    (*) jump to a computed address would be never considered for the DWARF
        tail-call records.

    So there could be:
            main(0x100) -> a(0x200) -> d(0x400)
            main(0x100) -> a(0x270) -> c(0x300) -> d(0x400)
    But then "a" frame itself is ambiguous and it must not be displayed.

    I did not realize that there can be self-tail-call:
            main(0x100) -> a(0x200) -> d(0x400)
            main(0x100) -> a(0x280) -> a(0x200) -> d(0x400)
    which intersects to:
            main(0x100) -> <???>? -> a(0x200) -> d(0x400)
    And so if the first chain was chosen the
            main(0x100) -> a(0x200) -> d(0x400)
    then the final intersection has callers+callees==length.

    > for example, if CALLERS is 3 and
    > CALLEES is 2, what does the chain look like?

    main(0x100) -> x(0x150) -> y(0x200) -> <???>? -> a(0x200) -> d(0x400)

    And if LENGTH is 7 then:
            call_site[0] = main(0x100)
            call_site[1] = x(0x150)
            call_site[2] = y(0x200)
            call_site[3] = garbage
            call_site[4] = garbage
            call_site[5] = a(0x200)
            call_site[6] = d(0x400)

    gdb/ChangeLog
    2015-06-01  Andreas Schwab  <schwab@linux-m68k.org>
            Jan Kratochvil  <jan.kratochvil@redhat.com>

        PR symtab/18392
        * dwarf2-frame-tailcall.c (pretended_chain_levels): Correct
        assertion.
        * dwarf2loc.c (chain_candidate): Likewise.

    gdb/testsuite/ChangeLog
    2015-06-01  Jan Kratochvil  <jan.kratochvil@redhat.com>

        PR symtab/18392
        * gdb.arch/amd64-tailcall-self.S: New file.
        * gdb.arch/amd64-tailcall-self.c: New file.
        * gdb.arch/amd64-tailcall-self.exp: New file.

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