[Bug backtrace/16215] SPARC: can't compute CFA for this frame

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https://sourceware.org/bugzilla/show_bug.cgi?id=16215

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https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=ec95c61a84a69b706714793a15bda0c5801b6db9

commit ec95c61a84a69b706714793a15bda0c5801b6db9
Author: Joel Brobecker <brobecker@adacore.com>
Date:   Fri Nov 21 04:59:35 2014 +0100

    Lift DWARF unwinder restriction in dwarf2-frame.c::dwarf2_frame_cfa

    GDB is currently broken on all SPARC targets when using GCC 4.9.
    When trying to print any local variable:

        (gdb) p x
        can't compute CFA for this frame

    This is related to the fact that the compiler now generates DWARF 4
    debugging info by default, and in particular that it now emits
    DW_OP_call_frame_cfa, which triggers a limitation in dwarf2_frame_cfa:

       /* This restriction could be lifted if other unwinders are known to
          compute the frame base in a way compatible with the DWARF
          unwinder.  */
       if (!frame_unwinder_is (this_frame, &dwarf2_frame_unwind)
           && !frame_unwinder_is (this_frame, &dwarf2_tailcall_frame_unwind))
         error (_("can't compute CFA for this frame"));

    We couldn't append the dwarf2 unwinder to all SPARC targets because
    it does not work properly with StackGhost:
        https://www.sourceware.org/ml/gdb-patches/2014-07/msg00012.html

    We also later discovered that using the DWARF2 unwinder means
    using it for computing the function's return address, which
    is buggy when it comes to functions returning a struct (where
    the return address is saved-pc+12 instead of saved-pc+8).
    This is because GCC is emitting the info about the return address
    as %o7/%i7 instead of the actual return address.  For functions
    that have debugging info, we compensate by looking at the function's
    return type and add the extra +4, but for function without debug
    info, we're stuck.

    EricB and I twisted the issue in all the directions we could think of,
    and unfortunately couldn't find a way to make it work without
    introduction one regression or another.

    But, stepping back a little, just removing the restriction seems to work
    well for us on all both sparc-elf and {sparc,sparc64}-solaris.
    After reviewing the previous discussions about this test, I could
    not figure out whether some unwinders were already known to have
    incompatible CFAs or if the concern was purely theoretical:
        https://www.sourceware.org/ml/gdb-patches/2009-06/msg00191.html
        https://www.sourceware.org/ml/gdb-patches/2009-07/msg00570.html
        https://www.sourceware.org/ml/gdb-patches/2009-09/msg00027.html

    At the moment, we took the approach of trying it out, and see what
    happens...

    gdb/ChangeLog:

            PR backtrace/16215:
            * dwarf2-frame.c (dwarf2_frame_cfa): Remove the restriction
            the frame unwinder must either be the dwarf2_frame_unwind
            or the dwarf2_tailcall_frame_unwind.  Verify that this_frame's
            stack_addr is valid before calling get_frame_base.  Throw
            an error if not valid.

    Tested on sparc-solaris and sparc-elf with AdaCore's testsuite
    (the FSF testsuite crashes all of AdaCore's solaris machines).

-----------------------------------------------------------------------

Summary of changes:
 gdb/ChangeLog      |    9 +++++++++
 gdb/dwarf2-frame.c |   12 ++++++------
 2 files changed, 15 insertions(+), 6 deletions(-)

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