[Bug symtab/29836] New: [gdb/symtab] Use current pc and location lists to evaluate DW_OP_entry_value for current frame

[Bug symtab/29836] New: [gdb/symtab] Use current pc and location lists to evaluate DW_OP_entry_value for current frame

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29836

            Bug ID: 29836
           Summary: [gdb/symtab] Use current pc and location lists to
                    evaluate DW_OP_entry_value for current frame
           Product: gdb
           Version: unknown
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: symtab
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

[ Spinoff PR of PR29813. ]

In some cases we can't evaluate a DW_OP_entry_value the usual way, by looking
at call site information.

We could try to use the .debug_loc information for the formal parameters of the
current function.

For instance, for a function f1, with parameter i, we find have location list
entries:
...
      0037 1000067c 10000680 (DW_OP_reg3 (r3))
      004a 10000680 10000690 (DW_OP_GNU_entry_value: (DW_OP_reg3 (r3));
                              DW_OP_stack_value)
...
and we could use the fact that the current pc is in the 1000067c-10000680
range, and that the range starts at the start of the function to deduce that
DW_OP_GNU_entry_value: (DW_OP_reg3 (r3)) == DW_OP_reg3 (r3).

Note that this only works for the current frame.

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[Bug symtab/29836] [gdb/symtab] Use current pc and location lists to evaluate DW_OP_entry_value for current frame

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29836

--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tom de Vries <vries@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=f3e033f32a0db3e81266a62b05f867b932a5995b

commit f3e033f32a0db3e81266a62b05f867b932a5995b
Author: Tom de Vries <tdevries@suse.de>
Date:   Tue Nov 29 22:47:31 2022 +0100

    [gdb/testsuite] Fix gdb.base/vla-optimized-out.exp for ppc64le

    On powerpc64le-linux, I run into:
    ...
    (gdb) PASS: gdb.base/vla-optimized-out.exp: o1: printed optimized out vla
    p sizeof (a)^M
    $2 = <optimized out>^M
    (gdb) FAIL: gdb.base/vla-optimized-out.exp: o1: \
      printed size of optimized out vla
    ...

    The problem happens as follows.

    In order to find the size of the optimized out vla, gdb needs to evaluate:
    ...
    <155> DW_AT_upper_bound : 13 byte block: f3 1 53 23 1 8 20 24 8 20 26 31 1c
\
      (DW_OP_GNU_entry_value: (DW_OP_reg3 (r3)); DW_OP_plus_uconst: 1;
       DW_OP_const1u: 32; DW_OP_shl; DW_OP_const1u: 32; DW_OP_shra; DW_OP_lit1;
       DW_OP_minus)
    ...

    When trying to evaluate DW_OP_GNU_entry_value, it looks for a call site
    matching the pc, but doesn't find it:
    ...
    $ gdb -q -batch outputs/gdb.base/vla-optimized-out/vla-optimized-out-o1 \
      -ex "break f1" -ex run -ex "set debug entry-values 1" -ex "print sizeof
(a)"
    Breakpoint 1 at 0x1000067c: file vla-optimized-out.c, line 34.

    Breakpoint 1, f1 (i=5) at vla-optimized-out.c:34
    34      }
    DW_OP_entry_value resolving cannot find DW_TAG_call_site 0x100006b0 in main
    $1 = <optimized out>
    ....

    The call site lookup fails because the call site label .LVL4:
    ...
            bl f1    # 11   *call_value_nonlocal_aixdi      [length = 8]
            nop
    .LVL4:
    ...
    is not placed directly after the bl insn.  This is gcc PR target/107909.

    However, after manually fixing the .s file we have instead:
    ...
    Cannot find matching parameter at DW_TAG_call_site 0x10000690 at main
    $1 = <optimized out>
    ...
    due to the fact that the call site has no call site parameters.

    The call site does have a reference to the corresponding function f1, with
    parameter i, for which we find location list entries:
    ...
      0037 1000067c 10000680 (DW_OP_reg3 (r3))
      004a 10000680 10000690 (DW_OP_GNU_entry_value: (DW_OP_reg3 (r3));
                              DW_OP_stack_value)
    ...
    and we could use the fact that the current pc is in the 1000067c-10000680
    range, and that that the range starts at the start of the function, to
deduce
    that DW_OP_GNU_entry_value: (DW_OP_reg3 (r3)) == DW_OP_reg3 (r3).
    But that's a non-trivial enhancement, filed as enhancement PR symtab/29836.

    Fix this by allowing <optimized out> for target powerpc and the gcc
compiler.

    Reviewed-By: Carl Love <cel@us.ibm.com>
    Tested-By: Carl Love <cel@us.ibm.com>
    PR testsuite/29813
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29813

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